0000950170-24-023183 | 10-K | XBRL Viewer | 4D Molecular Therapeutics INC

Stock-based Compensation (Tables)

12 Months Ended

Dec. 31, 2023

Share-Based Payment Arrangement [Abstract]

The following table summarizes the RSU activity for the year ended December 31, 2023:


	Number of Shares		Weighted Average Grant Date Fair Value		Weighted- Average Remaining Contractual Term (in years)		Aggregate Intrinsic Value (in thousands)
Unvested balance at December 31, 2022		—		—		—		—
Granted		181,181	$	14.42
Unvested balance at December 31, 2023		181,181	$	14.42		2.1	$	3,671

Summary of Stock Options Activity

The following table summarizes the stock options activity for the year ended December 31, 2023:


	Options Outstanding
	Number of Shares Underlying Outstanding Options			Weighted- Average Exercise Price		Weighted- Average Remaining Contractual Term (in years)		Aggregate Intrinsic Value (in thousands)
Balances at December 31, 2022		5,778,929		$	16.21		7.97	$	48,050
Options granted		3,824,931			16.04
Options exercised		(574,740	)		12.72
Options expired		(129,147	)		27.58
Options forfeited		(659,819	)		16.06
Balances at December 31, 2023		8,240,154		$	16.21		7.92	$	46,863
Shares exercisable, December 31, 2023		3,130,485		$	17.07		6.26	$	19,188
Shares vested and expected to vest, December 31, 2023		8,240,154		$	16.21		7.90	$	46,863

Summary of Stock Compensation Expense for Employees and Nonemployees by Function

The following table is a summary of stock compensation expense for employees and nonemployees by function (in thousands):


	Year Ended December 31,
	2023		2022
Research and development	$	9,740	$	9,221
General and administrative		9,925		7,900
Total stock-based compensation expense	$	19,665	$	17,121

Schedule of Assumptions Used in Black-Scholes Valuation Model to Estimate Fair Value of Stock Options

The Company estimates the fair value of employee and nonemployee stock options using the Black-Scholes option pricing model. The fair value of employee and nonemployee options is recognized on a straight-line basis over the requisite service period of the awards. The fair value of the Company's stock options was estimated using the following assumptions for the years ended December 31, 2023 and 2022:


	Year Ended December 31, 2023		Year Ended December 31, 2022
	Employee	Nonemployee	Employee	Nonemployee
Expected term	5.9 – 6.1 years	0	5.9 – 6.1 years	6.0 years
Expected volatility	79.8% – 81.8%	0.0%	80.1% – 84.3%	80.7%
Risk-free interest rate	3.7% – 4.6%	0.0%	1.5% – 4.3%	3.0%
Expected dividend yield	0%	0%	0%	0%

Common Stock - Additional Information (Details) - USD ($)	1 Months Ended		12 Months Ended
	Oct. 31, 2021	Apr. 30, 2020	Dec. 31, 2023	Dec. 31, 2022
Class of Stock [Line Items]
Common stock, shares authorized			300,000,000	300,000,000
Common stock, par value			$ 0.0001	$ 0.0001
Common stock voting rights			one vote per share
Common stock dividend			$ 0	$ 0
Funding Agreement \| CFF
Class of Stock [Line Items]
Additional funding upon acceptance of investigational New Drug application	$ 4,000,000	$ 4,000,000
Additional investment in cash	$ 4,000,000
Funding Agreement \| CFF \| Common Stock
Class of Stock [Line Items]
Number of shares sold and issued	125,715
Funding Agreement \| CFF \| Series C Redeemable Convertible Preferred Stock
Class of Stock [Line Items]
Investment in redeemable convertible preferred stock financing		$ 10,000,000

Statements of Operations (Parenthetical) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Income Statement [Abstract]
Research and development expense, related party	$ 183	$ 306

Marketable Securities

12 Months Ended

Dec. 31, 2023

Investments, Debt and Equity Securities [Abstract]

Marketable Securities

4. Marketable Securities

Marketable securities, which are classified as available-for-sale, consisted of the following as of December 31, 2023 and 2022 (in thousands):


	Amortized Cost Basis		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value as of December 31, 2023
Short-term marketable securities:
Commercial paper	$	17,603	$	1	$	(26	)	$	17,578
Corporate bonds		18,641		12		(3	)		18,650
U.S. government sponsored agencies		2,900		—		(4	)		2,896
Total short-term marketable securities	$	39,144	$	13	$	(33	)	$	39,124
Long-term marketable securities:
Commercial paper	$	661	$	4	$	—		$	665
Corporate bonds		10,257		32		—			10,289
Total long-term marketable securities	$	10,918	$	36	$	—		$	10,954


	Amortized Cost Basis		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value as of December 31, 2022
Short-term marketable securities:
Commercial paper	$	43,988	$	—	$	(24	)	$	43,964
Corporate bonds		47,565		—		(553	)		47,012
U.S. treasuries		63,502		1		(444	)		63,059
U.S. government sponsored agencies		7,185		12		(29	)		7,168
Total short-term marketable securities	$	162,240	$	13	$	(1,050	)	$	161,203
Long-term marketable securities:
Commercial paper	$	5,067	$	—	$	(159	)	$	4,908
Total long-term marketable securities	$	5,067	$	—	$	(159	)	$	4,908

All marketable securities held as of December 31, 2023 and 2022 had contractual maturities of less than two years. There have been no material realized gains or losses on marketable securities for the periods presented.

Aggregate fair values of marketable securities with unrealized losses and gains were as follows as of December 31, 2023 and 2022 (in thousands):


	As of December 31,
	2023		2022
Aggregate fair value of marketable securities in a continuous loss position for less than twelve months	$	20,494	$	97,472
Aggregate fair value of marketable securities in a continuous loss position for more than twelve months		4,986		53,716
Aggregate fair value of marketable securities in unrealized gain position		24,598		14,923
Total marketable securities	$	50,078	$	166,111

The Company manages credit risk associated with its investment portfolio through its investment policy, which limits purchases to high-quality issuers and also limits the amount of its portfolio that can be

invested in a single issuer. The Company did not record an allowance for credit losses or other impairment charges related to its marketable securities for any period presented. The Company has determined that (i) it does not have the intent to sell any of these investments, and (ii) it is not more likely than not that it will be required to sell any of these investments before recovery of the entire amortized cost basis. The Company further considered the maximum unrealized loss amounts both at the individual instrument level, $20 thousand, as well as in aggregate, $35 thousand, as of December 31, 2023, as immaterial. These unrealized losses were not attributed to credit risk and were associated with changes in market conditions. The Company periodically reviews its marketable securities for indications of credit losses. The Company anticipates that it will recover the entire amortized cost basis of such securities, and therefore, no credit loss existed as of December 31, 2023.

Subsequent Events - Additional Information (Details) - Common Stock - USD ($) $ / shares in Units, $ in Millions		1 Months Ended
	Feb. 09, 2024	Feb. 29, 2024	Dec. 31, 2020
Exercise of Overallotment Option
Subsequent Event [Line Items]
Number of shares sold and issued			1,260,000
Subsequent Event \| Exercise of Overallotment Option \| Maximum
Subsequent Event [Line Items]
Number of shares sold and issued	1,525,423	1,525,423
Subsequent Event \| 2024 Follow On Public Offering
Subsequent Event [Line Items]
Number of shares priced under public offering	6,586,015	6,586,015
Stock sold and issued price per share	$ 29.5	$ 29.5
Aggregate gross proceeds, before deducting underwriting discounts and commissions and offering costs	$ 300.0
Aggregate net proceeds, after deducting underwriting discounts and commissions and offering costs	$ 281.4	$ 281.4
Subsequent Event \| Pre-Funded Warrants \| 2024 Follow On Public Offering
Subsequent Event [Line Items]
Warrants to purchase shares of common stock	3,583,476	3,583,476
Common stock offering price	$ 29.4999	$ 29.4999

Related Party Transactions - Additional Information (Details) $ in Millions	1 Months Ended		12 Months Ended
	Mar. 31, 2021 USD ($)	Apr. 30, 2019 USD ($) Agreement	Dec. 31, 2023 USD ($)	Dec. 31, 2022 USD ($)
UC Regents \| SRAs
Related Party Transaction [Line Items]
Number of sponsored research agreements entered \| Agreement		2
Agreement term		3 years
Amount due to related party		$ 1.4
Payment to related party				$ 1.4
UC Regents \| Machine Learning SRA
Related Party Transaction [Line Items]
Agreement term	3 years
Amount due to related party	$ 1.4
Payment to related party				0.4
Agreement terminated			2022-03
UC Regents \| SRAs and Machine Learning SRA
Related Party Transaction [Line Items]
Accounts payable, related party			$ 0.0	0.0
Related party expenses			0.0	$ 0.3
Noriyuki Kasahara \| Company's Chief Development Officer
Related Party Transaction [Line Items]
Payment for services			$ 0.2

Marketable Securities (Tables)

12 Months Ended

Dec. 31, 2023

Investments, Debt and Equity Securities [Abstract]

Summary of Available for Sale Securities

Marketable securities, which are classified as available-for-sale, consisted of the following as of December 31, 2023 and 2022 (in thousands):


	Amortized Cost Basis		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value as of December 31, 2023
Short-term marketable securities:
Commercial paper	$	17,603	$	1	$	(26	)	$	17,578
Corporate bonds		18,641		12		(3	)		18,650
U.S. government sponsored agencies		2,900		—		(4	)		2,896
Total short-term marketable securities	$	39,144	$	13	$	(33	)	$	39,124
Long-term marketable securities:
Commercial paper	$	661	$	4	$	—		$	665
Corporate bonds		10,257		32		—			10,289
Total long-term marketable securities	$	10,918	$	36	$	—		$	10,954


	Amortized Cost Basis		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value as of December 31, 2022
Short-term marketable securities:
Commercial paper	$	43,988	$	—	$	(24	)	$	43,964
Corporate bonds		47,565		—		(553	)		47,012
U.S. treasuries		63,502		1		(444	)		63,059
U.S. government sponsored agencies		7,185		12		(29	)		7,168
Total short-term marketable securities	$	162,240	$	13	$	(1,050	)	$	161,203
Long-term marketable securities:
Commercial paper	$	5,067	$	—	$	(159	)	$	4,908
Total long-term marketable securities	$	5,067	$	—	$	(159	)	$	4,908

Summary of Aggregate Fair Values of Marketable Securities with Unrealized Losses and Gains

Aggregate fair values of marketable securities with unrealized losses and gains were as follows as of December 31, 2023 and 2022 (in thousands):


	As of December 31,
	2023		2022
Aggregate fair value of marketable securities in a continuous loss position for less than twelve months	$	20,494	$	97,472
Aggregate fair value of marketable securities in a continuous loss position for more than twelve months		4,986		53,716
Aggregate fair value of marketable securities in unrealized gain position		24,598		14,923
Total marketable securities	$	50,078	$	166,111

- Definition

Tabular disclosure of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Research and Collaboration Arrangements - CRF Agreement - Additional Information (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Accounts payable	$ 3,515	$ 3,322

Summary of Significant Accounting Policies - Schedule of Revenues by Geographic Region (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Disaggregation Of Revenue [Line Items]
Total revenue	$ 20,723	$ 3,129
Netherlands
Disaggregation Of Revenue [Line Items]
Total revenue	567	3,094
United States
Disaggregation Of Revenue [Line Items]
Total revenue	$ 20,156	$ 35

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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- Definition

Amount of revenue recognized from goods sold, services rendered, insurance premiums, or other activities that constitute an earning process. Includes, but is not limited to, investment and interest income before deduction of interest expense when recognized as a component of revenue, and sales and trading gain (loss).

+ References

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Marketable Securities - Summary of Aggregate fair values of marketable securities with unrealized losses and gains (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Debt Securities, Available-for-Sale [Line Items]
Aggregate fair value of marketable securities in a continuous loss position for less than twelve months	$ 20,494	$ 97,472
Aggregate fair value of marketable securities in a continuous loss position for more than twelve months	4,986	53,716
Aggregate fair value of marketable securities in unrealized gain position	24,598	14,923
Total marketable securities	$ 50,078	$ 166,111

- Definition

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Research and Collaboration Arrangements - Summary of Deferred Revenue (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Total deferred revenue	$ 1,245	$ 1,960
uniQure
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Total deferred revenue	0	566
CFF
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Total deferred revenue	$ 1,245	$ 1,394

Common Stock - Schedule of Common Share Reserved for Future Issuance (Details) - shares	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2020
Class Of Stock [Line Items]
Common stock reserved	9,314,262	8,357,910
2020 Equity Incentive Plan
Class Of Stock [Line Items]
Common stock reserved	524,078	2,112,039
2020 Employee Stock Share Purchase Plan
Class Of Stock [Line Items]
Common stock reserved	313,034	413,273	252,337
Options Issued and Outstanding
Class Of Stock [Line Items]
Common stock reserved	8,423,481	5,778,929
Common Stock Warrants
Class Of Stock [Line Items]
Common stock reserved	53,669	53,669

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-URI https://asc.fasb.org//1943274/2147481112/505-10-50-2

Reference 14: http://www.xbrl.org/2003/role/disclosureRef
-Topic 210
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(29))
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480566/210-10-S99-1

Reference 15: http://www.xbrl.org/2003/role/disclosureRef
-Topic 946
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 3
-Subparagraph (SX 210.6-03(i)(2))
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479886/946-10-S99-3

Reference 16: http://www.xbrl.org/2003/role/disclosureRef
-Topic 946
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 3
-Subparagraph (SX 210.6-03(i)(1))
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479886/946-10-S99-3

Reference 17: http://www.xbrl.org/2003/role/disclosureRef
-Topic 946
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 3
-Subparagraph (SX 210.6-03(i)(2)(i))
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479886/946-10-S99-3

Reference 18: http://www.xbrl.org/2003/role/disclosureRef
-Topic 946
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 3
-Subparagraph (SX 210.6-03(i)(2)(ii))
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479886/946-10-S99-3

+ Details

Income Taxes (Tables)

12 Months Ended

Dec. 31, 2023

Income Tax Disclosure [Abstract]

Schedule of Effective Tax Rate of Income Tax Expense (Benefit) Differs from Federal Statutory Rate

The effective tax rate of the Company’s income tax expense (benefit) differs from the federal statutory rate as follows:


	Year ended December 31,
	2023			2022 (1)
Federal statutory income tax rate		21.0	%		21.0	%
State tax rate		15.8	%		8.3	%
Research tax credit		2.4	%		1.3	%
Permanent differences		(0.1	)%		0.0	%
Stock-based compensation		(1.7	)%		(1.0	)%
Officer's compensation		(1.4	)%		(0.8	)%
Valuation allowance		(36.0	)%		(28.8	)%
Provision for income taxes		0.0	%		0.0	%

________________________________________

(1) The 2022 valuation allowance, research tax credit, state tax, stock based compensation, and officer’s compensation lines have been updated to conform to the 2023 presentation.

Schedule of Tax Effects of Temporary Differences to Significant Components of Deferred Taxes

The tax effects of temporary differences that give rise to significant components of the deferred taxes are as follows (in thousands):


	December 31,
	2023			2022
Deferred Tax Assets
Net operating loss carryforwards	$	59,547		$	47,050
Other accrued liabilities		2,497			1,921
Deferred revenue		348			539
Research tax credits		13,961			10,546
Stock-based compensation expense		5,340			4,825
Intangible asset basis		1,014			1,092
Operating lease liabilities		4,106			4,437
Capitalized research expenditures		35,784			16,172
Total deferred tax assets	$	122,597		$	86,582

Deferred Tax Liabilities
Operating lease right-of-use assets	$	(3,229	)	$	(3,600	)
Prepaid expenses		(1,843	)		(1,556	)
Total deferred tax liabilities	$	(5,072	)	$	(5,156	)
Less: valuation allowance		(117,525	)		(81,426	)
Total net deferred tax	$	—		$	—

Reconciliation of Beginning and Ending Unrecognized Tax Benefits

The reconciliation of the beginning and ending unrecognized tax benefits amounts is as follows (in thousands):


	Unrecognized Income Tax Benefits
Balance as of December 31, 2021	$	5,586
Additions for current year tax positions		2,687
Additions for tax positions of prior years		—
Balance as of December 31, 2022		8,273
Additions for current year tax positions		3,629
Additions for tax positions of prior years		—
Balance as of December 31, 2023	$	11,902

Research and Collaboration Arrangements - CFF - Additional Information (Details 1) - CFF	Dec. 31, 2023
Minimum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue, estimated performance period	1 year
Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue, estimated performance period	3 years

Research and Collaboration Arrangements - Additional Information (Details 1)	Jan. 31, 2014
uniQure \| uniQure Agreement \| Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date: 2024-01-01
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue, estimated performance period	4 years

Accrued and Other Current Liabilities - Summary of Accrued and Other Current Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Payables and Accruals [Abstract]
Payroll and related expenses	$ 8,149	$ 5,689
Accrued clinical and preclinical study costs	2,180	1,355
Consulting and professional	1,581	1,444
Other accrued expenses	104	382
Total accrued and other current liabilities	$ 12,014	$ 8,870

Property and Equipment, Net

12 Months Ended

Dec. 31, 2023

Property, Plant and Equipment [Abstract]

Property and Equipment, Net

5. Property and Equipment, Net

Property and equipment, net, consisted of the following (in thousands):


	December 31,
	2023			2022
Machinery and equipment	$	12,996		$	11,191
Leasehold improvements		16,902			16,902
Furniture and fixtures		566			566
Office equipment		239			239
Computer equipment and software		805			616
Construction in progress		550			476
Total property and equipment		32,058			29,990
Less: Accumulated depreciation and amortization		(11,932	)		(7,728	)
Property and equipment, net	$	20,126		$	22,262

All property and equipment are maintained in the United States. Depreciation expense was $4.4 million and $2.4 million for the years ended December 31, 2023 and 2022, respectively.

Summary of Significant Accounting Policies - Schedule of Concentration of Risk (Details) (Parenthetical) - Revenue - Customer Concentration Risk	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Customer B
Concentration Risk [Line Items]
Concentration risk, percentage		99.00%
Maximum \| Customer A
Concentration Risk [Line Items]
Concentration risk, percentage	10.00%	10.00%
Maximum \| Customer B
Concentration Risk [Line Items]
Concentration risk, percentage	10.00%

Stock-based Compensation

12 Months Ended

Dec. 31, 2023

Share-Based Payment Arrangement [Abstract]

Stock-based Compensation

12. Stock-based Compensation

2020 Incentive Award Plan

In December 2020, the Company adopted the 2020 Incentive Award Plan (“2020 Plan”), which became effective on December 10, 2020. The 2020 Plan initially reserved 2,606,546 shares of common stock for the issuance of stock options, stock appreciation rights, restricted stock awards, restricted stock units, performance bonus awards, performance stock units, dividend equivalents or other stock or cash based award granted to employees, directors and consultants of the Company. The number of shares reserved for future issuance under the 2020 Plan will increase annually on the first day of each fiscal year beginning in 2021 and ending in 2030 by the lesser of (i) 5% of the shares of common stock outstanding

(on an as converted basis) on the last day of the immediately preceding fiscal year and (ii) such number of shares of common stock as determined by the Company's board of directors, provided, however, no more than 18,000,000 shares of the Company's common stock may be issued upon the exercise of incentive stock options. On January 1, 2023, an additional 1,631,331 shares of common stock became available for issuance under the 2020 Plan, as a result of the operation of the automatic annual increase provision. All options are exercisable over a period not to exceed the contractual term of ten years from the date the stock options were issued. As of December 31, 2023 there were 190,619 shares available for grant under the 2020 Plan.

Following the effectiveness of the 2020 Plan, the Company will not make any further grants under the 2015 Equity Incentive Plan (the “2015 Plan”). However, the 2015 Plan continues to govern the terms of options that remain outstanding under the 2015 Plan. As of December 31, 2023, there were 335,605 shares available for grant under the 2015 Plan.

2015 Equity Incentive Plan

The 2015 Plan provided for grants of stock options, stock appreciation rights, restricted stock and restricted stock unit awards to employees, directors and consultants of the Company. As of December 31, 2023, options to purchase 1,620,457 shares of common stock were outstanding under the 2015 Plan. All options are exercisable over a period not to exceed the contractual term of ten years from the date the stock options were issued and are granted at prices not less than the estimated fair market value of the Company’s common stock on the grant date as determined by the board of directors. If an individual owns stock representing more than 10% of the Company’s outstanding shares, the exercise price of each share shall be at least 110% of the fair market value on the date of grant.

No additional grants will be made under the 2015 Plan, and all outstanding grants under the 2015 Plan that are repurchased, forfeited, expire or are cancelled are returned back to the 2015 Plan and are not available for grant under the 2020 Plan.

Employee Stock Purchase Plan

In December 2020, the Company adopted the 2020 Employee Stock Purchase Plan (the “2020 ESPP”). Under the 2020 ESPP, 252,337 shares of the Company's common stock were initially reserved for employee purchases of the Company's common stock under terms and provisions established by the Company's board of directors and approved by the Company's stockholders. The number of shares reserved for future issuance under the 2020 ESPP will increase annually on the first day of each fiscal year beginning in 2021 and ending in 2030 by the lesser of (i) 1% of the shares of common stock outstanding (on an as converted basis) on the last day of the immediately preceding fiscal year and (ii) such number of shares of common stock as determined by the Company's board of directors, provided, however, no more than 15,000,000 shares the Company's common stock may be issued under the 2020 ESPP. On December 9, 2022, the Company's Board of Directors approved an increase of 50,000 shares of common stock under the 2020 ESPP, effective as of January 1, 2023.

Under the 2020 ESPP the Company's employees may purchase common stock through payroll deductions at a price equal to 85% of the lower of the fair market value of the stock at the beginning of the offering period or at the end of each applicable purchase period. The 2020 ESPP provides for a series of overlapping 24-month offering periods comprising four six-month purchase periods. The initial offering period under the 2020 ESPP is longer than 24 months, commencing February 15, 2021 and ending on May 14, 2023. Contributions under the 2020 ESPP are limited to a maximum of 15% of an employee's eligible compensation.

Restricted Stock Units (RSU)

The following table summarizes the RSU activity for the year ended December 31, 2023:


	Number of Shares		Weighted Average Grant Date Fair Value		Weighted- Average Remaining Contractual Term (in years)		Aggregate Intrinsic Value (in thousands)
Unvested balance at December 31, 2022		—		—		—		—
Granted		181,181	$	14.42
Unvested balance at December 31, 2023		181,181	$	14.42		2.1	$	3,671

Stock Options

The following table summarizes the stock options activity for the year ended December 31, 2023:


	Options Outstanding
	Number of Shares Underlying Outstanding Options			Weighted- Average Exercise Price		Weighted- Average Remaining Contractual Term (in years)		Aggregate Intrinsic Value (in thousands)
Balances at December 31, 2022		5,778,929		$	16.21		7.97	$	48,050
Options granted		3,824,931			16.04
Options exercised		(574,740	)		12.72
Options expired		(129,147	)		27.58
Options forfeited		(659,819	)		16.06
Balances at December 31, 2023		8,240,154		$	16.21		7.92	$	46,863
Shares exercisable, December 31, 2023		3,130,485		$	17.07		6.26	$	19,188
Shares vested and expected to vest, December 31, 2023		8,240,154		$	16.21		7.90	$	46,863

The following table is a summary of stock compensation expense for employees and nonemployees by function (in thousands):


	Year Ended December 31,
	2023		2022
Research and development	$	9,740	$	9,221
General and administrative		9,925		7,900
Total stock-based compensation expense	$	19,665	$	17,121

During the years ended December 31, 2023 and 2022, the Company granted 3,824,931 and 2,700,437 stock options to employees with a weighted-average grant date fair value of $16.96 and $11.20 per share, respectively and zero and 10,000 stock options to nonemployees with a weighted-average grant date fair value of zero and $12.05 per share, respectively. The total fair value of options vested during the years ended December 31, 2023 and 2022 was $18.3 million and $15.1 million, respectively. As of December 31, 2023, the unrecognized stock-based compensation expense of unvested options was $55.0 million and is expected to be recognized over a weighted-average period of 2.1 years.

Stock-based compensation expense recorded for employee options was $19.5 million and $16.9 million for the years ended December 31, 2023 and 2022, respectively. Stock-based compensation

expense recorded for nonemployee consultants was $0.2 million for each of the years ended December 31, 2023 and 2022, respectively.


	Year Ended December 31, 2023		Year Ended December 31, 2022
	Employee	Nonemployee	Employee	Nonemployee
Expected term	5.9 – 6.1 years	0	5.9 – 6.1 years	6.0 years
Expected volatility	79.8% – 81.8%	0.0%	80.1% – 84.3%	80.7%
Risk-free interest rate	3.7% – 4.6%	0.0%	1.5% – 4.3%	3.0%
Expected dividend yield	0%	0%	0%	0%

Expected Term. The expected term for employee options is calculated using the simplified method as the Company does not have sufficient historical information to provide a basis for estimate. The simplified method is based on the average of the vesting tranches and the contractual life of each grant. The expected term for nonemployee options is the contractual term of the options.

Expected Volatility. The expected volatility was estimated based on a study of publicly traded peer companies as the Company did not have sufficient trading history for its common stock. The Company selected the peer group based on similarities in industry, stage of development, size and financial leverage with the Company’s principal business operations. For each grant, the Company measured historical volatility over a period equivalent to the expected term.

Risk-free Interest Rate. The risk-free interest rate is based on the yield available on U.S. Treasury zero-coupon issues whose term is similar in duration to the expected term of the respective stock option.

Expected Dividend Yield. The Company has not paid and does not anticipate paying any dividends on its common stock in the future. Accordingly, the Company has estimated the dividend yield to be zero.

Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
Statements of Cash Flows - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Cash flows from operating activities
Net loss	$ (100,837)	$ (107,494)
Adjustments to reconcile net loss to net cash used in operating activities
Stock-based compensation expense	19,665	17,121
Vesting of common stock warrants in return for services	88	87
Change in fair value of derivative liability	157	(2)
Depreciation and amortization	4,204	2,403
Amortization of right-of-use assets	1,545	1,477
Net amortization (accretion) of premium (discount) on marketable securities	(1,244)	1,059
Changes in operating assets and liabilities
Accounts receivable	0	47
Prepaid expenses and other current assets	(1,399)	1,235
Other assets	(70)	(11)
Accounts payable	896	(1,638)
Accrued and other liabilities	3,371	2,485
Deferred revenue	(715)	(3,130)
Operating lease liabilities	(1,453)	(324)
Net cash used in operating activities	(75,792)	(86,685)
Cash flows from investing activities
Acquisition of property and equipment	(2,771)	(11,536)
Purchase of marketable securities	(54,802)	(153,313)
Maturities of marketable securities	173,290	147,799
Net cash provided by (used in) investing activities	115,717	(17,050)
Cash flows from financing activities
Issuance of common stock upon exercise of stock options and warrants	7,310	2,730
Issuance of common stock through 2020 ESPP	1,178	823
Payment of offering costs	0	(468)
Net cash provided by financing activities	156,832	3,085
Net increase (decrease) in cash and cash equivalents	196,757	(100,650)
Cash and cash equivalents, beginning of period	52,351	153,001
Cash and cash equivalents, end of period	249,108	52,351
Supplemental disclosures of noncash investing and financing information
Purchases of property and equipment in accounts payable and accrued and other liabilities	234	937
Deferred ATM costs reclassification	468	0
Public offering
Cash flows from financing activities
Issuance of common stock for the offering program, net of issuance costs	129,208	0
ATM Offering Program
Cash flows from financing activities
Issuance of common stock for the offering program, net of issuance costs	$ 19,136	$ 0

Property and Equipment, Net - Schedule of Property and Equipment, Net (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Property Plant And Equipment [Line Items]
Total property and equipment	$ 32,058	$ 29,990
Less: Accumulated depreciation and amortization	(11,932)	(7,728)
Property and equipment, net	20,126	22,262
Machinery and Equipment
Property Plant And Equipment [Line Items]
Total property and equipment	12,996	11,191
Leasehold Improvements
Property Plant And Equipment [Line Items]
Total property and equipment	16,902	16,902
Furniture and Fixtures
Property Plant And Equipment [Line Items]
Total property and equipment	566	566
Office Equipment
Property Plant And Equipment [Line Items]
Total property and equipment	239	239
Computer Equipment and Software
Property Plant And Equipment [Line Items]
Total property and equipment	805	616
Construction in Progress
Property Plant And Equipment [Line Items]
Total property and equipment	$ 550	$ 476

Accrued and Other Current Liabilities (Tables)

12 Months Ended

Dec. 31, 2023

Payables and Accruals [Abstract]

Summary of Accrued and Other Current Liabilities

Accrued and other current liabilities consisted of the following (in thousands):


	December 31,
	2023		2022
Payroll and related expenses	$	8,149	$	5,689
Accrued clinical and preclinical study costs		2,180		1,355
Consulting and professional		1,581		1,444
Other accrued expenses		104		382
Total accrued and other current liabilities	$	12,014	$	8,870

Stock-based Compensation - Additional Information (Details) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2023 USD ($) Period $ / shares shares	Dec. 31, 2022 USD ($) $ / shares shares	Jan. 01, 2023 shares	Dec. 09, 2022 shares	Dec. 31, 2020 shares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of shares reserved for issuance	9,314,262	8,357,910
Number of options outstanding	8,240,154	5,778,929
Number of stock options granted	3,824,931
Fair value of options vested \| $	$ 18,300	$ 15,100
Unrecognized stock-based compensation of unvested options \| $	$ 55,000
Unrecognized stock-based compensation, expected to be recognized over weighted-average period	2 years 1 month 6 days
Stock-based compensation expense \| $	$ 19,665	$ 17,121
Expected dividend yield	0.00%	0.00%
Employee
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of stock options granted	3,824,931	2,700,437
Weighted-average grant date fair value \| $ / shares	$ 16.96	$ 11.2
Stock-based compensation expense \| $	$ 19,500	$ 16,900
Expected dividend yield	0.00%	0.00%
Nonemployee
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of stock options granted	0	10,000
Weighted-average grant date fair value \| $ / shares	$ 0	$ 12.05
Stock-based compensation expense \| $	$ 200	$ 200
Expected dividend yield	0.00%	0.00%
2020 Incentive Award Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of shares reserved for issuance					2,606,546
Percentage of common stock outstanding increased annually for future issuance	5.00%
Number of shares available for grant	190,619
Number of additional shares available for grant			1,631,331
2020 Incentive Award Plan \| Maximum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Options exercisable period	10 years
Number of shares authorized	18,000,000
2015 Equity Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Options exercisable period	10 years
Number of shares available for grant	335,605
Number of additional shares available for grant	0
Number of options outstanding	1,620,457
Minimum percentage of outstanding shares held by individual	10.00%
Exercise price as percentage of fair market value, minimum	110.00%
2020 ESPP
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of shares reserved for issuance	313,034	413,273			252,337
Increase in shares available for issuance percentage	1.00%
Number of shares available for grant				50,000
Employee purchase price of common stock as percentage of fair market value	85.00%
Common stock overlapping offering period	24 months
Number of duration purchase period \| Period	4
Duration of purchase period	6 months
Percentage of maximum contributions of employee's eligible compensation	15.00%
2020 ESPP \| Maximum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of shares authorized	15,000,000

Pay vs Performance Disclosure - USD ($) $ in Thousands	12 Months Ended
Pay vs Performance Disclosure - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Pay vs Performance Disclosure
Net Income (Loss)	$ (100,837)	$ (107,494)

- Definition

The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 235
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08(g)(1)(ii))
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480678/235-10-S99-1

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 323
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 3
-Subparagraph (c)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481687/323-10-50-3

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 825
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 28
-Subparagraph (f)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482907/825-10-50-28

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Topic 220
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 6
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482765/220-10-50-6

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Topic 250
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 3
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147483443/250-10-50-3

Reference 6: http://www.xbrl.org/2003/role/disclosureRef
-Topic 250
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Subparagraph (b)(2)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147483443/250-10-50-1

Reference 7: http://www.xbrl.org/2003/role/disclosureRef
-Topic 815
-SubTopic 40
-Name Accounting Standards Codification
-Section 65
-Paragraph 1
-Subparagraph (f)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480175/815-40-65-1

Reference 8: http://www.xbrl.org/2003/role/disclosureRef
-Topic 250
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 8
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147483443/250-10-50-8

Reference 9: http://www.xbrl.org/2003/role/disclosureRef
-Topic 250
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 9
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147483443/250-10-50-9

Reference 10: http://www.xbrl.org/2003/role/disclosureRef
-Topic 250
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 11
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147483443/250-10-50-11

Reference 11: http://www.xbrl.org/2003/role/disclosureRef
-Topic 250
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 11
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147483443/250-10-50-11

Reference 12: http://www.xbrl.org/2003/role/disclosureRef
-Topic 250
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 4
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147483443/250-10-50-4

Reference 13: http://www.xbrl.org/2003/role/exampleRef
-Topic 946
-SubTopic 830
-Name Accounting Standards Codification
-Section 55
-Paragraph 10
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480167/946-830-55-10

Reference 14: http://www.xbrl.org/2003/role/disclosureRef
-Topic 946
-SubTopic 220
-Name Accounting Standards Codification
-Section 45
-Paragraph 7
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147483581/946-220-45-7

Reference 15: http://www.xbrl.org/2003/role/disclosureRef
-Topic 944
-SubTopic 220
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-04(18))
-Publisher FASB
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Reference 16: http://www.xbrl.org/2003/role/disclosureRef
-Topic 280
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
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-URI https://asc.fasb.org//1943274/2147482810/280-10-50-22

Reference 17: http://www.xbrl.org/2003/role/disclosureRef
-Topic 946
-SubTopic 220
-Name Accounting Standards Codification
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Reference 18: http://www.xbrl.org/2003/role/disclosureRef
-Topic 946
-SubTopic 220
-Name Accounting Standards Codification
-Section S99
-Paragraph 3
-Subparagraph (SX 210.6-09(1)(d))
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+ Details

The Company

12 Months Ended

Dec. 31, 2023

Organization, Consolidation and Presentation of Financial Statements [Abstract]

The Company

1. The Company

4D Molecular Therapeutics, Inc. (the “Company”) was formed as a limited liability company in September 2013 under the name 4D Molecular Therapeutics, LLC. The Company changed its name and converted into a corporation which was incorporated in the state of Delaware in March 2015. The Company is a clinical-stage biotherapeutics company harnessing the power of directed evolution for targeted genetic medicines.

Initial Public Offering

In December 2020, the Company sold and issued 9,660,000 shares of common stock at a price to the public of $23.00 per share, which included shares sold upon the underwriters’ exercise of their overallotment option to purchase 1,260,000 additional shares. The Company received an aggregate of $204.7 million in net proceeds, after deducting underwriting discounts and commissions and offering costs.

Upon the closing of the Company's initial public offering in December 2020 (the "IPO"), all outstanding shares of redeemable convertible preferred stock automatically converted into 11,575,984 shares of common stock. Subsequent to the closing of the IPO, there were no shares of redeemable convertible preferred stock outstanding.

2021 Follow On Public Offering

In November 2021, the Company completed an underwritten public offering ("2021 Offering") in which 4,750,000 shares of the Company's common stock were sold at an offering price of $25.00 per share pursuant to an effective Registration Statement on Form S-1. The net proceeds from the 2021 Offering were $111.1 million, after deducting underwriting discounts and commissions and offering expenses.

2023 Follow On Public Offering

In May 2023, the Company completed its third underwritten public offering (the “2023 Offering”) in which 8,625,000 shares of the Company’s common stock were sold at an offering price of $16.00 per share pursuant to an effective Registration Statement on Form S-3. The net proceeds from the 2023 Offering were $129.2 million, after deducting underwriting discounts and commissions and offering expenses.

2024 Follow On Public Offering

In February 2024, the Company completed an underwritten public offering (the “2024 Offering”) in which 6,586,015 shares of the Company’s common stock were sold at an offering price of $29.50 per share, as well as pre-funded warrants to purchase 3,583,476 shares of the Company’s common stock at an offering price of $29.4999 per underlying share pursuant to an effective Registration Statement on Form S-3. The net proceeds from the 2024 Offering were $281.4 million, after deducting underwriting discounts and commissions and other offering expenses. The Company also granted the underwriters the option to purchase up to 1,525,423 additional shares of common stock in connection with the offering.

Liquidity

The Company has incurred significant losses and negative cash flows from operations and had an accumulated deficit of $415.3 million as of December 31, 2023. The Company believes that its cash and cash equivalents and marketable securities as of December 31, 2023 are sufficient for the Company to fund planned operations for at least one year from the issuance date of these financial statements for the year ended December 31, 2023. The Company has historically financed its operations primarily through the sale of equity securities, and to a lesser extent, from cash received pursuant to its collaboration and

license agreements. To date, none of the Company’s product candidates have been approved for sale, and therefore, the Company has not generated any revenue from product sales. Management expects operating losses and negative cash flows from operations to continue for the foreseeable future. The Company plans to raise additional funding as required based on the status of its clinical trials and projected cash flows. There can be no assurance that, in the event the Company requires additional financing, such financing will be available on terms acceptable to the Company, if at all. Failure to generate sufficient cash flows from operations, raise additional capital and reduce discretionary spending should additional capital not become available could have a material adverse effect on the Company’s ability to achieve its business objectives.

Related Party Transactions

12 Months Ended

Dec. 31, 2023

Related Party Transactions

16. Related Party Transactions

In April 2019, the Company entered into two sponsored research agreements (“SRAs”) with the UC Regents to conduct research in a research facility on the U.C. Berkeley campus, under the direction of Dr. Schaffer. The SRAs had a three year term that ended in May 2022. Under the SRAs, the Company had an option to license (on a royalty-bearing basis) all intellectual property generated under the SRAs. The total amount the Company was committed to pay to the UC Regents under the SRAs was $1.4 million, which was fully paid as of December 31, 2022. In March 2021, the Company entered into another sponsored research agreement with the UC Regents to conduct research in laboratories on the U.C. Berkeley campus that are under the direction of Dr. Schaffer and another U.C. Berkeley professor covering investigations into how machine learning approaches may enhance AAV capsid engineering (the “Machine Learning SRA”). Pursuant to the Machine Learning SRA, the Company committed to pay the UC Regents a total of $1.4 million, of which $0.4 million was paid as of December 31, 2022. The Machine Learning SRA had a three-year term ending in 2024. The Company could terminate the Machine Learning SRA for convenience and without cause with 60 days’ notice. The Machine Learning SRA was terminated by the Company in March 2022. While the Machine Learning SRA was between the Company and the UC Regents, the payments under the SRA were used to fund the lab under the direction of Dr. Schaffer. As of December 31, 2023 and 2022, there were no accounts payable to the UC Regents related to the SRAs and Machine Learning SRA. Any patent prosecution costs incurred under the Machine Learning SRA and the SRAs were borne by the Company. During the years ended December 31, 2023 and 2022, the Company recorded zero and $0.3 million, respectively, of expense related to these agreements.

As of February 2022, the Machine Learning SRA and the SRAs ceased to be related party transactions upon the resignation of David Schaffer, Ph.D., from the board of directors of the Company.

In 2023, while the Company's Chief Development Officer ("CDO") was on leave, the Company engaged the services of Noriyuki Kasahara, M.D., Ph.D., a member of the Company's Board of Directors, as interim CDO. Dr. Kasahara was paid $0.2 million in 2023 for his services as interim CDO.

- Definition

The entire disclosure for related party transactions. Examples of related party transactions include transactions between (a) a parent company and its subsidiary; (b) subsidiaries of a common parent; (c) and entity and its principal owners; and (d) affiliates.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 946
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480990/946-20-50-2

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
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Reference 3: http://www.xbrl.org/2003/role/disclosureRef
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Reference 5: http://www.xbrl.org/2003/role/disclosureRef
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-Subparagraph (e)
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Reference 8: http://www.xbrl.org/2003/role/disclosureRef
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Reference 9: http://www.xbrl.org/2003/role/disclosureRef
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-URI https://asc.fasb.org//850/tableOfContent

Reference 11: http://www.xbrl.org/2003/role/disclosureRef
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+ Details

Property and Equipment, Net (Tables)

12 Months Ended

Dec. 31, 2023

Property, Plant and Equipment [Abstract]

Schedule of Property and Equipment, Net

Property and equipment, net, consisted of the following (in thousands):


	December 31,
	2023			2022
Machinery and equipment	$	12,996		$	11,191
Leasehold improvements		16,902			16,902
Furniture and fixtures		566			566
Office equipment		239			239
Computer equipment and software		805			616
Construction in progress		550			476
Total property and equipment		32,058			29,990
Less: Accumulated depreciation and amortization		(11,932	)		(7,728	)
Property and equipment, net	$	20,126		$	22,262

Stock-based Compensation - Schedule of Assumptions Used in Black-Scholes Valuation Model to Estimate Fair Value of Stock Options (Details)	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected dividend yield	0.00%	0.00%
Employee
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected volatility, minimum	79.80%	80.10%
Expected volatility, maximum	81.80%	84.30%
Risk-free interest rate, minimum	3.70%	1.50%
Risk-free interest rate, maximum	4.60%	4.30%
Expected dividend yield	0.00%	0.00%
Nonemployee
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected term	0 years	6 years
Expected volatility	0.00%	80.70%
Risk-free interest rate	0.00%	3.00%
Expected dividend yield	0.00%	0.00%
Minimum \| Employee
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected term	5 years 10 months 24 days	5 years 10 months 24 days
Maximum \| Employee
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected term	6 years 1 month 6 days	6 years 1 month 6 days

Summary of Significant Accounting Policies - Schedule of Concentration of Risk (Details) - Revenue - Customer Concentration Risk	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Customer B
Concentration Risk [Line Items]
Concentration risk, percentage		99.00%
Customer C
Concentration Risk [Line Items]
Concentration risk, percentage	97.00%	0.00%
Customer
Concentration Risk [Line Items]
Concentration risk, percentage	97.00%	99.00%

Research and Collaboration Arrangements

12 Months Ended

Dec. 31, 2023

Research And Collaboration Arrangements [Abstract]

Research and Collaboration Arrangements

7. Research and Collaboration Arrangements

Collaboration and license revenue for each period was as follows (in thousands):


	Year Ended December 31,
	2023		2022
uniQure	$	567	$	3,094
CFF		148		35
Astellas (see Note 8)		20,008		—
	$	20,723	$	3,129

Deferred revenue for each period was as follows (in thousands):


	December 31,
	2023		2022
uniQure	$	—	$	566
CFF		1,245		1,394
	$	1,245	$	1,960

The total amount of revenue in the year ended December 31, 2023, which was included in deferred revenue at January 1, 2023, was $0.7 million. The total amount of revenue in the year ended December 31, 2022, which was included in deferred revenue at January 1, 2022, was $3.1 million.

uniQure

In January 2014, the Company and uniQure biopharma B.V. (“uniQure”) entered into a Collaboration and License Agreement (the “uniQure Agreement”) to collaborate on the discovery and non-clinical research activities related to the Company’s Therapeutic Vector Evolution platform in order to generate and validate vectors for gene delivery to treat diseases within the central nervous system and liver (together, the “uniQure Field”).

The uniQure Agreement provided uniQure with a research license as well as an exclusive development and commercialization license for each project variant selected for further development. The initial research term is three years with an option for uniQure to extend the research term one time for an additional year. Once the Company’s research plan has concluded, uniQure is solely responsible for the continued development, manufacturing and commercialization of the project variants as potential product candidates. In October 2016, uniQure exercised its option to extend the research term for an additional year to January 2018. The Company was also required to work exclusively with uniQure in the uniQure Field (the “uniQure Exclusivity Clause”).

Pursuant to the uniQure Agreement, the Company received upfront payments of $0.2 million, and was entitled to receive (i) contingent payments for the achievement of research and development milestones of up to $5.0 million for each licensed product selected under the arrangement, and (ii) royalties in the single digit range on future sales of the potential product candidates and sublicense consideration in the low teens to low thirties range on any future sublicensing arrangements. The Company also received capped research and development service fees based on contractual full-time employee rates per year. In connection with the performance obligations under the uniQure Agreement, the founders of 4D Molecular Therapeutics, LLC received equity options to purchase an aggregate of 609,744 of uniQure ordinary shares that vested over the initial three-year term of the agreement.

The upfront payment of $0.2 million was recorded as deferred revenue and was recognized on a ratable basis over the estimated performance period of four years. Payments and reimbursements for research costs were recognized on an as-incurred basis. The options to purchase uniQure shares were deemed to be a noncash component of the arrangement consideration, as the vesting of options is linked to the uniQure Agreement and there is a requirement for the holders of the options to provide services under the agreement. The fair value of the uniQure options, which was estimated to be $10.6 million, was recognized ratably as revenue over the estimated performance period of four years and the associated compensation expense related to the stock options was recorded as research and development expense.

In August 2019, the Company and uniQure entered into an Amended and Restated Collaboration and License Agreement (the “Amended uniQure Agreement”), which amended and restated the uniQure Agreement, and a separate Collaboration and License Agreement (the “Second uniQure Agreement”). Under these agreements, the Company agreed to transfer incremental rights and services to uniQure in exchange for uniQure eliminating the uniQure Exclusivity Clause and transferring other rights back to the Company.

Under the Amended uniQure Agreement, uniQure continues to have an exclusive license to select AAV capsid variants (the “Selected Variants”) in the uniQure Field. uniQure continues to be solely responsible, at its cost, to develop and commercialize the compounds and products containing the Selected Variants. The amended uniQure Agreement eliminated the uniQure Exclusivity Clause in the uniQure Agreement. Furthermore, the contingent payments that the Company was entitled to from uniQure for the achievement of research and development milestones of up to $5.0 million for each licensed product selected under the uniQure Agreement were eliminated and sublicense consideration on any future sublicensing arrangements was reduced from the low teens to low thirties percentages to mid-single digit to mid-twenties percentages.

Under the Second uniQure Agreement, the parties agreed to research and develop new AAV capsid variants (the “New Variants”) that are not Selected Variants that affect certain targets selected by uniQure (the “uniQure Targets”) in the uniQure Field. The Company is solely responsible, at its cost, for the research of the New Variants. The Company granted uniQure an exclusive license to a certain number of the New Variants (the “uniQure New Variants”) that affect the uniQure Targets. uniQure is solely responsible, at its cost, to develop and commercialize the compounds and products containing the uniQure New Variants that affect the uniQure Targets (the “Licensed Products”). The Company retains all rights to New Variants in the uniQure Field that affect targets other than the uniQure Targets.

Under both the Amended uniQure Agreement and the Second uniQure Agreement, uniQure will be required to pay the Company royalties on worldwide annual net sales of Licensed Products at a mid-single digit percentage rate, subject to certain specified reductions. uniQure will also be required to pay the Company sublicensing consideration for sublicensing the Company’s intellectual property rights licensed under the Amended uniQure Agreement or the Second uniQure Agreement to third parties at a rate between the mid-single digit to mid-twenties. The Company has reciprocal obligations, at the same percentage rates as uniQure, to pay uniQure royalties and sublicensing consideration for sublicensing certain intellectual property rights licensed under the Amended uniQure Agreement or the Second uniQure Agreement to third parties.

The Company concluded that the Amended uniQure Agreement and the Second uniQure Agreement should be accounted for as one combined contract that should be accounted for as a separate contract from the uniQure Agreement given that the incremental licensed intellectual property rights and research and development services are distinct from the rights and services previously transferred to uniQure under the uniQure Agreement and the transaction price increased by an amount that equals the standalone selling price of the incremental rights and services to be transferred to uniQure under the Amended uniQure Agreement and Second uniQure Agreement.

Neither party was required to pay monetary consideration in connection with the execution of the Amended uniQure Agreement or the Second uniQure Agreement or for subsequent performance by the parties under those agreements, notwithstanding the potential future royalty and sublicense consideration described above. The fair value of the non-monetary consideration given by uniQure to the Company, for the intellectual property right is $5.1 million. This intellectual property right is considered to be an in-process research and development asset with no alternative future use and, accordingly, was written off as acquired in-process research and development expense in the year ended December 31, 2019.

The incremental transaction price described in the paragraph above was recorded as deferred revenue given that the Company identified one single combined performance obligation under ASC 606, which includes the licenses to the New Variants, research services and participation in the joint steering committee (“JSC”). Revenue is being recognized using the input method based on actual costs incurred as a percentage of total budgeted costs as the Company completes its performance obligation. The Company completed its performance obligation during the third quarter of 2023 and the deferred revenue was recognized as revenue in the same period.

The Company determined the transaction price using the risk adjusted net present value analysis (“rNPV”) methodology to value the elimination of the uniQure exclusivity clause and other material rights received by the Company, including the potential royalties the Company would receive from uniQure. The

rNPVs incorporate estimates and assumptions including the number of products the Company and uniQure would develop, the risk-adjusted probability of successfully developing a biopharmaceutical product, the probability that uniQure will develop a product, the research and development costs, the potential worldwide sales and associated commercialization costs, corporate tax rate, and discount rate.

During the years ended December 31, 2023 and 2022, the Company recognized revenue of $0.6 million and $3.1 million under the Amended uniQure Agreement and the Second uniQure Agreement, respectively. As of December 31, 2023 and 2022, deferred revenue relating to uniQure was zero and $0.6 million, respectively. During the year ended December 31, 2022, the decrease in total budgeted costs for the remaining performance obligation resulted in a $2.2 million increase in revenue recognized related to the performance obligation partially satisfied prior to January 1, 2022. There were no amounts due from uniQure under the uniQure Agreement, Amended uniQure Agreement or Second uniQure Agreement as of December 31, 2023 and 2022. As of December 31, 2023 and 2022, the aggregate amount of the transaction price allocated to the remaining performance obligation was zero and $0.6 million, respectively.

CFF

In September 2016, the Company entered into an award agreement for the Optimized Adeno-Associated Virus for Lung Epithelia Gene Delivery Development Program with CFF, a non-profit organization dedicated to finding a cure for cystic fibrosis, an inherited disorder that causes disease in the pulmonary airways leading to morbidity and mortality. Under this agreement, CFF contributes funding to help advance the Company’s CF research program. The September 2016 grant award agreement was incorporated into a new grant award agreement with the CFF in September 2017 with the same objectives, which was subsequently amended in August 2018 and February 2021. In August 2023, the Company executed a third amendment to the agreement (the “August 2023 Amendment”), which modified the research plan, increased the aggregate milestone payments from $3.5 million to $6.3 million and extended the estimated project completion date. The aforementioned September 2017 agreement and three amendments are collectively referred to as the “CFF Agreement”. The August 2023 Amendment represents a contract modification to an existing contract under Topic 606, given the amendment did not include any additional goods or services, and the remaining research activities are not distinct from those previously provided. The August 2023 Amendment did not impact the transaction price, given the increased award amount relates to variable consideration for future milestones that are fully constrained. Accordingly, the contract modification did not result in a revenue adjustment. As of each of December 31, 2023 and 2022, the Company had achieved milestones totaling $1.7 million under the CFF Agreement. The remaining award amount will be paid by CFF based on achievement of certain development milestones by the Company.

The Company expects to make payments to CFF equal to six times the actual award received by the Company in three installments within the first four years of the first commercial sale of a product developed under this agreement. The Company also has agreed to make future sales-based milestone payments to CFF of up to three times the actual award received upon achieving specified commercialization milestones with respect to the first of any product developed utilizing any compound covered under the CFF Agreement. The CFF Agreement also requires the Company to pay to CFF royalties of a mid-single digit percentage, up to six times the actual award received, on any amounts received by the Company from the sale, license or transfer to a third-party of rights in the technology developed as a result of this collaboration. Any such royalty payments shall be credited against the payments owed by the Company upon first commercial sale. In the event of a change of control of the Company, CFF will receive certain payments, depending on the timing of the change of control and the size of the transaction.

To date, the Company has not developed a commercial product in connection with the CFF Agreement, and it has not licensed, sold or otherwise transferred to another party the product developed under the CFF Agreement or the underlying technology.

If at any time prior to the first commercial sale of a product developed as a result of the CFF Agreement, the Company ceases to use commercially reasonable efforts to develop or commercialize any product under the CFF Agreement for a continuous period of 180 consecutive days and fails to present a

reasonable plan to resume commercially reasonable efforts, the Company will grant to CFF an irrevocable, exclusive worldwide interruption license under all of the Company’s interest in the research plan technology to exploit such product. Any third-party license granted by the Company shall be subject to such interruption license.

The Company identified one performance obligation within the CFF grant agreement for research activities. The Company’s contract with CFF does not include a significant financing component.

The Company concluded that the transaction price should not include the variable consideration related to future research milestones as they were considered to be constrained as it is probable that the inclusion of such variable consideration could result in a significant reversal of cumulative revenue in the future. The Company re-evaluates the transaction price and estimated period of performance at each reporting period.

Revenue recognized during the year ended December 31, 2023 was $0.1 million, while revenue recognized during the year December 31, 2022 was immaterial. As of December 31, 2023 and 2022, deferred revenue relating to the CFF Agreement was $1.2 million and $1.4 million, respectively. There were no accounts receivable from CFF under the CFF Agreement as of December 31, 2023 and 2022. As of December 31, 2023 and 2022, the aggregate amount of the transaction price allocated to the remaining performance obligation was $1.2 million and $1.4 million, respectively. Based on current timelines, the deferred revenue is expected to be recognized as revenue over the next one to three years as the Company performs research services through the completion of IND-enabling studies.

The obligation to make payments to CFF upon a change of control meets the definition of an embedded derivative that is required to be bifurcated and separately accounted for as a derivative liability. See Note 15 for further discussion of the embedded derivative.

Arbor Biotechnologies, Inc.

On December 20, 2023 (the “Effective Date”), the Company entered into a co-development and co-commercialization agreement (the “Arbor Agreement”) with Arbor Biotechnologies, Inc. (“Arbor”), pursuant to which the Company and Arbor agree to co-develop and co-commercialize on a fifty-fifty cost-and-profit-sharing basis up to six genetic medicine products to treat central nervous system (“CNS”) indications, based on combining the Company’s CNS-targeting vectors and Arbors’ Cas enzymes and Guide RNAs. The first product candidate under the Arbor Agreement will address a molecular target implicated in amyotrophic lateral sclerosis (ALS).

The Company will account for its costs related to the co-development and co-commercialization of product candidates under the Arbor Agreement as research and development expenses in the periods in which they are incurred. As of December 31, 2023, no Shared Product has been designated by the JSC (each as defined in the Arbor Agreement) under the Arbor Agreement, and no research and development expenses were incurred related to the Arbor Agreement during the year ended December 31, 2023.

Summary of Significant Accounting Policies - Additional Information (Details)	12 Months Ended
	Dec. 31, 2023 USD ($) Financial_Institution Segment	Dec. 31, 2022 USD ($)
Summary Of Significant Accounting Policies [Line Items]
Number of reportable segments \| Segment	1
Number of operating segments \| Segment	1
Number of financial institutions cash is held \| Financial_Institution	2
Allowance for doubtful accounts	$ 0	$ 0
Write-offs relating to uncollectible accounts receivable	0	0
Impairments of long-lived assets	$ 0	$ 0
Award requisite service period	4 years
Accounting Standards Update 2016-13
Summary Of Significant Accounting Policies [Line Items]
Change in accounting principle accounting standards update, adopted	true
Change in accounting principle accounting standards update, adoption date	Jan. 01, 2023
Change in accounting principle accounting standards update, immaterial effect	true
Minimum
Summary Of Significant Accounting Policies [Line Items]
Property and equipment, estimated useful life	3 years
Maximum
Summary Of Significant Accounting Policies [Line Items]
Property and equipment, estimated useful life	5 years
Accounts Receivable \| Credit Concentration Risk \| Customer
Summary Of Significant Accounting Policies [Line Items]
Concentration risk, percentage	0.00%	0.00%

Research and Collaboration Arrangements - CFF - Additional Information (Details)	1 Months Ended		12 Months Ended
	Aug. 31, 2023 USD ($)	Sep. 30, 2016 USD ($) Installment	Dec. 31, 2023 USD ($)	Dec. 31, 2022 USD ($)
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue recognized			$ 700,000	$ 3,100,000
CFF Agreement \| CFF
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaborative arrangement total amount of award	$ 6,300,000	$ 3,500,000
Collaborative arrangement achieved milestones			1,700,000	1,700,000
Number of installments to pay actual award \| Installment		3
Actual award payment period upon first commercial sale of product		4 years
Continuous period where the Company ceases to develop or commercialize any product		180 days
Collaborative arrangements special term		If at any time prior to the first commercial sale of a product developed as a result of the CFF Agreement, the Company ceases to use commercially reasonable efforts to develop or commercialize any product under the CFF Agreement for a continuous period of 180 consecutive days and fails to present a reasonable plan to resume commercially reasonable efforts, the Company will grant to CFF an irrevocable, exclusive worldwide interruption license under all of the Company’s interest in the research plan technology to exploit such product. Any third-party license granted by the Company shall be subject to such interruption license.
Deferred revenue			1,200,000	1,400,000
Accounts receivable			0	0
Aggregate amount of transaction price allocated to remaining performance obligation			1,200,000	$ 1,400,000
Revenue recognized			$ 100,000
CFF Agreement \| CFF \| Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Expected payments multiples to equal actual award received		6
Future sales-based milestone payments multiples		3
Royalties multiples on actual award received		6

Stock-based Compensation - Summary of Restricted Stock Units (RSU) (Details) - Restricted Stock Units (RSU) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2023 USD ($) $ / shares shares
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Number of Shares, Granted \| shares	181,181
Number of Shares, Unvested Ending Balance \| shares	181,181
Weighted Average Grant Date Fair Value, Granted \| $ / shares	$ 14.42
Weighted Average Grant Date Fair Value, Unvested Ending Balance \| $ / shares	$ 14.42
Weighted- Average Remaining Contractual Term (in years), Unvested balance	2 years 1 month 6 days
Aggregate Intrinsic Value, Unvested balance \| $	$ 3,671

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

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+ Details

Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Current assets:
Cash and cash equivalents	$ 249,108	$ 52,351
Marketable securities	39,124	161,203
Prepaid expenses and other current assets	8,356	6,957
Total current assets	296,588	220,511
Marketable securities, long-term	10,954	4,908
Property and equipment, net	20,126	22,262
Operating lease right-of-use assets, net	11,539	13,085
Other assets	684	1,080
Total assets	339,891	261,846
Current liabilities
Accounts payable	3,515	3,322
Accrued and other current liabilities	12,014	8,870
Deferred revenue	273	884
Operating lease liabilities, current portion	3,149	2,655
Total current liabilities	18,951	15,731
Deferred revenue, net of current portion	972	1,076
Derivative liability	369	212
Operating lease liabilities, long-term portion	11,522	13,469
Other liabilities	248	21
Total liabilities	32,062	30,509
Commitments and contingencies (Note 9)
Stockholders' equity
Preferred stock, $0.0001 par value, 10,000,000 shares authorized at December 31, 2023 and 2022; no shares issued and outstanding at December 31, 2023 and 2022	0	0
Common stock, $0.0001 par value, 300,000,000 shares authorized at December 31, 2023 and 2022; 43,075,218 and 32,626,627 shares issued and outstanding at December 31, 2023 and 2022, respectively	4	3
Additional paid-in-capital	723,136	547,020
Accumulated other comprehensive gain (loss)	16	(1,196)
Accumulated deficit	(415,327)	(314,490)
Total stockholders' equity	307,829	231,337
Total liabilities and stockholders' equity	$ 339,891	$ 261,846

License Arrangements

12 Months Ended

Dec. 31, 2023

License Arrangements [Abstract]

License Arrangements

8. License Arrangements

Astellas Gene Therapies, Inc.

On July 5, 2023, the Company entered into a licensing agreement (the “License Agreement”) with Astellas Gene Therapies, Inc. (“AGT”), pursuant to which the Company granted to AGT a license to utilize its intravitreal R100 vector (“4D Vector”) to develop and commercialize licensed compounds and licensed products for one genetic target implicated in rare monogenic ophthalmic disease(s), with options to add up to two additional targets implicated in rare monogenic ophthalmic diseases after paying additional option exercise fees. Under the terms of the License Agreement, the Company has provided its 4D vector technology to Astellas to deliver Astellas’ genetic payloads for the treatment of rare monogenic diseases. Astellas will conduct all subsequent research, development, manufacturing, and commercialization

activities. As partial consideration for the rights and licenses granted to AGT by the Company under this Agreement, AGT paid the Company an upfront amount of $20 million, which was received in July 2023. The Company may receive potential future option fees and milestones of up to $942.5 million including potential near-term development milestones of $15 million for the initial target. In addition, the Company is entitled to receive mid-single digit to double-digit, sub-teen royalties on net sales of all licensed products.

Under the License Agreement, the Company’s performance obligation is to grant and make available to AGT the Licensed IP and Licensed Know-How (each as defined in the License Agreement) with respect to the 4D Vector. In connection with the grant of the license, in July 2023, the Company delivered to AGT the Transferred Material (as defined in the License Agreement). As of December 31, 2023, there was no deferred revenue and no accounts receivable relating to the License Agreement.

Aevitas Therapeutics, Inc. and The Trustees of the University of Pennsylvania

On April 21, 2023, the Company entered into an agreement (the “Agreement”) with Aevitas Therapeutics, Inc. (“Aevitas”), pursuant to which the Company acquired all of Aevitas’ worldwide rights to short-form human complement factor H ("sCFH"), which the Company plans to use for its 4D-175 product candidate research program. The asset purchase was accounted for as an asset acquisition. As consideration for the Agreement, the Company shall pay Aevitas up to approximately $144 million in cash upon certain late-stage development, regulatory and sales milestones being achieved plus royalties in the low single digits range on sales of 4D-175. In addition, as part of the Agreement, the Company was assigned a License Agreement for sCFH with the Trustees of the University of Pennsylvania, under which the Company shall pay the University of Pennsylvania up to approximately $42 million in cash upon certain late-stage development, regulatory and sales milestones being achieved plus royalties in the low single digits range on sales of 4D-175. No upfront consideration was paid under the Agreement.

As of December 31, 2023, the Company has not recorded a liability related to contingent consideration for future milestone and royalty payments to either Aevitas or the University of Pennsylvania, as the achievement of such milestones has not occurred and was not deemed probable and product sales have not commenced.

Regents of the University of California

The Company has exclusive, worldwide license agreements (the “UC Agreements”) with the Regents of the University of California (the “UC Regents”) relating to the use of certain patents and intellectual property surrounding its core technologies, including Therapeutic Vector Evolution. Pursuant to each of the UC Agreements executed prior to January 2019, the Company was obligated to pay a (i) non-refundable license fee of $5,000 upon execution, (ii) a non-refundable license fee of $5,000 each year thereafter, until sales of a licensed product are made and royalties are paid to the UC Regents, (iii) reimbursement of domestic and foreign patent filing, prosecution and maintenance fees, and (iv) either $50,000 or issuance of a 3% equity interest in the Company upon the closing of the first qualified financing at the option of the UC Regents. The Company’s first qualified financing occurred in 2015 and at the election of the UC Regents, the Company issued the UC Regents in January 2016 an amount of common stock equal to 6% of the equity interests in the Company pursuant to the applicable clause in each of the UC Agreements.

Pursuant to an agreement with the UC Regents executed in January 2019 the Company paid a non-refundable license fee of $50,000 to the UC Regents upon execution of the agreement. The Company is obligated to pay a non-refundable license fee of $5,000 on the one-year anniversary of the contract effective date and each year thereafter, until sales of a licensed product are made and royalties are paid to the UC Regents.

In addition, the Company is obligated to make certain contingent payments including (i) development milestones up to $3.1 million, (ii) low single digit royalties on the net sales of its developed products that

consists of a minimum annual royalty of up to $0.1 million per year for the term of the agreement beginning in the first calendar year after the year in which net sales first occurred, and (iii) sublicense consideration in the mid-teens to the mid-twenties-range on any future sublicensing arrangements the Company may enter into with third-party licensees.

Regents of the University of California and The Trustees of the University of Pennsylvania

In July 2021, the Company entered into an exclusive license agreement with the UC Regents and the Trustees of the University Pennsylvania to license intellectual property related to certain vectors. In July 2021, the Company paid a non-refundable license fee of $100,000 to the UC Regents upon execution of the agreement. The Company is obligated to pay a non-refundable license maintenance fee of $10,000 on the one-year anniversary of the contract effective date and each year thereafter, except years for which the Company has paid royalties on the net sales of a licensed product. In addition, the Company is obligated to make certain contingent payments including (i) development milestones up to $3.9 million, (ii) low single digit percentage rate royalties on the net sales of its licensed products that consists of a minimum annual royalty of up to $0.1 million per year for the term of the agreement beginning in the first calendar year after the year in which net sales first occurred and (iii) sublicense consideration in the mid-single digits to the low twenties percentage rate range on any future sublicensing arrangements the Company may enter into with third-party licensees. On February 22, 2023, the Company provided 90 days written notice to the UC Regents and the Trustees of the University Pennsylvania to terminate said agreement in its entirety at-will. Accordingly, the effective termination date of this agreement is May 23, 2023.

During the year ended December 31, 2023, the Company incurred no expenses under the provisions of the outstanding license arrangements, while $0.2 million was incurred during the year ended December 31, 2022.

Net Loss Per Share, Basic and Diluted - Computation of Basic and Diluted Net Loss Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Numerator
Net loss attributable to common stockholders	$ (100,837)	$ (107,494)
Denominator
Weighted-average shares outstanding used in computing net loss per share, basic	39,130,067	32,351,221
Weighted-average shares outstanding used in computing net loss per share, diluted	39,130,067	32,351,221
Net loss per share, basic	$ (2.58)	$ (3.32)
Net loss per share, diluted	$ (2.58)	$ (3.32)

- Definition

The amount of net income (loss) for the period per each share of common stock or unit outstanding during the reporting period.

+ References

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The amount of net income (loss) for the period available to each share of common stock or common unit outstanding during the reporting period and to each share or unit that would have been outstanding assuming the issuance of common shares or units for all dilutive potential common shares or units outstanding during the reporting period.

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+ Details

Commitments and Contingencies - Summary of Maturities of Lease Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Commitments and Contingencies Disclosure [Abstract]
2024	$ 3,149
2025	3,244
2026	3,135
2027	2,810
2028	2,894
Thereafter	2,981
Total future minimum lease payments	18,213
Less: Amount representing interest	(3,542)
Present value of future minimum lease payments	14,671
Less: Current portion of operating lease liabilities	3,149	$ 2,655
Long-term portion of operating lease liabilities	$ 11,522	$ 13,469

Derivative Liability

12 Months Ended

Dec. 31, 2023

Derivative Instruments and Hedging Activities Disclosure [Abstract]

Derivative Liability

15. Derivative Liability

The Company identified an embedded derivative resulting from the change of control provision in the CFF Agreement. Embedded derivatives that are required to be bifurcated from the underlying host instrument are accounted for and valued as separate financial instruments. At the inception of the derivative in 2017, the Company recognized this derivative as a liability and revenue was reduced by the initial fair value of the derivative liability. The Company remeasures the derivative liability to fair value at each reporting period and records the change in fair value of the derivative liability as other income (expense), net. The Company uses a present value analysis with multiple scenarios, which incorporates assumptions and estimates to value the derivative instrument. The Company assesses these assumptions and estimates on a periodic basis as additional information impacting the assumptions is obtained. Estimates and assumptions impacting the fair value measurement include the change of control payment to CFF (range of zero to $18.9 million at December 31, 2023 and zero to $10.6 million at December 31, 2022), the probability of a change of control event, the probability of the product achieving development or commercial status at time of change of control (range of 4.8% to 17.2% at December 31, 2023 and 2022) and the discount rate (15% at December 31, 2023 and 2022). The Company determined the estimated fair value of this liability as of the inception date of the CFF Agreement and concluded that the amount was immaterial. The Company determined the fair value of this derivative liability was $0.4 million and $0.2 million as of December 31, 2023 and 2022, respectively.

Statements of Comprehensive Loss - USD ($) $ in Thousands	12 Months Ended
Statements of Comprehensive Loss - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Statement of Comprehensive Income [Abstract]
Net Income (Loss)	$ (100,837)	$ (107,494)
Other comprehensive loss:
Net unrealized gain (loss) on marketable securities	1,212	(773)
Total comprehensive loss	$ (99,625)	$ (108,267)

- Definition

The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

+ References

+ Details

Income Taxes - Schedule of Effective Tax Rate of Income Tax Expense (Benefit) Differs from Federal Statutory Rate (Details)	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Income Tax Disclosure [Abstract]
Federal statutory income tax rate	21.00%	21.00%
State tax rate	15.80%	8.30%
Research tax credit	2.40%	1.30%
Permanent differences	(0.10%)	0.00%
Stock-based compensation	(1.70%)	(1.00%)
Officer's compensation	(1.40%)	(0.80%)
Valuation allowance	(36.00%)	(28.80%)
Provision for income taxes	0.00%	0.00%

Statements of Operations - USD ($) $ in Thousands	12 Months Ended
Statements of Operations - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Revenue:
Collaboration and license revenue	$ 20,723	$ 3,129
Operating expenses:
Research and development (includes $183 and $306 for the years ended December 31, 2023 and 2022, respectively, attributable to related parties)	97,096	80,253
General and administrative	36,494	32,908
Total operating expenses	133,590	113,161
Loss from operations	(112,867)	(110,032)
Other income (expense):
Interest income	12,211	2,573
Other expense, net	(181)	(35)
Total other income (expense), net	12,030	2,538
Net loss	$ (100,837)	$ (107,494)
Net loss per share, basic	$ (2.58)	$ (3.32)
Net loss per share, diluted	$ (2.58)	$ (3.32)
Weighted-average shares outstanding used in computing net loss per share, basic	39,130,067	32,351,221
Weighted-average shares outstanding used in computing net loss per share, diluted	39,130,067	32,351,221

Research and Collaboration Arrangements (Tables)

12 Months Ended

Dec. 31, 2023

Research And Collaboration Arrangements [Abstract]

Summary of Collaboration and License Revenue

Collaboration and license revenue for each period was as follows (in thousands):


	Year Ended December 31,
	2023		2022
uniQure	$	567	$	3,094
CFF		148		35
Astellas (see Note 8)		20,008		—
	$	20,723	$	3,129

Summary of Deferred Revenue

Deferred revenue for each period was as follows (in thousands):


	December 31,
	2023		2022
uniQure	$	—	$	566
CFF		1,245		1,394
	$	1,245	$	1,960

Common Stock

12 Months Ended

Dec. 31, 2023

Equity [Abstract]

Common Stock

11. Common Stock

As of December 31, 2023 and 2022, the Company’s certificate of incorporation authorized the Company to issue 300,000,000 shares of common stock, at the par value of $0.0001 per share. The holder of each share of common stock is entitled to one vote per share.

Common stockholders are entitled to dividends if and when declared by the board of directors, subject to the prior rights of the preferred stockholders. As of December 31, 2023 and 2022, no dividends on common stock had been declared by the board of directors.

As of December 31, 2023 and 2022, the Company has reserved common stock, on an as-converted basis, for future issuance as follows:


	December 31,
	2023		2022
Issuance of common stock under the 2020 Equity Incentive Plan		524,078		2,112,039
Issuance of common stock under the 2020 Employee Stock Share Purchase Plan		313,034		413,273
Exercise of options issued and outstanding		8,423,481		5,778,929
Exercise of common stock warrants		53,669		53,669
Total common stock reserved		9,314,262		8,357,910

Funding Agreement with CFF — In April 2020, CFF made a $10.0 million investment in the Company's Series C redeemable convertible preferred stock financing. In return for the investment, CFF received shares of Series C redeemable convertible preferred stock, and the Company and CFF entered into a Funding Agreement (the “Funding Agreement”). Pursuant to the terms of the Funding Agreement, except in the event of a technical failure, the $10.0 million received from CFF will be used to advance the development program for 4D-710, the Company’s lead product in cystic fibrosis, or any other therapeutic approved by the Program Advisory Group (“PAG”) to alleviate pulmonary complications of cystic fibrosis (the “Funding Agreement Product”).

CFF committed to provide an additional $4.0 million of funding upon acceptance of an Investigational New Drug (“IND”) application or its equivalent to allow for human testing of the Funding Agreement Product (“Acceptance”).

In October 2021, the IND was cleared by the U.S. Food and Drug Administration and CFF made the additional investment of $4.0 million in cash for the issuance of 125,715 shares of the Company's common stock to CFF.

The Company was committed to providing an amount equal to the funding provided by CFF to be used solely to advance the Funding Agreement Product. As of December 31, 2023, the funding commitment has been fulfilled. Under the terms of the Funding Agreement, neither the $10.0 million investment in the Series C redeemable convertible preferred stock, which converted to common stock as of December 31, 2020, nor the $4.0 million of funding upon Acceptance are restricted as to withdrawal or usage.

- Definition

The entire disclosure for equity.

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-URI https://asc.fasb.org//1943274/2147481112/505-10-50-18

Reference 18: http://www.xbrl.org/2003/role/disclosureRef
-Topic 505
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 18
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481112/505-10-50-18

Reference 19: http://www.xbrl.org/2003/role/disclosureRef
-Topic 505
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 18
-Subparagraph (d)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481112/505-10-50-18

+ Details

Income Taxes - Schedule of Tax Effects of Temporary Differences to Significant Components of Deferred Taxes (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Deferred Tax Assets
Net operating loss carryforwards	$ 59,547	$ 47,050
Other accrued liabilities	2,497	1,921
Deferred revenue	348	539
Research tax credits	13,961	10,546
Stock-based compensation expense	5,340	4,825
Intangible asset basis	1,014	1,092
Operating lease liabilities	4,106	4,437
Capitalized research expenditures	35,784	16,172
Total deferred tax assets	122,597	86,582
Deferred Tax Liabilities, Net [Abstract]
Operating lease right-of-use assets	(3,229)	(3,600)
Prepaid expenses	(1,843)	(1,556)
Total deferred tax liabilities	(5,072)	(5,156)
Less: valuation allowance	(117,525)	(81,426)
Total net deferred tax	$ 0	$ 0

Insider Trading Arrangements	12 Months Ended
Insider Trading Arrangements	Dec. 31, 2023 shares
Trading Arrangements, by Individual
Material Terms of Trading Arrangement	During the fiscal quarter ended December 31, 2023, the following officer, as defined in Rule 16a-1(f), adopted a “Rule 10b5-1 trading arrangement,” as defined in Regulation S-K Item 408, as follows: On October 10, 2023, Scott Bizily, our Chief Legal Officer, adopted a Rule 10b5-1 trading arrangement providing for the sale from time to time of up to 100,922 shares of our common stock. The trading arrangement is intended to satisfy the affirmative defense in Rule 10b5-1(c). The duration of the trading arrangement is until June 30, 2025. None of our other directors or officers (as defined in Section 16 of the Securities Exchange Act of 1934, as amended) adopted or terminated any contract, instruction or written plan for the purchase or sale of our securities that was intended to satisfy the affirmative defense conditions of Rule 10b5-1(c) or any “non-Rule 10b5-1 trading arrangement,” as defined in Item 408(a) of Regulation S-K.
Name	Scott Bizily
Title	Chief Legal Officer
Rule 10b5-1 Arrangement Adopted	true
Adoption Date	October 10, 2023
Aggregate Available	100,922

Net Loss Per Share, Basic and Diluted - Outstanding Potentially Dilutive Securities Excluded From Computation of Diluted Net Loss Per Share (Details) - shares	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive securities	8,755,652	6,225,887
Options Issued and Outstanding
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive securities	8,423,481	5,778,929
2020 ESPP
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive securities	278,502	393,289
Common Stock Warrants
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive securities	53,669	53,669

Commitments and Contingencies - Summary of Components of Lease Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Commitments and Contingencies Disclosure [Abstract]
Operating lease cost	$ 2,793	$ 2,787
Variable lease cost	1,244	1,010
Total	$ 4,037	$ 3,797

Commitments and Contingencies

12 Months Ended

Dec. 31, 2023

Commitments and Contingencies Disclosure [Abstract]

Commitments and Contingencies

9. Commitments and Contingencies

5980 Horton Street Building Lease

In May 2015, the Company executed a lease agreement for office and laboratory space in Emeryville, California. In January 2016, the Company executed the first amendment to the lease agreement for additional rentable office and laboratory space which extended the lease term to March 31, 2023 and updated the option to renew the lease at market rent from the prior additional term of three years to five years. In October 2018, the Company executed a second amendment to extend the lease to September 2026. Additionally, the second amendment provided a tenant improvement allowance of $0.2 million, which was paid to the Company in November 2018.

The 5980 Horton Street Building Lease is considered an operating lease under ASC 842 as it does not meet the criteria of a finance lease. As of December 31, 2023, the operating lease right-of-use asset and operating lease liability was $1.2 million and $1.4 million, respectively. The discount rate used to determine the lease liability was 7.5%.

5858 Horton Street Building Lease and Expansion

In October 2018, the Company executed a second lease agreement for additional office and laboratory space in Emeryville, California. The new lease has an initial term of 87 months beginning on the rent commencement date with the option to renew the lease for one additional term of five years at market rent. The Company did not have to pay rent until October 2019. This lease agreement also provided for a tenant improvement allowance of $0.4 million, which was paid to the Company in December 2019. The Company amortizes the tenant improvement allowance on a straight-line basis over the remaining term of the lease as a reduction of rent expense.

In May 2019, the Company amended the second lease agreement executed in October 2018 to add additional office, manufacturing and laboratory space (the “Expansion”). The amendment extended the term of the lease to December 31, 2029. The Company did not have to pay rent until December 2019. The lease agreement also included a tenant improvement allowance, which was increased to $2.3 million in February 2021 pursuant to a second amendment to the second lease agreement. As of December 31, 2023, the Company has received the $2.3 million tenant improvement allowance. The tenant improvement allowance funds improvements to the additional office, manufacturing and laboratory space, which the Company has determined to be lessee owned. The allowance is treated as a reduction of lease payments used to measure the lease liability.

The 5858 Horton Street Building Lease and Expansion is considered an operating lease under ASC 842 as it does not meet the criteria of a finance lease. As of December 31, 2023, the operating lease right-of-use asset and operating lease liability was $10.3 million and $13.2 million, respectively. The discount rate used to determine the lease liability was 7.8%.

Total operating lease cost for each of the years ended December 31, 2023 and 2022 was $2.8 million. Cash paid for amounts included in the measurement of lease liabilities was $3.1 million for the year ended December 31, 2023.

Neither of the leases include a general option for the Company to terminate the leases.

The following table summarizes the components of lease expense for the years ended December 31, 2023 and 2022, which are included in operating expenses in the Company’s statements of operations (in thousands):


	Year Ended December 31,
	2023		2022
Operating lease cost	$	2,793	$	2,787
Variable lease cost		1,244		1,010
Total	$	4,037	$	3,797

Variable lease payments include amounts relating to common area maintenance and are recognized in the statements of operations as incurred.

The following table summarizes supplemental information related to operating leases:


	Year Ended December 31,
	2023			2022
Weighted-average remaining lease term (in years)		5.7			6.7
Weighted-average discount rate		7.8	%		7.8	%

The following table summarizes the maturities of lease liabilities as of December 31, 2023 (in thousands):


2024	$	3,149
2025		3,244
2026		3,135
2027		2,810
2028		2,894
Thereafter		2,981
Total future minimum lease payments		18,213
Less: Amount representing interest		(3,542	)
Present value of future minimum lease payments		14,671
Less: Current portion of operating lease liabilities		3,149
Long-term portion of operating lease liabilities	$	11,522

Indemnification Agreements

In the ordinary course of business, the Company enters into agreements that may include indemnification provisions, such as with vendors and other parties. Pursuant to such agreements, the Company may indemnify, hold harmless and defend an indemnified party for losses suffered or incurred by the indemnified party. Some of the provisions will limit losses to those arising from third-party actions. In some cases, the indemnification will continue after the termination of the agreement. The maximum potential amount of future payments the Company could be required to make under these provisions is not determinable. The Company has never incurred material costs to defend lawsuits or settle claims related to these indemnification provisions. The Company has also entered into indemnification agreements with its directors and officers that require the Company, among other things, to indemnify them against certain liabilities that may arise by reason of their status or service as directors or officers to the fullest extent permitted by Delaware corporate law. The Company currently maintains directors’ and officers’ liability insurance that would generally enable it to recover a portion of any future amounts paid. The Company believes the estimated fair value of its indemnification agreements in excess of applicable insurance coverage is not material.

Legal Proceedings

From time to time, the Company may become involved in legal proceedings arising from the ordinary course of its business. If applicable, the Company records a legal liability when it believes that it is both probable that a liability may be imputed, and the amount of the liability can be reasonably estimated. Significant judgment by the Company is required to determine both probability and the estimated amount. There are no material legal proceedings outstanding at December 31, 2023. See Note 13 for details of a settlement with a former employee in April 2021.

Net Loss Per Share, Basic and Diluted

12 Months Ended

Dec. 31, 2023

Earnings Per Share [Abstract]

Net Loss Per Share, Basic and Diluted

14. Net Loss Per Share, Basic and Diluted

The following table sets forth the computation of basic and diluted net loss per share (in thousands, except share and per share data):


	Year Ended December 31,
	2023			2022
Numerator
Net loss attributable to common stockholders	$	(100,837	)	$	(107,494	)
Denominator
Weighted-average shares outstanding used in computing net loss per share, basic and diluted		39,130,067			32,351,221
Net loss per share, basic and diluted	$	(2.58	)	$	(3.32	)

The following outstanding shares of potentially dilutive securities were excluded from the computation of diluted net loss per share for the periods presented because including them would have been antidilutive:


	December 31,
	2023		2022
Options issued and outstanding		8,423,481		5,778,929
2020 ESPP		278,502		393,289
Common stock warrants		53,669		53,669
Total		8,755,652		6,225,887

Fair Value Measurements - Fair Value Hierarchy for Financial Assets and Financial Liabilities Measured at Fair Value on Recurring Basis (Details) - Fair Value on Recurring Basis - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets	$ 267,722	$ 210,558
Liabilities	369	212
Derivative Liability
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Liabilities	369	212
Commercial Paper
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets	18,243	48,872
Corporate Bonds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets	28,939	47,012
U.S. Treasuries
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets		63,059
U.S. Government Sponsored Agencies
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets	2,896	7,168
Money Market Funds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets	217,644	44,447
Level 1
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets	217,644	44,447
Level 1 \| Money Market Funds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets	217,644	44,447
Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets	50,078	166,111
Level 2 \| Commercial Paper
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets	18,243	48,872
Level 2 \| Corporate Bonds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets	28,939	47,012
Level 2 \| U.S. Treasuries
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets		63,059
Level 2 \| U.S. Government Sponsored Agencies
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets	2,896	7,168
Level 3
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Liabilities	369	212
Level 3 \| Derivative Liability
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Liabilities	$ 369	$ 212

License Arrangements - Additional Information (Details) - USD ($)			1 Months Ended			12 Months Ended
	Jul. 05, 2023	Apr. 21, 2023	Jul. 31, 2021	Jan. 31, 2019	Jan. 31, 2016	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2018
Astellas GeneTherapies Inc
Research And Development Arrangement Contract To Perform For Others [Line Items]
Upfront payments received	$ 20,000,000
Contingent payments development milestones initial target	15,000,000
Deferred revenue						$ 0
Accounts receivable						0
Astellas GeneTherapies Inc \| Maximum
Research And Development Arrangement Contract To Perform For Others [Line Items]
Future option fees and milestones	$ 942,500,000
Aevitas Therapeutics, Inc. Agreement
Research And Development Arrangement Contract To Perform For Others [Line Items]
Upfront consideration payment		$ 0
Liability related to contingent consideration						0
Aevitas Therapeutics, Inc. Agreement \| Maximum
Research And Development Arrangement Contract To Perform For Others [Line Items]
Potential cash payment upon achievement of certain late-stage development, regulatory and sales milestones		144,000,000
License Agreement for sCFH with University of Pennsylvania
Research And Development Arrangement Contract To Perform For Others [Line Items]
Liability related to contingent consideration						0
License Agreement for sCFH with University of Pennsylvania \| Maximum
Research And Development Arrangement Contract To Perform For Others [Line Items]
Potential cash payment upon achievement of certain late-stage development, regulatory and sales milestones		$ 42,000,000
UC Regents and the Trustees of the University of Pennsylvania
Research And Development Arrangement Contract To Perform For Others [Line Items]
Amount paid non-refundable license fee			$ 100,000
Non-refundable license fee payable annually			10,000
Incurred expenses						$ 0	$ 200,000
UC Regents and the Trustees of the University of Pennsylvania \| Maximum
Research And Development Arrangement Contract To Perform For Others [Line Items]
Contingent payments development milestones			3,900,000
UC Regents and the Trustees of the University of Pennsylvania \| Minimum
Research And Development Arrangement Contract To Perform For Others [Line Items]
Annual royalty			$ 100,000
UC Agreements
Research And Development Arrangement Contract To Perform For Others [Line Items]
Non-refundable license fee payable upon execution								$ 5,000
Non-refundable license fee payable upon execution every year thereafter until sales of a licensed product and payment of royalties								5,000
Amount obligated to pay upon closing first qualified financing at option of UC Regents								$ 50,000
Percentage of equity interest upon closing of first qualified financing at option of UC Regents								3.00%
Percentage of equity interests issued as common stock					6.00%
Amount paid non-refundable license fee				$ 50,000
Non-refundable license fee payable annually				5,000
UC Agreements \| Maximum
Research And Development Arrangement Contract To Perform For Others [Line Items]
Contingent payments development milestones				3,100,000
UC Agreements \| Minimum
Research And Development Arrangement Contract To Perform For Others [Line Items]
Annual royalty				$ 100,000

Accrued and Other Current Liabilities

12 Months Ended

Dec. 31, 2023

Payables and Accruals [Abstract]

Accrued and Other Current Liabilities

6. Accrued and Other Current Liabilities

Accrued and other current liabilities consisted of the following (in thousands):


	December 31,
	2023		2022
Payroll and related expenses	$	8,149	$	5,689
Accrued clinical and preclinical study costs		2,180		1,355
Consulting and professional		1,581		1,444
Other accrued expenses		104		382
Total accrued and other current liabilities	$	12,014	$	8,870

Stock-based Compensation - Summary of Stock Compensation Expense for Employees and Nonemployees by Function (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total stock-based compensation expense	$ 19,665	$ 17,121
Research and Development
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total stock-based compensation expense	9,740	9,221
General and Administrative
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total stock-based compensation expense	$ 9,925	$ 7,900

Research and Collaboration Arrangements - Additional Information (Details) - USD ($)	1 Months Ended	12 Months Ended
	Jan. 31, 2014	Dec. 31, 2023	Dec. 31, 2022	Aug. 31, 2019
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue recognized		$ 700,000	$ 3,100,000
Deferred revenue		1,245,000	1,960,000
Arbor \| Arbor Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Research and development expenses		0
Shared product designated by JSC		0
uniQure
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue		0	566,000
Accounts receivable		0	0
uniQure \| uniQure Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Increase in revenue recognized related to performance obligations partially satisfied in the beginning of the current year			2,200,000
Collaborative arrangement, initial research term	3 years
Upfront payments received	$ 200,000
Options to purchase ordinary shares	609,744
Ordinary shares vesting period	3 years
Fair value of options	$ 10,600,000
Fair value of non monetary consideration of intellectual property right				$ 5,100,000
uniQure \| uniQure Agreement \| Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contingent payments entitled to receive for achievement of research and development milestones	$ 5,000,000
uniQure \| Amended uniQure Agreement \| Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contingent payments eliminated				$ 5,000,000
uniQure \| Amended uniQure Agreement and Second uniQure Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue recognized		600,000	3,100,000
Remaining performance obligations		$ 0	$ 600,000

Income Taxes - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Operating Loss Carryforwards [Line Items]
Income tax expense	$ 0	$ 0
Increase in valuation allowance, deferred tax asset	36,100,000	31,200,000
Net operating loss carryforwards	228,200,000	212,900,000
Accrued interest and penalties related to unrecognized tax benefits	0	0
Capitalized research expenditures	$ 35,784,000	16,172,000
Inflation Reduction Act of 2022
Operating Loss Carryforwards [Line Items]
Alternative minimum tax description	A new corporate alternative minimum tax imposed on certain corporations meeting average annual financial statement income of more than $1 billion for the three tax-year period
United States
Operating Loss Carryforwards [Line Items]
Research and experimental expenses amortized period years	5 years
Foreign
Operating Loss Carryforwards [Line Items]
Research and experimental expenses amortized period years	15 years
Federal
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	$ 9,500,000
Net operating loss carryforwards, expiration year	2037
Net operating loss carryforward indefinitely	$ 218,700,000
Research and development credit carryforward	$ 17,100,000	11,900,000
Tax credit carryforward beginning expiration year	2035
State
Operating Loss Carryforwards [Line Items]
Research and development credit carryforward	$ 10,000,000	$ 7,900,000
Tax credit carryforward, expiration description	indefinitely

Common Stock Warrants - Additional Information (Details) - USD ($) $ / shares in Units, $ in Millions	1 Months Ended				12 Months Ended
	Oct. 31, 2021	Apr. 30, 2021	Dec. 31, 2020	May 31, 2018	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2020	Dec. 31, 2016	Dec. 31, 2018
Class Of Warrant Or Right [Line Items]
Number of warrant shares issued		40,000	30,000	23,669				45,000
Exercise price		$ 9.41	$ 18	$ 3.19			$ 18	$ 1.14
Warrant shares, exercised	45,000
Warrant expiration year				2025			2027	2023
Warrant vesting period							4 years
Warrant exercised month and year	2021-10	2021-05
Maximum
Class Of Warrant Or Right [Line Items]
Fair value of warrant shares					$ 0.1	$ 0.1
Series B Financing
Class Of Warrant Or Right [Line Items]
Warrant shares become exercisable									15,000
Private Placement
Class Of Warrant Or Right [Line Items]
Warrant shares become exercisable								15,000
Proceeds from offering of securities in a private placement								$ 25.0
Initial Public Offering
Class Of Warrant Or Right [Line Items]
Warrant shares become exercisable			30,000				30,000

Commitments and Contingencies (Tables)

12 Months Ended

Dec. 31, 2023

Commitments and Contingencies Disclosure [Abstract]

Summary of Components of Lease Expense


	Year Ended December 31,
	2023		2022
Operating lease cost	$	2,793	$	2,787
Variable lease cost		1,244		1,010
Total	$	4,037	$	3,797

Summary of Supplemental Information Related to Leases

The following table summarizes supplemental information related to operating leases:


	Year Ended December 31,
	2023			2022
Weighted-average remaining lease term (in years)		5.7			6.7
Weighted-average discount rate		7.8	%		7.8	%

Summary of Maturities of Lease Liabilities

The following table summarizes the maturities of lease liabilities as of December 31, 2023 (in thousands):


2024	$	3,149
2025		3,244
2026		3,135
2027		2,810
2028		2,894
Thereafter		2,981
Total future minimum lease payments		18,213
Less: Amount representing interest		(3,542	)
Present value of future minimum lease payments		14,671
Less: Current portion of operating lease liabilities		3,149
Long-term portion of operating lease liabilities	$	11,522

Research and Collaboration Arrangements - Summary of Collaboration and License Revenue (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration and license revenue	$ 20,723	$ 3,129
Collaboration and License Revenue
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration and license revenue	20,723	3,129
Collaboration and License Revenue \| uniQure
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration and license revenue	567	3,094
Collaboration and License Revenue \| CFF
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration and license revenue	148	35
Collaboration and License Revenue \| Astellas
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration and license revenue	$ 20,008	$ 0

- Definition

+ References

+ Details

Common Stock Warrants

12 Months Ended

Dec. 31, 2023

Equity [Abstract]

Common Stock Warrants

13. Common Stock Warrants

In 2016, the Company issued a warrant for 45,000 shares of the Company’s common stock to a service provider with an exercise price of $1.14 per share, of which 15,000 warrant shares become exercisable upon completion of an offering of securities in a private placement by the Company with net proceeds in excess of $25.0 million and 30,000 warrant shares become exercisable upon completion of an IPO by the Company. The warrant expires in 2023. The fair value of the warrant was determined at the issuance date using the Black-Scholes option pricing model. 15,000 of these warrant shares became exercisable upon the completion of the Series B financing in 2018 and 30,000 of these warrants became exercisable upon completion of the IPO in 2020. 45,000 shares of this warrant were exercised in October 2021.

In May 2018, the Company issued a warrant for 23,669 shares of the Company’s common stock to a service provider with an exercise price of $3.19 per share. The fair value of the warrant was determined at the issuance date using the Black-Scholes option pricing model, expires in 2025 and upon issuance was fully vested.

In December 2020, the Company issued a warrant for 30,000 shares of the Company’s common stock to a service provider with an exercise price of $18.00 per share. This warrant vests over a period of four years and expires in 2027. The fair value of the warrant was determined at the issuance date using the Black-Scholes option pricing model.

In April 2021, the Company issued a warrant for 40,000 shares of the Company’s common stock to a former employee with an exercise price of $9.41 per share as a result of a settlement agreement with such former employee. The warrant was exercised in May 2021.

The Company recorded less than $0.1 million expense for the above warrants within operating expenses in the statements of operations during the years ended December 31, 2023 and 2022.

Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2023	Dec. 31, 2022
Statement of Financial Position [Abstract]
Preferred stock, par value	$ 0.0001	$ 0.0001
Preferred stock, shares authorized	10,000,000	10,000,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value	$ 0.0001	$ 0.0001
Common stock, shares authorized	300,000,000	300,000,000
Common stock, shares, issued	43,075,218	32,626,627
Common stock, shares, outstanding	43,075,218	32,626,627

Document and Entity Information - USD ($)	12 Months Ended
Document and Entity Information - USD ($)	Dec. 31, 2023	Feb. 23, 2024	Jun. 30, 2023
Cover [Abstract]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2023
Document Fiscal Year Focus	2023
Document Fiscal Period Focus	FY
Document Annual Report	true
Document Transition Report	false
Entity Registrant Name	4D Molecular Therapeutics, Inc.
Entity Central Index Key	0001650648
Current Fiscal Year End Date	--12-31
Entity Well-known Seasoned Issuer	No
Entity Current Reporting Status	Yes
Entity Voluntary Filers	No
Entity Filer Category	Non-accelerated Filer
Entity Small Business	true
Entity Emerging Growth Company	true
Entity Ex Transition Period	false
Document Financial Statement Error Correction [Flag]	false
ICFR Auditor Attestation Flag	false
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		49,778,127
Entity Public Float			$ 630,270,306
Entity Interactive Data Current	Yes
Entity File Number	001-39782
Entity Tax Identification Number	47-3506994
Entity Address, Address Line One	5858 Horton Street #455
Entity Address, City or Town	Emeryville
Entity Address, State or Province	CA
Entity Address, Postal Zip Code	94608
City Area Code	(510)
Local Phone Number	505-2680
Entity Incorporation, State or Country Code	DE
Title of 12(b) Security	Common Stock, par value $0.0001 per share
Trading Symbol	FDMT
Security Exchange Name	NASDAQ
Documents Incorporated by Reference	DOCUMENTS INCORPORATED BY REFERENCE Portions of the registrant’s definitive Proxy Statement relating to the 2023 Annual Meeting of Stockholders are incorporated herein by reference in Part III of this Annual Report on Form 10-K to the extent stated herein. The proxy statement will be filed with the Securities and Exchange Commission within 120 days of the registrant’s fiscal year ended December 31, 2023.
Auditor Firm ID	238
Auditor Name	PricewaterhouseCoopers LLP
Auditor Location	San Jose, California

Common Stock (Tables)

12 Months Ended

Dec. 31, 2023

Equity [Abstract]

Schedule of Common Shares Reserved for Future Issuance

As of December 31, 2023 and 2022, the Company has reserved common stock, on an as-converted basis, for future issuance as follows:


	December 31,
	2023		2022
Issuance of common stock under the 2020 Equity Incentive Plan		524,078		2,112,039
Issuance of common stock under the 2020 Employee Stock Share Purchase Plan		313,034		413,273
Exercise of options issued and outstanding		8,423,481		5,778,929
Exercise of common stock warrants		53,669		53,669
Total common stock reserved		9,314,262		8,357,910

401(K) Plan - Additional Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
2014 401(k) Plan
Defined Contribution Plan Disclosure [Line Items]
Employer contribution	$ 1.1	$ 0.9

Fair Value Measurements

12 Months Ended

Dec. 31, 2023

Fair Value Disclosures [Abstract]

Fair Value Measurements

3. Fair Value Measurements

The following tables represent the Company’s fair value hierarchy for financial assets and financial liabilities measured at fair value on a recurring basis as of December 31, 2023 and 2022 (in thousands):


	Basis for Fair Value Measurements						Fair Value as of
	Level 1		Level 2		Level 3		December 31, 2023
Assets
Money market funds	$	217,644	$	—	$	—	$	217,644
Commercial paper		—		18,243		—		18,243
Corporate bonds		—		28,939		—		28,939
U.S. government sponsored agencies		—		2,896				2,896
Total	$	217,644	$	50,078	$	—	$	267,722
Liabilities
Derivative liability	$	—	$	—	$	369	$	369
Total	$	—	$	—	$	369	$	369


	Basis for Fair Value Measurements						Fair Value as of
	Level 1		Level 2		Level 3		December 31, 2022
Assets
Money market funds	$	44,447	$	—	$	—	$	44,447
Commercial paper		—		48,872		—		48,872
Corporate bonds		—		47,012		—		47,012
U.S. treasuries		—		63,059		—		63,059
U.S. government sponsored agencies		—		7,168		—		7,168
Total	$	44,447	$	166,111	$	—	$	210,558
Liabilities
Derivative liability	$	—	$	—	$	212	$	212
Total	$	—	$	—	$	212	$	212

Level 3 Inputs

The fair value of the derivative liability is based on significant inputs not observable in the market, which represents a Level 3 measurement within the fair value hierarchy. The fair value of the derivative liability was determined using a present value analysis with multiple scenarios. In determining the fair value of the derivative liability, the inputs impacting fair value include the change of control payment to CFF, the probability of a change of control event, the product status at time of a change of control event and the discount rate. See Note 15 for further discussion on embedded derivative.

There were no transfers between Level 1, 2 and 3 during the periods presented.

The following table sets forth a summary of the changes in the fair value of the Company’s Level 3 derivative liability (in thousands):


Balance as of December 31, 2021	$	214
Change in fair value included in other income (expense), net		(2	)
Balance as of December 31, 2022		212
Change in fair value included in other income (expense), net		157
Balance as of December 31, 2023	$	369

- Definition

The entire disclosure for the fair value of financial instruments (as defined), including financial assets and financial liabilities (collectively, as defined), and the measurements of those instruments as well as disclosures related to the fair value of non-financial assets and liabilities. Such disclosures about the financial instruments, assets, and liabilities would include: (1) the fair value of the required items together with their carrying amounts (as appropriate); (2) for items for which it is not practicable to estimate fair value, disclosure would include: (a) information pertinent to estimating fair value (including, carrying amount, effective interest rate, and maturity, and (b) the reasons why it is not practicable to estimate fair value; (3) significant concentrations of credit risk including: (a) information about the activity, region, or economic characteristics identifying a concentration, (b) the maximum amount of loss the entity is exposed to based on the gross fair value of the related item, (c) policy for requiring collateral or other security and information as to accessing such collateral or security, and (d) the nature and brief description of such collateral or security; (4) quantitative information about market risks and how such risks are managed; (5) for items measured on both a recurring and nonrecurring basis information regarding the inputs used to develop the fair value measurement; and (6) for items presented in the financial statement for which fair value measurement is elected: (a) information necessary to understand the reasons for the election, (b) discussion of the effect of fair value changes on earnings, (c) a description of [similar groups] items for which the election is made and the relation thereof to the balance sheet, the aggregate carrying value of items included in the balance sheet that are not eligible for the election; (7) all other required (as defined) and desired information.

+ References + Details

Property and Equipment, Net - Additional Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Property, Plant and Equipment [Abstract]
Depreciation expense	$ 4.4	$ 2.4

Subsequent Events

12 Months Ended

Dec. 31, 2023

Subsequent Events [Abstract]

Subsequent Events

18. Subsequent Events

On February 9, 2024, subsequent to the reporting period for the fiscal year ended December 31, 2023, the Company completed an underwritten public follow-on offering. The offering consisted of 6,586,015 shares of the Company’s common stock at an offering price to the public of $29.50 per share, as well as pre-funded warrants to purchase 3,583,476 shares of the Company’s common stock at a public offering price of $29.4999 per underlying share, in each case before underwriting discounts and commissions. Pursuant to the offering, the Company received aggregate gross proceeds of approximately $300 million, before deducting underwriting discounts and commissions and other offering expenses, resulting in net proceeds of approximately $281.4 million, after deducting underwriting discounts and commissions and other offering expenses. The Company also granted the underwriters the option to purchase up to 1,525,423 additional shares of common stock in connection with the offering.

Derivative Liability - Additional Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Derivative [Line Items]
Change in control discount rate	15.00%	15.00%
Estimated fair value of derivative liability	$ 0.4	$ 0.2
Minimum
Derivative [Line Items]
Change of control payment	$ 0.0	$ 0.0
Change in control event percentage	4.80%	4.80%
Maximum
Derivative [Line Items]
Change of control payment	$ 18.9	$ 10.6
Change in control event percentage	17.20%	17.20%

Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	ATM Offering Program	Public offering	Common Stock	Common Stock ATM Offering Program	Common Stock Public offering	Additional Paid-in Capital	Additional Paid-in Capital ATM Offering Program	Additional Paid-in Capital Public offering	Accumulated Other Comprehensive Loss	Accumulated Deficit
Stockholders' Equity, beginning balances at Dec. 31, 2021	$ 319,107			$ 3			$ 526,523			$ (423)	$ (206,996)
Stockholders' Equity, beginning balances (in shares) at Dec. 31, 2021				32,224,524
Issuance of common stock upon exercise of stock options and warrants	2,466						2,466
Issuance of common stock upon exercise of stock options and warrants (in shares)				291,629
Stock-based compensation expense	17,121						17,121
Issuance of common stock - 2020 ESPP	823						823
Issuance of common stock - 2020 ESPP (in shares)				110,474
Vesting of common stock warrants issued for services	87						87
Net unrealized gain (loss) on marketable securities	(773)									(773)
Net loss	(107,494)										(107,494)
Stockholders' Equity, ending balances at Dec. 31, 2022	231,337			$ 3			547,020			(1,196)	(314,490)
Stockholders' Equity, ending balances (in shares) at Dec. 31, 2022				32,626,627
Issuance of common stock upon exercise of stock options and warrants	7,310						7,310
Issuance of common stock upon exercise of stock options and warrants (in shares)				574,740
Issuance of common stock for the offering program, net of issuance costs		$ 19,348	$ 129,208			$ 1		$ 19,348	$ 129,207
Issuance of common stock for the offering program, net of issuance costs (in shares)					1,098,612	8,625,000
ATM offering costs		$ (680)						$ (680)
Stock-based compensation expense	19,665						19,665
Issuance of common stock - 2020 ESPP	1,178						1,178
Issuance of common stock - 2020 ESPP (in shares)				150,239
Vesting of common stock warrants issued for services	88						88
Net unrealized gain (loss) on marketable securities	1,212									1,212
Net loss	(100,837)										(100,837)
Stockholders' Equity, ending balances at Dec. 31, 2023	$ 307,829			$ 4			$ 723,136			$ 16	$ (415,327)
Stockholders' Equity, ending balances (in shares) at Dec. 31, 2023				43,075,218

Summary of Significant Accounting Policies

12 Months Ended

Dec. 31, 2023

Accounting Policies [Abstract]

Summary of Significant Accounting Policies

2. Summary of Significant Accounting Policies

Basis of Presentation

The accompanying financial statements have been prepared in accordance with United States Generally Accepted Accounting Principles (“U.S. GAAP”).

Use of Estimates and Judgements

The preparation of financial statements in conformity with U.S. GAAP requires management to make estimates and judgements that affect the reported amounts of assets, liabilities, revenue and expenses; and disclosure of contingent assets and liabilities as of the date of the financial statements. Such estimates include the determination of useful lives for property and equipment, the contract term, transaction price and costs of collaboration agreements, as well as estimates of the fair value of common stock (prior to the IPO), stock options and derivative instruments and income tax uncertainties. Actual results could differ from those estimates.

Due to the coronavirus (“COVID-19”) pandemic, the war in Ukraine, rising interest rates and inflation, there has been uncertainty and disruption in the global economy and financial markets. The Company is not aware of any specific event or circumstance that would require an update to its estimates or judgments or a revision of the carrying value of its assets or liabilities as of December 31, 2023. While there was not a material impact to the Company’s financial statements as of December 31, 2023, these estimates may change, as new events occur and additional information is obtained, as well as other factors that could result in material impacts to the financial statements in future reporting periods.

Segment Information

The Company operates and manages its business as one reportable and operating segment. The Company’s chief executive officer, who is the chief operating decision maker, reviews financial information on a company-wide basis for purposes of allocating resources and assessing financial performance.

As of and for the years ended December 31, 2023 and 2022, all of the Company’s long-lived assets were located in the United States and all revenue was earned in the United States.

Concentration of Credit Risk

Financial instruments that potentially subject the Company to a concentration of credit risk consist of cash and cash equivalents, marketable securities and accounts receivable. The Company’s cash is held at two financial institutions in the United States of America. The Company’s cash equivalents are invested in money market funds. The Company also invests in U.S. Treasuries, U.S. government sponsored agencies, commercial paper and corporate bonds. The Company has not experienced any losses on its deposits of cash and cash equivalents. Such deposits may, at times, exceed federally insured limits.

The Company’s partners in collaboration and license agreements who represent 10% or more of the Company’s total revenue are as follows:


	Year Ended December 31,
	2023	2022
Customer B	*	99%
Customer C	97%	0%
Total	97%	99%

* Less than 10%

The Company did not have accounts receivable from its partners in collaboration and license agreements as of December 31, 2023 and 2022.

The Company’s total revenues by geographic region, based on the location of the customer, are as follows (in thousands):


	Year Ended December 31,
	2023		2022
Netherlands	$	567	$	3,094
United States		20,156		35
Total revenue	$	20,723	$	3,129

Cash and Cash Equivalents

The Company considers all highly liquid investments with original maturities of three months or less at the date of purchase to be cash equivalents. Cash equivalents consist of money market funds.

Marketable Securities

Marketable securities consist of commercial paper, corporate bonds, U.S. Treasuries and U.S. government sponsored agencies and are included in current and noncurrent assets. The Company classifies its marketable securities as available-for-sale and carries them at fair value on its balance sheet. Fair value is estimated using independent pricing sources based on quoted prices in active markets for similar securities. Unrealized gains and losses on the marketable securities are reported as a component of stockholders' equity (deficit) in accumulated other comprehensive loss. The amortized cost of marketable securities is adjusted for amortization of premiums and accretion of discounts to maturity, which is included in interest income on the statements of operations. Realized gains and losses are included in interest income on the statements of operations.

The Company periodically evaluates its marketable securities to assess whether those with unrealized loss positions are other than temporarily impaired. The Company considers various factors in determining whether to recognize an impairment charge. If the Company determines that the decline in an investment's fair value is other-than-temporary, the difference is recognized as an impairment loss in the statements of operations.

Acquisitions

The Company first determines whether a set of assets acquired constitute a business and should be accounted for as a business combination. If the assets acquired do not constitute a business, the Company accounts for the transaction as an asset acquisition where the cost of the acquisition is allocated to the assets acquired and liabilities based on their relative fair values. In-process research and development ("IPR&D") projects with no alternative future use are recorded as research and development expense upon acquisition, and contingent consideration obligations incurred in connection with an asset acquisition are recorded when it is probable that they will occur and they can be reasonably estimated.

Business combinations are accounted for by means of the acquisition method of accounting. Under the acquisition method, assets acquired, including IPR&D projects, and liabilities assumed are recorded at their respective fair values as of the acquisition date. The excess of the fair value of consideration transferred over the fair value of the net assets acquired is recorded as goodwill.

Other Risks and Uncertainties

The Company is subject to risks and uncertainties common to early-stage companies in the biotechnology industry, including, but not limited to, development by competitors of new technological innovations, protection of proprietary technology, dependence on key personnel, suppliers for key raw materials, contract manufacturing organizations (“CMOs”) and contract research organizations (“CROs”), compliance with government regulations and the need to obtain additional financing to fund operations. Product candidates currently under development will require significant additional research and development efforts, including extensive preclinical studies, clinical trials and regulatory approval, prior to commercialization. These efforts require significant amounts of additional capital, adequate personnel and infrastructure and extensive compliance and reporting.

There can be no assurance that the Company’s research and development will be successfully completed, that adequate protection for the Company’s intellectual property will be obtained or maintained, that any products developed will obtain necessary government regulatory approval or that any approved products will be commercially viable. Even if the Company’s product development efforts are successful, it is uncertain when, if ever, the Company will generate significant revenue from product sales. The Company operates in an environment of rapid change in technology and substantial competition from other pharmaceutical and biotechnology companies. In addition, the Company is dependent upon the services of its employees, consultants and other third parties (including for clinical trials and some aspects of research and preclinical testing).

Fair Value Measurements

The Company applies fair value accounting for all financial assets and liabilities and nonfinancial assets and liabilities that are recognized or disclosed at fair value in the financial statements on a recurring basis. Fair value is an exit price, representing the amount that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants on the measurement date. As such, fair value is a market-based measurement that should be determined based on assumptions that market participants would use in pricing an asset or liability.

As a basis for considering such assumptions, a three-level fair value hierarchy has been established, which prioritizes the inputs used in measuring fair value as follows:

•

Level 1 — Observable inputs that reflect unadjusted quoted market prices in active markets for identical assets or liabilities that are accessible at the measurement date.

•

Level 2 — Inputs other than Level 1 that are observable, either directly or indirectly, such as quoted prices for similar assets or liabilities; quoted prices in markets that are not active; or other inputs that are observable or can be corroborated by observable market data for substantially the full term of the assets or liabilities.

•

Level 3 — Unobservable inputs that are supported by little or no market activity and that are significant to the fair value of the assets or liabilities.

The Company accounts for transfers of financial instruments between levels of the fair value hierarchy on the date of the event or change in circumstance that caused the transfer.

Accounts Receivable—Allowance for Doubtful Accounts

The Company regularly reviews accounts receivable for collectability and establishes an allowance for probable credit losses and writes off uncollectible accounts as necessary. The Company has determined that no allowance was required at December 31, 2023 and 2022. The Company did not have any write-offs relating to uncollectible accounts receivable during the years ended December 31, 2023 and 2022.

Property and Equipment, Net

Property and equipment are stated at cost less accumulated depreciation for acquired assets. Depreciation is computed using the straight-line method over the estimated useful lives of assets, ranging from three to five years. Leasehold improvements are amortized over the shorter of the useful life of the assets or the lease term. Upon sale or retirement of assets, the costs and related accumulated depreciation are removed from the balance sheet and the resulting gain or loss is reflected within operating expenses in the statements of operations. Maintenance and repairs are charged to expense as incurred.

Impairment of Long-Lived Assets

The Company reviews long-lived assets for impairment whenever events or changes in circumstances indicate that the carrying amount of an asset may not be recoverable. Recoverability is measured by comparison of the carrying amount to the future undiscounted net cash flows, which the assets are expected to generate. If such assets are considered to be impaired, the impairment to be recognized is typically measured by the amount by which the carrying amount of the assets exceeds the projected discounted future net cash flows arising from the asset. There have been no such impairments of long-lived assets in the years ended December 31, 2023 and 2022.

Common Stock Warrants

The Company accounts for common stock warrants which meet the definition of a derivative as liabilities if the warrant requires net cash settlement or gives the holder the option of net cash settlement. The Company accounts for common stock warrants as equity if the contract requires physical settlement or net physical settlement or if the Company has the option of physical settlement or net physical settlement. Common stock warrants classified as liabilities are initially recorded at fair value and remeasured at fair value each balance sheet date with the offset adjustments recorded in other income (expense), net within the statements of operations. Common stock warrants classified as equity are initially measured at fair value on the grant date and are not subsequently remeasured.

Leases

At contract inception, the Company determines if an arrangement is or contains a lease. A contract is or contains a lease if the contract conveys the right to control the use of an identified asset for a period of time in exchange for consideration. If determined to be or contain a lease, the lease is assessed for classification as either an operating or finance lease at the lease commencement date, defined as the date on which the leased asset is made available for use by the Company, based on the economic characteristics of the lease.

A right-of-use asset represents the economic benefit conveyed to the Company by the right to use the underlying asset over the lease term. A lease liability represents the obligation to make lease payments arising from the lease. Operating lease right-of-use assets represent the Company's right to use an underlying asset for the lease term and operating lease liabilities represent the Company's obligation to make payments arising from the lease. Operating right-of-use assets and liabilities are recognized at the commencement date of the lease and are measured at the present value of the fixed payments due over the expected lease term less the present value of any incentives, rebates, or abatements the Company expects to receive from the lessor. The Company records amortization of operating right-of-use assets and accretion of lease liabilities as a single lease cost on a straight-line basis over the lease term. No lease renewal options are recognized as part of the right-of-use assets and lease liabilities.

The Company's operating leases are presented in the balance sheet as operating lease right-of-use assets, classified as noncurrent assets, and operating lease liabilities, classified as current and noncurrent based on the discounted lease payments to be made within the proceeding twelve months.

As the Company’s leases do not provide an implicit rate, the Company uses its incremental borrowing rate to discount lease payments. The incremental borrowing rate represents an estimated rate of interest that the Company would have to pay to borrow equivalent funds on a collateralized basis at the lease commencement date.

Revenue Recognition

The Company determines revenue recognition for arrangements within the scope of Accounting Standards Update 2014-09, Revenue from Contracts with Customers (Topic 606) (“ASC 606”) by performing the following five steps: (i) identification of the promised goods or services in the contract; (ii) determination of whether the promised goods or services are performance obligations including whether they are distinct in the context of the contract; (iii) measurement of the transaction price, including the constraint on variable consideration; (iv) allocation of the transaction price to the performance obligations based on estimated selling prices; and (v) recognition of revenue when (or as) the Company satisfies each performance obligation.

The Company’s revenue is primarily derived through its license, research, development and commercialization agreements. The terms of these types of agreements may include (i) licenses to the Company’s technology, (ii) research and development services, and (iii) services or obligations in connection with participation in research or steering committees. Payments to the Company under these arrangements typically include one or more of the following: nonrefundable upfront and license fees, research funding, milestone and other contingent payments to the Company for the achievement of defined collaboration objectives and certain preclinical, clinical, regulatory and sales-based events, as well as royalties on sales of any commercialized products. Arrangements that include upfront payments are recorded as deferred revenue upon receipt or when due and are recognized as revenue as performance conditions are met. The event-based milestone payments, royalties and cost reimbursements represent variable consideration, and the Company uses the most likely amount method to estimate this variable consideration. Royalty payments are recognized when earned or as the sales occur. The Company records cost reimbursements as accounts receivable when right to consideration is unconditional.

A performance obligation is a promise in a contract to transfer a distinct good or service to the customer and is the unit of account in ASC 606. The Company allocates the total transaction price to each performance obligation based on the estimated standalone selling price and recognizes revenue when, or as, the performance obligation is satisfied. The Company includes the unconstrained amount of estimated variable consideration in the transaction price. At the end of each reporting period, the Company re-evaluates the estimated variable consideration included in the transaction price and any related constraint, and if necessary, adjusts its estimate of the overall transaction price.

Significant management judgment is required to determine the level of effort required under an arrangement and the period over which the Company expects to complete its performance obligations under the arrangement. Changes in these estimates can have a material effect on revenue recognized.

Research and Development Expenses

Costs related to research, design and development of programs are charged to research and development expense as incurred. Research and development costs include, but are not limited to, payroll and personnel expenses including stock-based compensation, materials, laboratory supplies, outside services and allocated overhead, including rent, insurance, repairs and maintenance, depreciation and utilities. The Company expenses all research and development costs in the period in which they are incurred.

Costs incurred in obtaining technology licenses are charged to research and development expense as acquired in-process research and development if the technology licensed has not reached technological feasibility and has no alternative future use.

Accrued Research and Development

The Company has entered into various agreements with CROs and CMOs. The Company’s research and development accruals are estimated based on the level of services performed, progress of the studies, including the phase or completion of events, and contracted costs. The estimated costs of research and development provided, but not yet invoiced, are included in accrued and other current liabilities on the balance sheet. If the actual timing of the performance of services or the level of effort varies from the original estimates, the Company will adjust the accrual accordingly. Payments made to CROs or CMOs under these arrangements in advance of the performance of the related services are recorded as prepaid expenses and other current assets until the services are rendered.

Stock-Based Compensation

The Company accounts for stock-based compensation for stock options granted to employees, directors and nonemployees as measured at grant date, based on the fair value of the award. The Company measures the fair value of awards granted using the Black-Scholes option pricing model and recognizes the expense in the statements of operations over the requisite service period, generally four years, using the straight-line method. Forfeitures are accounted for as they occur. The Company’s policy for issuing stock upon stock option exercise is to issue new common stock.

Income Taxes

The Company accounts for income taxes under the asset and liability method, which requires, among other things, that deferred income taxes be provided for temporary differences between the tax basis of the Company’s assets and liabilities and their financial statement reported amounts. In addition, deferred tax assets are recorded for the future benefit of utilizing net operating losses and research and development credit carryforwards and are measured using the enacted tax rates and laws that will be in effect when such items are expected to reverse. A valuation allowance is provided against deferred tax assets unless it is more likely than not that they will be realized.

The Company accounts for uncertain tax positions by assessing all material positions taken in any assessment or challenge by relevant taxing authorities. Assessing an uncertain tax position begins with the initial determination of the position’s sustainability and is measured at the largest amount of benefit that is greater than fifty percent likely of being realized upon ultimate settlement. The Company’s policy is to recognize interest and penalties related to the underpayment of income taxes as a component of income tax expense or benefit. To date, there have been no interest or penalties charged in relation to the unrecognized tax benefits.

Embedded Derivative

Embedded derivatives that are required to be bifurcated from the underlying host instrument are accounted for and valued as a separate financial instrument. An embedded derivative exists in the award agreement with the Cystic Fibrosis Foundation (“CFF”). As described in Note 15, the embedded derivative has been bifurcated and is classified as a liability on the balance sheet and separately accounted for at its fair value. The derivative liability is subject to remeasurement to fair value each reporting period. Changes in the fair value of the derivative liability are recognized as a component of other income (expense), net within the statements of operations.

Deferred Offering Costs

The Company capitalizes certain legal, accounting and other third-party fees that are directly related to the Company’s in-process financings, until such financings are consummated. After consummation of the financing, these costs are recorded as a reduction of the proceeds received as a result of the offering. In the event that a planned offering does not occur or is significantly delayed, all related deferred offering costs will be expensed immediately within the Company’s statements of operations.

Net Loss Per Share, Basic and Diluted

Basic net loss per share is calculated by dividing the net loss by the weighted-average number of shares of common stock outstanding during the period. Diluted net loss per share is computed by giving effect to all potentially dilutive common shares outstanding for the period. For purposes of this calculation, stock options to acquire shares of common stock, common stock warrants, and common stock expected to be issued under the ESPP, are considered potentially dilutive common shares, but have been excluded from the calculation of diluted net loss per share as their effect is antidilutive.

Recently Adopted Accounting Pronouncements

In June 2016, the FASB issued ASU No. 2016-13, Financial Instruments-Credit Losses (Topic 326), Measurement of Credit Losses on Financial Instruments, as clarified in subsequent amendments. ASU 2016-13 changes the impairment model for certain financial instruments. The new model is a forward-looking expected loss model and will apply to financial assets subject to credit losses and measured at amortized cost and certain off-balance sheet credit exposures. This includes loans, held-to-maturity debt securities, loan commitments, financial guarantees and net investments in leases, as well as trade receivables. For available-for-sale debt securities with unrealized losses, credit losses will be measured in a manner similar to today, except that the losses will be recognized as allowances rather than reductions in the amortized cost of the securities. In October 2019, the FASB voted to delay the effective date of this standard. Topic 326 will be effective for the Company for fiscal years beginning after December 15, 2022, including interim periods within those fiscal years. Early adoption is permitted. The Company adopted ASU 2016-13 on January 1, 2023 and the adoption did not have a material impact on the Company’s consolidated financial statements and related disclosures.

Recently Issued Accounting Pronouncements Not Yet Adopted

On December 14, 2023, the FASB issued ASU No. 2023-09, Improvements to Income Tax Disclosures. The final guidance adds clarifications related to the presentation of rate reconciliation for public business entities and definitions of specific categories in rate reconciliation. These amendments are effective for public business entities for annual periods beginning after December 15, 2024. For other entities, amendments are effective for annual periods beginning after December 15, 2025. The Company is not early adopting ASU 2023-09.

401(K) Plan

12 Months Ended

Dec. 31, 2023

Retirement Benefits [Abstract]

401(K) Plan

17. 401(k) Plan

In 2014, the Company adopted a 401(k) plan for all employees who have met certain eligibility requirements. The 401(k) plan allows employees to make pre-tax and post-tax contributions up to the maximum allowable amount set by the Internal Revenue Service. The Company made contributions to the Plan for eligible participants, and recorded contribution expenses of $1.1 million and $0.9 million for the years ended December 31, 2023 and 2022, respectively.

Income Taxes - Reconciliation of Beginning and Ending Unrecognized Tax Benefits (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Income Tax Disclosure [Abstract]
Balance at beginning of year	$ 8,273	$ 5,586
Additions for current year tax positions	3,629	2,687
Additions for tax positions of prior years	0	0
Balance at end of year	$ 11,902	$ 8,273

Commitments and Contingencies - Additional Information (Details) - USD ($) $ in Thousands	1 Months Ended					12 Months Ended
	Feb. 28, 2021	Dec. 31, 2019	Nov. 30, 2018	Oct. 31, 2018	May 31, 2015	Dec. 31, 2023	Dec. 31, 2022	Jan. 31, 2016
Commitment And Contingencies [Line Items]
Operating lease right-of-use ("ROU") assets						$ 11,539	$ 13,085
Operating lease liability						14,671
Operating lease cost						2,793	$ 2,787
Cash paid for amounts included in the measurement of lease liabilities						3,100
5980 Horton Street Building Lease
Commitment And Contingencies [Line Items]
Lease extended term date					Mar. 31, 2023
Lease extended term period				2026-09
Tenant improvement allowance			$ 200
Operating lease right-of-use ("ROU") assets						1,200
Operating lease liability						$ 1,400
Operating lease, discount rate						7.50%
Lease renewal term					3 years			5 years
5858 Horton Street Building Lease and Expansion
Commitment And Contingencies [Line Items]
Tenant improvement allowance	$ 2,300	$ 400				$ 2,300
Operating lease right-of-use ("ROU") assets						10,300
Operating lease liability						$ 13,200
Operating lease, discount rate						7.80%
Lease initial term				87 months
Lease option to renew term description						The new lease has an initial term of 87 months beginning on the rent commencement date with the option to renew the lease for one additional term of five years at market rent.
Lease existence of option to renew term				true
Lease renewal term				5 years

Fair Value Measurements - Additional Information (Details) - USD ($)	Dec. 31, 2023	Dec. 31, 2022
Fair Value Disclosures [Abstract]
Assets, Level 1 and Level 2 transfers	$ 0	$ 0
Liabilities, Level 1 and Level 2 transfers	$ 0	$ 0

Summary of Significant Accounting Policies (Policies)

12 Months Ended

Dec. 31, 2023

Accounting Policies [Abstract]

Basis of Presentation

The accompanying financial statements have been prepared in accordance with United States Generally Accepted Accounting Principles (“U.S. GAAP”).

Use of Estimates and Judgements

Segment Information

As of and for the years ended December 31, 2023 and 2022, all of the Company’s long-lived assets were located in the United States and all revenue was earned in the United States.

Concentration of Credit Risk

The Company’s partners in collaboration and license agreements who represent 10% or more of the Company’s total revenue are as follows:


	Year Ended December 31,
	2023	2022
Customer B	*	99%
Customer C	97%	0%
Total	97%	99%

* Less than 10%

The Company did not have accounts receivable from its partners in collaboration and license agreements as of December 31, 2023 and 2022.

The Company’s total revenues by geographic region, based on the location of the customer, are as follows (in thousands):


	Year Ended December 31,
	2023		2022
Netherlands	$	567	$	3,094
United States		20,156		35
Total revenue	$	20,723	$	3,129

Cash and Cash Equivalents

The Company considers all highly liquid investments with original maturities of three months or less at the date of purchase to be cash equivalents. Cash equivalents consist of money market funds.

Marketable Securities

Acquisitions

Other Risks and Uncertainties

Fair Value Measurements

As a basis for considering such assumptions, a three-level fair value hierarchy has been established, which prioritizes the inputs used in measuring fair value as follows:

•

Level 1 — Observable inputs that reflect unadjusted quoted market prices in active markets for identical assets or liabilities that are accessible at the measurement date.

•

Level 3 — Unobservable inputs that are supported by little or no market activity and that are significant to the fair value of the assets or liabilities.

The Company accounts for transfers of financial instruments between levels of the fair value hierarchy on the date of the event or change in circumstance that caused the transfer.

Accounts Receivable—Allowance for Doubtful Accounts

Property and Equipment, Net

Impairment of Long-Lived Assets

Common Stock Warrants

Leases

Revenue Recognition

Research and Development Expenses

Accrued Research and Development

Stock-Based Compensation

Income Taxes

Embedded Derivative

Deferred Offering Costs

Net Loss Per Share, Basic and Diluted

Recently Adopted and Not Yet Adopted Issued Accounting Pronouncements

Recently Adopted Accounting Pronouncements

Recently Issued Accounting Pronouncements Not Yet Adopted

Income Taxes

12 Months Ended

Dec. 31, 2023

Income Tax Disclosure [Abstract]

Income Taxes

10. Income Taxes

The Company did not record any income tax expense during the years ended December 31, 2023 and 2022. The Company has a net operating loss and has provided a valuation allowance against net deferred tax assets due to uncertainties regarding the Company’s ability to realize these assets. All losses before income taxes arose in the United States.

The effective tax rate of the Company’s income tax expense (benefit) differs from the federal statutory rate as follows:


	Year ended December 31,
	2023			2022 (1)
Federal statutory income tax rate		21.0	%		21.0	%
State tax rate		15.8	%		8.3	%
Research tax credit		2.4	%		1.3	%
Permanent differences		(0.1	)%		0.0	%
Stock-based compensation		(1.7	)%		(1.0	)%
Officer's compensation		(1.4	)%		(0.8	)%
Valuation allowance		(36.0	)%		(28.8	)%
Provision for income taxes		0.0	%		0.0	%

________________________________________

(1) The 2022 valuation allowance, research tax credit, state tax, stock based compensation, and officer’s compensation lines have been updated to conform to the 2023 presentation.

The tax effects of temporary differences that give rise to significant components of the deferred taxes are as follows (in thousands):


	December 31,
	2023			2022
Deferred Tax Assets
Net operating loss carryforwards	$	59,547		$	47,050
Other accrued liabilities		2,497			1,921
Deferred revenue		348			539
Research tax credits		13,961			10,546
Stock-based compensation expense		5,340			4,825
Intangible asset basis		1,014			1,092
Operating lease liabilities		4,106			4,437
Capitalized research expenditures		35,784			16,172
Total deferred tax assets	$	122,597		$	86,582

Deferred Tax Liabilities
Operating lease right-of-use assets	$	(3,229	)	$	(3,600	)
Prepaid expenses		(1,843	)		(1,556	)
Total deferred tax liabilities	$	(5,072	)	$	(5,156	)
Less: valuation allowance		(117,525	)		(81,426	)
Total net deferred tax	$	—		$	—

ASC 740, Income Taxes, requires that the tax benefit of net operating losses, temporary differences and credit carryforwards be recorded as an asset to the extent that management assesses that realization is “more likely than not.” Realization of the future tax benefits is dependent on the Company’s ability to generate sufficient taxable income within the carryforward period. Because of the Company’s recent history of operating losses, management believes that recognition of the deferred tax assets arising from future tax benefits is currently not likely to be realized and, accordingly, has provided a full valuation allowance. The valuation allowance increased by $36.1 million during the year ended December 31, 2023 and $31.2 million during the year ended December 31, 2022. The increase in the valuation allowance for each of the years ended December 31, 2023 and 2022 was primarily driven by net losses incurred, capitalized research expenditures, stock-based compensation expense and tax credits generated within the U.S.

The Company had net operating loss (“NOL”) carryforwards of $228.2 million and $212.9 million as of December 31, 2023 and 2022, respectively, available to reduce future taxable income, if any, for federal income tax purposes. $9.5 million of the federal NOL carryforwards expire in 2037 and the remaining $218.7 million carryforward indefinitely.

As of December 31, 2023 and 2022, the Company had federal research and development credit carryforwards of $17.1 million and $11.9 million, respectively, and state research and development credit carryforwards of $10.0 million and $7.9 million, respectively, available to reduce future taxable income, if

any, for federal and California state income tax purposes, respectively. The federal credit carryforwards begin expiring in 2035 and the state credits carryforward indefinitely.

Utilization of the NOL carryforwards and research credit carryforwards may be subject to an annual limitation due to the ownership percentage change limitations provided by the Internal Revenue Code and similar state provisions. Annual limitations may result in the expiration of the NOL and tax credit carryforwards before they are utilized. The Company has experienced ownership changes in the past as a result of its Series B redeemable convertible preferred stock financing. As a result of the ownership changes, some of the tax attribute carryforwards may be permanently limited as they will expire unused and such amounts are excluded from our NOLs as of December 31, 2023. Subsequent ownership changes may result in additional limitations.

The reconciliation of the beginning and ending unrecognized tax benefits amounts is as follows (in thousands):


	Unrecognized Income Tax Benefits
Balance as of December 31, 2021	$	5,586
Additions for current year tax positions		2,687
Additions for tax positions of prior years		—
Balance as of December 31, 2022		8,273
Additions for current year tax positions		3,629
Additions for tax positions of prior years		—
Balance as of December 31, 2023	$	11,902

The entire amount of the unrecognized tax benefits would not impact the Company’s effective tax rate if recognized. During the years ended December 31, 2023 and 2022, the Company did not recognize accrued interest and penalties related to unrecognized tax benefits. The Company does not anticipate that the amount of existing unrecognized tax benefits will materially increase or decrease during the next 12 months.

The Company files income tax returns in the U.S. federal and California tax jurisdictions. In general, the Company is no longer subject to tax examination by the Internal Revenue Service or state taxing authorities for years before 2017. Although the federal and state statutes are closed for purposes of assessing additional income tax in those prior years, the taxing authorities may still make adjustments to the NOL and credit carryforwards used in open years. Therefore, the tax statutes should be considered open as it relates to the NOL and credit carryforwards used in open years. The Company has no ongoing income tax examinations by tax authorities at this time.

In accordance with the 2017 Tax Act, research and experimental ("R&E") expenses under Internal Revenue Code Section 174 are required to be capitalized beginning in 2022. R&E expenses are required to be amortized over a period of five years for domestic expenses and 15 years for foreign expenses. The Company recognized a deferred tax asset of $35.8 million for capitalized R&E expenses as of December 31, 2023.

The Inflation Reduction Act of 2022, which includes certain tax measures, was signed into law by President Joe Biden on August 16, 2022. The act contains two main tax provisions:

A new corporate alternative minimum tax imposed on certain corporations meeting average annual financial statement income of more than $1 billion for the three tax-year period, and

An excise tax imposed upon share repurchased by certain publicly traded corporations.

This provision is effective for tax years beginning after December 31, 2022. The Company does not believe the act will have a material impact.

- Definition

The entire disclosure for income taxes. Disclosures may include net deferred tax liability or asset recognized in an enterprise's statement of financial position, net change during the year in the total valuation allowance, approximate tax effect of each type of temporary difference and carryforward that gives rise to a significant portion of deferred tax liabilities and deferred tax assets, utilization of a tax carryback, and tax uncertainties information.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 946
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 13
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480990/946-20-50-13

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 235
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08(h)(2))
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480678/235-10-S99-1

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 740
-Name Accounting Standards Codification
-Publisher FASB
-URI https://asc.fasb.org//740/tableOfContent

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Topic 740
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 14
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482685/740-10-50-14

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Topic 740
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 21
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482685/740-10-50-21

Reference 6: http://www.xbrl.org/2003/role/disclosureRef
-Topic 740
-SubTopic 270
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482526/740-270-50-1

Reference 7: http://www.xbrl.org/2003/role/disclosureRef
-Topic 740
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 17
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482685/740-10-50-17

Reference 8: http://www.xbrl.org/2003/role/disclosureRef
-Topic 740
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SAB TOPIC 6.I.5.Q1)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479360/740-10-S99-1

Reference 9: http://www.xbrl.org/2003/role/disclosureRef
-Topic 740
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 2
-Subparagraph (SAB Topic 11.C)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479360/740-10-S99-2

Reference 10: http://www.xbrl.org/2003/role/disclosureRef
-Topic 740
-SubTopic 30
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482603/740-30-50-2

+ Details

Fair Value Measurements - Summary of Changes in Fair Value of Level 3 Derivative Liability (Details) - Derivative Liability - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Beginning Balance	$ 212	$ 214
Change in fair value included in other income (expense), net	$ 157	$ (2)
Fair Value, Recurring Basis, Unobservable Input Reconciliation, Liability, Gain (Loss), Statement of Income [Extensible List]	Other expense, net	Other expense, net
Ending Balance	$ 369	$ 212

Marketable Securities - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Debt Securities, Available-for-Sale [Line Items]
Realized gains or losses on marketable securities	$ 0	$ 0
Maximum unrealized loss at individual instrument level	20,000
Other impairment charges related to marketable securities	0	0
Aggregate unrealized loss	35,000
Allowance for credit loss	$ 0	$ 0
Maximum
Debt Securities, Available-for-Sale [Line Items]
Maturity period of marketable securities	2 years	2 years

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

+ Details

Summary of Significant Accounting Policies (Tables)

12 Months Ended

Dec. 31, 2023

Accounting Policies [Abstract]

Schedule of Concentration of Risk

The Company’s partners in collaboration and license agreements who represent 10% or more of the Company’s total revenue are as follows:


	Year Ended December 31,
	2023	2022
Customer B	*	99%
Customer C	97%	0%
Total	97%	99%

* Less than 10%

The Company did not have accounts receivable from its partners in collaboration and license agreements as of December 31, 2023 and 2022.

Schedule of Revenues by Geographic Region

The Company’s total revenues by geographic region, based on the location of the customer, are as follows (in thousands):


	Year Ended December 31,
	2023		2022
Netherlands	$	567	$	3,094
United States		20,156		35
Total revenue	$	20,723	$	3,129

The Company - Additional Information (Details) - USD ($) $ / shares in Units, $ in Thousands		1 Months Ended
	Feb. 09, 2024	Feb. 29, 2024	May 31, 2023	Nov. 30, 2021	Dec. 31, 2020	Dec. 31, 2023	Dec. 31, 2022	Jan. 01, 2021
Organization And Nature Of Business [Line Items]
Outstanding shares of redeemable convertible preferred stock								0
Accumulated deficit						$ 415,327	$ 314,490
Initial Public Offering
Organization And Nature Of Business [Line Items]
Warrants to purchase shares of common stock					30,000
Common Stock \| Initial Public Offering
Organization And Nature Of Business [Line Items]
Number of shares sold and issued					9,660,000
Stock sold and issued price per share					$ 23
Aggregate net proceeds, after deducting underwriting discounts and commissions and offering costs					$ 204,700
Number shares issued upon conversion of redeemable convertible preferred shares					11,575,984
Common Stock \| Exercise of Overallotment Option
Organization And Nature Of Business [Line Items]
Number of shares sold and issued					1,260,000
Common Stock \| 2021 Follow On Public Offering
Organization And Nature Of Business [Line Items]
Number of shares priced under public offering				4,750,000
Stock sold and issued price per share				$ 25
Aggregate net proceeds, after deducting underwriting discounts and commissions and offering costs				$ 111,100
Common Stock \| 2023 Follow On Public Offering
Organization And Nature Of Business [Line Items]
Number of shares priced under public offering			8,625,000
Stock sold and issued price per share			$ 16
Aggregate net proceeds, after deducting underwriting discounts and commissions and offering costs			$ 129,200
Common Stock \| 2024 Follow On Public Offering \| Subsequent Event
Organization And Nature Of Business [Line Items]
Number of shares priced under public offering	6,586,015	6,586,015
Stock sold and issued price per share	$ 29.5	$ 29.5
Aggregate net proceeds, after deducting underwriting discounts and commissions and offering costs	$ 281,400	$ 281,400
Common Stock \| 2024 Follow On Public Offering \| Pre-funded Warrants [Member] \| Subsequent Event
Organization And Nature Of Business [Line Items]
Warrants to purchase shares of common stock	3,583,476	3,583,476
Common stock offering price	$ 29.4999	$ 29.4999
Common Stock \| Maximum \| Exercise of Overallotment Option \| Subsequent Event
Organization And Nature Of Business [Line Items]
Number of shares sold and issued	1,525,423	1,525,423

Fair Value Measurements (Tables)

12 Months Ended

Dec. 31, 2023

Fair Value Disclosures [Abstract]

Fair Value Hierarchy for Financial Assets and Financial Liabilities Measured at Fair Value on Recurring Basis


	Basis for Fair Value Measurements						Fair Value as of
	Level 1		Level 2		Level 3		December 31, 2023
Assets
Money market funds	$	217,644	$	—	$	—	$	217,644
Commercial paper		—		18,243		—		18,243
Corporate bonds		—		28,939		—		28,939
U.S. government sponsored agencies		—		2,896				2,896
Total	$	217,644	$	50,078	$	—	$	267,722
Liabilities
Derivative liability	$	—	$	—	$	369	$	369
Total	$	—	$	—	$	369	$	369


	Basis for Fair Value Measurements						Fair Value as of
	Level 1		Level 2		Level 3		December 31, 2022
Assets
Money market funds	$	44,447	$	—	$	—	$	44,447
Commercial paper		—		48,872		—		48,872
Corporate bonds		—		47,012		—		47,012
U.S. treasuries		—		63,059		—		63,059
U.S. government sponsored agencies		—		7,168		—		7,168
Total	$	44,447	$	166,111	$	—	$	210,558
Liabilities
Derivative liability	$	—	$	—	$	212	$	212
Total	$	—	$	—	$	212	$	212

Summary of Changes in Fair Value of Level 3 Derivative Liability

The following table sets forth a summary of the changes in the fair value of the Company’s Level 3 derivative liability (in thousands):


Balance as of December 31, 2021	$	214
Change in fair value included in other income (expense), net		(2	)
Balance as of December 31, 2022		212
Change in fair value included in other income (expense), net		157
Balance as of December 31, 2023	$	369

Marketable Securities - Summary of Available for Sale Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Short-term Marketable Securities
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost Basis	$ 39,144	$ 162,240
Gross Unrealized Gains	13	13
Gross Unrealized Losses	(33)	(1,050)
Fair Value	39,124	161,203
Short-term Marketable Securities \| Commercial Paper
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost Basis	17,603	43,988
Gross Unrealized Gains	1	0
Gross Unrealized Losses	(26)	(24)
Fair Value	17,578	43,964
Short-term Marketable Securities \| Corporate Bonds
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost Basis	18,641	47,565
Gross Unrealized Gains	12	0
Gross Unrealized Losses	(3)	(553)
Fair Value	18,650	47,012
Short-term Marketable Securities \| U.S. Treasuries
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost Basis		63,502
Gross Unrealized Gains		1
Gross Unrealized Losses		(444)
Fair Value		63,059
Short-term Marketable Securities \| U.S. Government Sponsored Agencies
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost Basis	2,900	7,185
Gross Unrealized Gains	0	12
Gross Unrealized Losses	(4)	(29)
Fair Value	2,896	7,168
Long-term Marketable Securities
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost Basis	10,918	5,067
Gross Unrealized Gains	36	0
Gross Unrealized Losses	0	(159)
Fair Value	10,954	4,908
Long-term Marketable Securities \| Commercial Paper
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost Basis	661	5,067
Gross Unrealized Gains	4	0
Gross Unrealized Losses	0	(159)
Fair Value	665	$ 4,908
Long-term Marketable Securities \| Corporate Bonds
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost Basis	10,257
Gross Unrealized Gains	32
Gross Unrealized Losses	0
Fair Value	$ 10,289

Net Loss Per Share, Basic and Diluted (Tables)

12 Months Ended

Dec. 31, 2023

Earnings Per Share [Abstract]

Computation of Basic and Diluted Net Loss Per Share

The following table sets forth the computation of basic and diluted net loss per share (in thousands, except share and per share data):


	Year Ended December 31,
	2023			2022
Numerator
Net loss attributable to common stockholders	$	(100,837	)	$	(107,494	)
Denominator
Weighted-average shares outstanding used in computing net loss per share, basic and diluted		39,130,067			32,351,221
Net loss per share, basic and diluted	$	(2.58	)	$	(3.32	)

Outstanding Shares of Potentially Dilutive Securities Excluded from Computation of Diluted Net Loss Per Share


	December 31,
	2023		2022
Options issued and outstanding		8,423,481		5,778,929
2020 ESPP		278,502		393,289
Common stock warrants		53,669		53,669
Total		8,755,652		6,225,887

Commitments and Contingencies - Summary of Supplemental Information Related to Leases (Details)	Dec. 31, 2023	Dec. 31, 2022
Commitments and Contingencies Disclosure [Abstract]
Weighted-average remaining lease term (in years)	5 years 8 months 12 days	6 years 8 months 12 days
Weighted-average discount rate	7.80%	7.80%

Stock-based Compensation - Summary of Stock Options Activity (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Number of Shares Underlying Outstanding Options, Beginning Balance	5,778,929
Number of Shares Underlying Outstanding Options, granted	3,824,931
Number of Shares Underlying Outstanding Options, exercised	(574,740)
Number of Shares Underlying Outstanding Options, expired	(129,147)
Number of Shares Underlying Outstanding Options, forfeited	(659,819)
Number of Shares Underlying Outstanding Options, Ending Balance	8,240,154	5,778,929
Number of Shares Underlying Outstanding Options, Shares exercisable, December 31, 2023	3,130,485
Number of Shares Underlying Outstanding Options, Shares vested and expected to vest, December 31, 2023	8,240,154
Weighted-Average Exercise Price, Beginning Balance	$ 16.21
Weighted-Average Exercise Price, Options granted	16.04
Weighted-Average Exercise Price, Options exercised	12.72
Weighted-Average Exercise Price, Options expired	27.58
Weighted-Average Exercise Price, Options forfeited	16.06
Weighted-Average Exercise Price, Ending Balance	16.21	$ 16.21
Weighted-Average Exercise Price, Shares exercisable, December 31, 2023	17.07
Weighted-Average Exercise Price, Shares vested and expected to vest, December 31, 2023	$ 16.21
Weighted-Average Remaining Contractual Term (in years), Outstanding	7 years 11 months 1 day	7 years 11 months 19 days
Weighted-Average Remaining Contractual Term (in years), Shares exercisable, December 31, 2023	6 years 3 months 3 days
Weighted-Average Remaining Contractual Term (in years), Shares vested and expected to vest, December 31, 2023	7 years 10 months 24 days
Aggregate Intrinsic Value, Outstanding Balance	$ 46,863	$ 48,050
Aggregate Intrinsic Value, Shares exercisable, December 31, 2023	19,188
Aggregate Intrinsic Value, Shares vested and expected to vest, December 31, 2023	$ 46,863