0000950170-23-062103 | 10-Q | XBRL Viewer | 4D Molecular Therapeutics INC

Summary of Significant Accounting Policies

9 Months Ended

Sep. 30, 2023

Summary of Significant Accounting Policies

2. Summary of Significant Accounting Policies

Basis of Presentation

The accompanying unaudited condensed financial statements have been prepared in accordance with United States Generally Accepted Accounting Principles (“U.S. GAAP”) and the rules and regulations of the United States Securities and Exchange Commission (“SEC”) for interim reporting.

Certain information and note disclosures normally included in annual financial statements prepared in accordance with U.S. GAAP have been condensed or omitted. Accordingly, the unaudited condensed financial statements should be read in conjunction with the audited financial statements and the related notes thereto included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2022 filed with the SEC.

The accompanying financial information for the three and nine months ended September 30, 2023 and 2022 is unaudited. The unaudited condensed financial statements have been prepared on the same basis as the annual audited financial statements and, in the opinion of management, reflect all adjustments, which include only normal recurring adjustments, necessary to present fairly the Company’s financial position as of September 30, 2023 and December 31, 2022 and its results of operations and cash flows for the nine months ended September 30, 2023 and 2022. The results for interim periods are not necessarily indicative of the results expected for the full fiscal year or any other periods.

Use of Estimates and Judgments

The preparation of condensed financial statements in conformity with U.S. GAAP requires management to make estimates and judgments 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 the derivative instrument 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 September 30, 2023. While there was not a material impact to the Company’s unaudited condensed financial statements as of September 30, 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 unaudited condensed 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.

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:


	Three Months Ended September 30,		Nine Months Ended September 30,
	2023	2022	2023	2022
Customer A	*	*	*	*
Customer B	*	99%	*	98%
Customer C	99%	*	96%	*
Total	99%	99%	96%	98%

* Less than 10%

The Company did not have accounts receivable from its partners in collaboration and license agreements as of September 30, 2023 and December 31, 2022.

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


	Three Months Ended September 30,				Nine Months Ended September 30,
	2023		2022		2023		2022
Netherlands	$	41	$	497	$	566	$	1,849
United States		20,163		3		20,176		33
Total revenue	$	20,204	$	500	$	20,742	$	1,882

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).

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 condensed consolidated financial statements and related disclosures.

Commitments and Contingencies- Summary of Components of Lease Expense - (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Commitments and Contingencies Disclosure [Abstract]
Operating lease cost	$ 698	$ 697	$ 2,095	$ 2,090
Variable lease cost	311	260	933	751
Lease, Cost, Total	$ 1,009	$ 957	$ 3,028	$ 2,841

Property and Equipment, Net - Schedule of Property and Equipment, Net (Details) - USD ($) $ in Thousands	Sep. 30, 2023	Dec. 31, 2022
Property Plant And Equipment [Line Items]
Total property and equipment	$ 31,569	$ 29,990
Less: Accumulated depreciation and amortization	(10,831)	(7,728)
Property and equipment, net	20,738	22,262
Machinery and Equipment
Property Plant And Equipment [Line Items]
Total property and equipment	11,995	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	240	239
Computer Equipment and Software
Property Plant And Equipment [Line Items]
Total property and equipment	836	616
Construction in Progress
Property Plant And Equipment [Line Items]
Total property and equipment	$ 1,030	$ 476

Summary of Significant Accounting Policies - Additional Information (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2023	Dec. 31, 2022
Summary Of Significant Accounting Policies [Line Items]
Operating lease, right-of-use assets	$ 11,950	$ 13,085
Operating lease liabilities, current portion	3,126	2,655
Operating lease liabilities, long-term portion	$ 12,031	$ 13,469
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
Accounts Receivable \| Credit Concentration Risk \| Customer
Summary Of Significant Accounting Policies [Line Items]
Concentration risk, percentage	0.00%	0.00%

Marketable Securities

9 Months Ended

Sep. 30, 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 September 30, 2023 and December 31, 2022 (in thousands):


	Amortized Cost Basis		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value as of September 30, 2023
Short-term marketable securities:
Commercial paper	$	24,452	$	312	$	(77	)	$	24,687
Corporate bonds		6,302		—		(23	)		6,279
U.S. treasuries		4,827		142		—			4,969
U.S. government sponsored agencies		7,871		195		(15	)		8,051
Total short-term marketable securities	$	43,452	$	649	$	(115	)	$	43,986


	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 September 30, 2023 had contractual maturities of less than one year. 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 September 30, 2023 and December 31, 2022 (in thousands):


	September 30,		December 31,
	2023		2022
Aggregate fair value of marketable securities in a continuous loss position for less than twelve months	$	12,783	$	97,472
Aggregate fair value of marketable securities in a continuous loss position for more than twelve months		11,232		53,716
Aggregate fair value of marketable securities in unrealized gain position		19,971		14,923
Total marketable securities	$	43,986	$	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, $0.1 million, as well as in aggregate, $0.1 million, as of September 30, 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 September 30, 2023.

License Arrangements - Additional Information (Details) - USD ($)			1 Months Ended			3 Months Ended	9 Months Ended
	Jul. 05, 2023	Apr. 21, 2023	Jul. 31, 2021	Jan. 31, 2019	Jan. 31, 2016	Sep. 30, 2023	Sep. 30, 2023	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
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	$ 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	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	$ 0
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

The Company

9 Months Ended

Sep. 30, 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 estimated 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.

Liquidity

The Company has incurred significant losses and negative cash flows from operations and had an accumulated deficit of $383.0 million as of September 30, 2023. The Company believes that its cash and cash equivalents and marketable securities as of September 30, 2023 are sufficient for the Company to fund planned operations for at least one year from the issuance date of these unaudited condensed financial statements. 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.

Summary of Significant Accounting Policies - Schedule of Concentration of Risk (Details) (Parenthetical) - Revenue - Customer Concentration Risk	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Customer B
Concentration Risk [Line Items]
Concentration risk, percentage		99.00%		98.00%
Maximum \| Customer A
Concentration Risk [Line Items]
Concentration risk, percentage	10.00%	10.00%	10.00%	10.00%
Maximum \| Customer B
Concentration Risk [Line Items]
Concentration risk, percentage	10.00%		10.00%

Condensed Statements of Comprehensive Loss (Unaudited) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Statement of Comprehensive Income [Abstract]
Net loss	$ (10,256)	$ (25,691)	$ (68,554)	$ (80,115)
Other comprehensive loss:
Net unrealized gain (loss) on marketable securities	189	(24)	1,730	(1,428)
Total Comprehensive loss	$ (10,067)	$ (25,715)	$ (66,824)	$ (81,543)

- Definition

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

+ References

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+ Details

Stock-based Compensation - Schedule of Assumptions Used in Black-Scholes Valuation Model to Estimate Fair Value of Stock Options (Details)	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected volatility, minimum	80.60%	81.30%	80.60%	80.00%
Expected volatility, maximum	82.20%	81.90%	84.40%	81.90%
Risk-free interest rate, minimum	4.10%	2.90%	3.50%	1.50%
Risk-free interest rate, maximum	4.40%	3.90%	4.40%	3.90%
Expected dividend yield			0.00%
Minimum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected term	6 years	5 years 10 months 24 days	6 years	5 years 10 months 24 days
Maximum
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	6 years 1 month 6 days	6 years 1 month 6 days

- 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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Reference 4: http://www.xbrl.org/2003/role/disclosureRef
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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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Reference 12: http://www.xbrl.org/2003/role/disclosureRef
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Reference 13: http://www.xbrl.org/2003/role/disclosureRef
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+ Details

Research and Collaboration Arrangements - CFF - Additional Information (Details 1) - CFF	Sep. 30, 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

Fair Value Measurements

9 Months Ended

Sep. 30, 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 September 30, 2023 and December 31, 2022 (in thousands):


	Basis for Fair Value Measurements						Fair Value as of
	Level 1		Level 2		Level 3		September 30, 2023
Assets
Money market funds	$	250,041	$	—	$	—	$	250,041
Commercial paper		—		24,687		—		24,687
Corporate bonds		—		6,279		—		6,279
U.S. Treasuries		—		4,969		—		4,969
U.S. government sponsored agencies		—		8,051		—		8,051
Total	$	250,041	$	43,986	$	—	$	294,027
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 14 for further discussion on the embedded derivative.

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

The following tables set forth a summary of the changes in the fair value of the Company’s Level 3 financial instrument during the nine-month periods ended September 30, 2023 and 2022 (in thousands):


	Derivative Liability
Balance as of December 31, 2022	$	212
Change in fair value included in other income (expense), net		157
Balance as of September 30, 2023	$	369


	Derivative Liability
Balance as of December 31, 2021	$	214
Change in fair value included in other income (expense), net		(1	)
Balance as of September 30, 2022	$	213

- 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

9 Months Ended

Sep. 30, 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):


	September 30, 2023			December 31, 2022
Machinery and equipment	$	11,995		$	11,191
Leasehold improvements		16,902			16,902
Furniture and fixtures		566			566
Office equipment		240			239
Computer equipment and software		836			616
Construction in progress		1,030			476
Total property and equipment		31,569			29,990
Less: Accumulated depreciation and amortization		(10,831	)		(7,728	)
Property and equipment, net	$	20,738		$	22,262

All property and equipment are maintained in the United States. Depreciation expense was $1.1 million and $0.5 million for the three months ended September 30, 2023 and 2022, respectively, and $3.1 million and $1.4 million for the nine months ended September 30, 2023 and 2022, respectively.

Net Loss Per Share, Basic and Diluted - Computation of Basic and Diluted Net Loss Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Numerator
Net loss	$ (10,256)	$ (25,691)	$ (68,554)	$ (80,115)
Denominator
Weighted-average shares outstanding used in computing net loss per share, basic	42,256,629	32,385,791	37,884,363	32,305,074
Weighted-average shares outstanding used in computing net loss per share, diluted	42,256,629	32,385,791	37,884,363	32,305,074
Net loss per share, basic	$ (0.24)	$ (0.79)	$ (1.81)	$ (2.48)
Net loss per share, diluted	$ (0.24)	$ (0.79)	$ (1.81)	$ (2.48)

- Definition

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

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 250
-SubTopic 10
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-Section 50
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-URI https://asc.fasb.org//1943274/2147482689/260-10-45-7

+ Details

- Definition

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.

+ References

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- Definition

Amount, after deduction of tax, noncontrolling interests, dividends on preferred stock and participating securities; of income (loss) available to common shareholders.

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+ Details

Condensed Balance Sheets (Unaudited) (Parenthetical) - $ / shares	Sep. 30, 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	42,752,452	32,626,627
Common stock, shares, outstanding	42,752,452	32,626,627

Accrued and Other Current Liabilities (Tables)

9 Months Ended

Sep. 30, 2023

Payables and Accruals [Abstract]

Summary of Accrued and Other Current Liabilities

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


	September 30, 2023		December 31, 2022
Payroll and related expenses	$	6,223	$	5,689
Accrued clinical and preclinical study costs		1,776		1,355
Consulting and professional		2,032		1,444
Other accrued expenses		163		382
Total accrued and other current liabilities	$	10,194	$	8,870

Common Stock (Tables)

9 Months Ended

Sep. 30, 2023

Equity [Abstract]

Schedule of Common Shares Reserved for Future Issuance

As of September 30, 2023 and December 31, 2022, the Company has reserved common stock, on an as-converted basis, for future issuance as follows:


	September 30, 2023		December 31, 2022
Issuance of common stock under the 2020 Equity Incentive Award Plan		3,181,340		2,112,039
Issuance of common stock under the 2020 Employee Stock Purchase Plan		380,636		413,273
Exercise of options issued and outstanding		6,021,383		5,778,929
Exercise of common stock warrants		53,669		53,669
Total common stock reserved for future issuance		9,637,028		8,357,910

Net Loss Per Share, Basic and Diluted - Outstanding Potentially Dilutive Securities Excluded From Computation of Diluted Net Loss Per Share (Details) - shares	9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive securities	6,344,154	6,089,125
Options Issued and Outstanding
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive securities	6,021,383	6,035,456
ESPP Shares Expected to be Issued
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive securities	269,102	0
Common Stock Warrants
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive securities	53,669	53,669

Condensed Statements of Stockholders' Equity (Unaudited) - 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 Gain (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	330						330
Issuance of common stock upon exercise of stock options (in shares)				39,541
Stock-based compensation expense	3,933						3,933
Vesting of common stock warrants issued for services	22						22
Net unrealized gain (loss) on marketable securities	(998)									(998)
Net loss	(26,338)										(26,338)
Stockholders' Equity, ending balances at Mar. 31, 2022	296,056			$ 3			530,808			(1,421)	(233,334)
Stockholders' Equity, ending balances (in shares) at Mar. 31, 2022				32,264,065
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
Net unrealized gain (loss) on marketable securities	(1,428)
Net loss	(80,115)
Stockholders' Equity, ending balances at Sep. 30, 2022	251,444			$ 3			540,403			(1,851)	(287,111)
Stockholders' Equity, ending balances (in shares) at Sep. 30, 2022				32,392,378
Stockholders' Equity, beginning balances at Mar. 31, 2022	296,056			$ 3			530,808			(1,421)	(233,334)
Stockholders' Equity, beginning balances (in shares) at Mar. 31, 2022				32,264,065
Issuance of common stock upon exercise of stock options	138						138
Issuance of common stock upon exercise of stock options (in shares)				51,261
Issuance of common stock - 2020 ESPP	471						471
Issuance of common stock - 2020 ESPP (in shares)				61,552
Stock-based compensation expense	4,322						4,322
Vesting of common stock warrants issued for services	22						22
Net unrealized gain (loss) on marketable securities	(406)									(406)
Net loss	(28,086)										(28,086)
Stockholders' Equity, ending balances at Jun. 30, 2022	272,517			$ 3			535,761			(1,827)	(261,420)
Stockholders' Equity, ending balances (in shares) at Jun. 30, 2022				32,376,878
Issuance of common stock upon exercise of stock options	100						100
Issuance of common stock upon exercise of stock options (in shares)				15,500
Stock-based compensation expense	4,520						4,520
Vesting of common stock warrants issued for services	22						22
Net unrealized gain (loss) on marketable securities	(24)									(24)
Net loss	(25,691)										(25,691)
Stockholders' Equity, ending balances at Sep. 30, 2022	251,444			$ 3			540,403			(1,851)	(287,111)
Stockholders' Equity, ending balances (in shares) at Sep. 30, 2022				32,392,378
Stockholders' Equity, beginning balances at Dec. 31, 2022	231,337			$ 3			547,020			(1,196)	(314,490)
Stockholders' Equity, beginning balances (in shares) at Dec. 31, 2022				32,626,627
Issuance of common stock upon exercise of stock options	1,135						1,135
Issuance of common stock upon exercise of stock options (in shares)				122,207
Issuance of common stock for the offering program, net of issuance costs		$ 9,647						$ 9,647
Issuance of common stock for the offering program, net of issuance costs (in shares)					487,934
Stock-based compensation expense	4,221						4,221
Vesting of common stock warrants issued for services	22						22
Net unrealized gain (loss) on marketable securities	926									926
Net loss	(28,682)										(28,682)
Stockholders' Equity, ending balances at Mar. 31, 2023	218,606			$ 3			562,045			(270)	(343,172)
Stockholders' Equity, ending balances (in shares) at Mar. 31, 2023				33,236,768
Stockholders' Equity, beginning balances at Dec. 31, 2022	$ 231,337			$ 3			547,020			(1,196)	(314,490)
Stockholders' Equity, beginning balances (in shares) at Dec. 31, 2022				32,626,627
Issuance of common stock upon exercise of stock options (in shares)	319,576
Net unrealized gain (loss) on marketable securities	$ 1,730
Net loss	(68,554)
Stockholders' Equity, ending balances at Sep. 30, 2023	329,934			$ 4			712,440			534	(383,044)
Stockholders' Equity, ending balances (in shares) at Sep. 30, 2023				42,752,452
Stockholders' Equity, beginning balances at Mar. 31, 2023	218,606			$ 3			562,045			(270)	(343,172)
Stockholders' Equity, beginning balances (in shares) at Mar. 31, 2023				33,236,768
Issuance of common stock upon exercise of stock options	894						894
Issuance of common stock upon exercise of stock options (in shares)				72,217
Issuance of common stock for the offering program, net of issuance costs			$ 129,208			$ 1			$ 129,207
Issuance of common stock for the offering program, net of issuance costs (in shares)						8,625,000
Issuance of common stock - 2020 ESPP	658						658
Issuance of common stock - 2020 ESPP (in shares)				82,637
ATM offering costs		(212)						(212)
Stock-based compensation expense	4,969						4,969
Vesting of common stock warrants issued for services	22						22
Net unrealized gain (loss) on marketable securities	615									615
Net loss	(29,616)										(29,616)
Stockholders' Equity, ending balances at Jun. 30, 2023	325,144			$ 4			697,583			345	(372,788)
Stockholders' Equity, ending balances (in shares) at Jun. 30, 2023				42,016,622
Issuance of common stock upon exercise of stock options	1,040						1,040
Issuance of common stock upon exercise of stock options (in shares)				125,152
Issuance of common stock for the offering program, net of issuance costs		9,701						9,701
Issuance of common stock for the offering program, net of issuance costs (in shares)					610,678
ATM offering costs		$ (468)						$ (468)
Stock-based compensation expense	4,562						4,562
Vesting of common stock warrants issued for services	22						22
Net unrealized gain (loss) on marketable securities	189									189
Net loss	(10,256)										(10,256)
Stockholders' Equity, ending balances at Sep. 30, 2023	$ 329,934			$ 4			$ 712,440			$ 534	$ (383,044)
Stockholders' Equity, ending balances (in shares) at Sep. 30, 2023				42,752,452

License Arrangements

9 Months Ended

Sep. 30, 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 retinotropic 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).

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 September 30, 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.

The Company did not incur any expense under the provisions of the outstanding license agreements during the three and nine months ended September 30, 2023. During the three and nine months ended September 30, 2022, the Company incurred immaterial expenses under the provisions of the outstanding license agreements.

Research and Collaboration Arrangements (Tables)

9 Months Ended

Sep. 30, 2023

Research And Collaboration Arrangements [Abstract]

Summary of Collaboration and License Revenue

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


	Three Months Ended September 30,				Nine Months Ended September 30,
	2023		2022		2023		2022
uniQure	$	41	$	497	$	566	$	1,849
CFF		155		3		168		33
Astellas		20,008		—		20,008		—
Total revenue	$	20,204	$	500	$	20,742	$	1,882

Summary of Deferred Revenue

Deferred revenue is summarized as follows (in thousands):


	September 30, 2023		December 31, 2022
uniQure	$	—	$	566
CFF		1,226		1,394
Total deferred revenue	$	1,226	$	1,960

Derivative Liability - Additional Information (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 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%

Stock-based Compensation (Tables)

9 Months Ended

Sep. 30, 2023

Share-Based Payment Arrangement [Abstract]

Summary of Stock Options Activity

The following table summarizes the stock options activity for the nine months ended September 30, 2023:


	Number of Shares Available for Grant			Number of Shares Underlying Outstanding Options
Balances at December 31, 2022		2,112,039			5,778,929
Options authorized		1,631,331			—
Options granted		(1,226,579	)		1,226,579
Options exercised		—			(319,576	)
Options expired		60,746			(60,746	)
Options forfeited		603,803			(603,803	)
Balances at September 30, 2023		3,181,340			6,021,383
Options exercisable at September 30, 2023					3,146,872
Options vested and expected to vest at September 30, 2023					6,021,383

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

The Company estimates the fair value of 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 three and nine months ended September 30, 2023 and 2022:


	Three Months Ended September 30,		Nine Months Ended September 30,
	2023	2022	2023	2022
Expected term	6.0 - 6.1 years	5.9 - 6.1 years	6.0 - 6.1 years	5.9 - 6.1 years
Expected volatility	80.6% - 82.2%	81.3% - 81.9%	80.6% - 84.4%	80.0% - 81.9%
Risk-free interest rate	4.1% - 4.4%	2.9% - 3.9%	3.5% - 4.4%	1.5% - 3.9%
Expected dividend yield	—%	—%	—%	—%

Summary of Stock-based Compensation Expense for Employees and Nonemployees by Function

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


	Three Months Ended September 30,				Nine Months Ended September 30,
	2023		2022		2023		2022
Research and development	$	2,314	$	2,438	$	7,463	$	6,931
General and administrative		2,248		2,082		6,289		5,844
Total stock-based compensation expense	$	4,562	$	4,520	$	13,752	$	12,775

Document and Entity Information - shares	9 Months Ended
Document and Entity Information - shares	Sep. 30, 2023	Nov. 02, 2023
Cover [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Sep. 30, 2023
Document Fiscal Year Focus	2023
Document Fiscal Period Focus	Q3
Document Quarterly Report	true
Document Transition Report	false
Entity Registrant Name	4D Molecular Therapeutics, Inc.
Entity Central Index Key	0001650648
Entity Current Reporting Status	Yes
Current Fiscal Year End Date	--12-31
Entity Filer Category	Non-accelerated Filer
Entity Small Business	true
Entity Emerging Growth Company	true
Entity Ex Transition Period	false
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		42,753,607
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

Related Party Transactions

9 Months Ended

Sep. 30, 2023

Related Party Transactions

15. 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 David Schaffer, Ph.D., a co-founder and former director and Chief Scientific Advisor of the Company. 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 March 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 David Schaffer, Ph.D., 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 March 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 David Schaffer, Ph.D. As of September 30, 2023 and December 31, 2022, there were no accounts payable 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 three and nine months ended September 30, 2023, the Company did not record any expense related to these agreements. During the three and nine months ended September 30, 2022, the Company recorded zero and $0.3 million, respectively, of expenses 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.

- 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
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-Name Accounting Standards Codification
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Reference 2: http://www.xbrl.org/2003/role/disclosureRef
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Reference 4: http://www.xbrl.org/2003/role/disclosureRef
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Reference 5: http://www.xbrl.org/2003/role/disclosureRef
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Reference 13: http://www.xbrl.org/2003/role/disclosureRef
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-URI https://asc.fasb.org//1943274/2147483326/850-10-50-1

+ Details

Summary of Significant Accounting Policies - Schedule of Concentration of Risk (Details) - Revenue - Customer Concentration Risk	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Customer B
Concentration Risk [Line Items]
Concentration risk, percentage		99.00%		98.00%
Customer C
Concentration Risk [Line Items]
Concentration risk, percentage	99.00%		96.00%
Customer
Concentration Risk [Line Items]
Concentration risk, percentage	99.00%	99.00%	96.00%	98.00%

Stock-based Compensation - Summary of Stock-based Compensation Expense for Employees and Nonemployees by Function (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total stock-based compensation expense	$ 4,562	$ 4,520	$ 13,752	$ 12,775
Research and Development
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total stock-based compensation expense	2,314	2,438	7,463	6,931
General and Administrative
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total stock-based compensation expense	$ 2,248	$ 2,082	$ 6,289	$ 5,844

Commitments and Contingencies

9 Months Ended

Sep. 30, 2023

Commitments and Contingencies Disclosure [Abstract]

Commitments and Contingencies

9. Commitments and Contingencies

Operating Lease Commitments

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 September 30, 2023, the operating lease right-of-use asset and operating lease liability were $1.3 million and $1.5 million, respectively. As of December 31, 2022, the operating lease right-of-use asset and operating lease liability were $1.6 million and $1.9 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 September 30, 2023, the Company has received $2.3 million of this 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 are considered operating leases under ASC 842 as they do not meet the criteria of a finance lease. As of September 30, 2023, the operating lease right-of-use asset and operating lease liability was $10.7 million and $13.6 million, respectively. As of December 31, 2022, the operating lease right-of-use asset and operating lease liability was $11.5 million and $14.3 million, respectively. The discount rate used to determine the lease liability was 7.8%.

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 three and nine months ended September 30, 2023 and 2022, which are included in operating expenses in the Company’s condensed statements of operations (in thousands):


	Three Months Ended September 30,				Nine Months Ended September 30,
	2023		2022		2023		2022
Operating lease cost	$	698	$	697	$	2,095	$	2,090
Variable lease cost		311		260		933		751
Total	$	1,009	$	957	$	3,028	$	2,841

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

The following table summarizes supplemental information related to operating leases:


	September 30, 2023			December 31, 2022
Weighted-average remaining lease term (in years)		6.0			6.7
Weighted-average discount rate:		7.8	%		7.8	%

The following table summarizes the maturities of lease liabilities as of September 30, 2023 (in thousands):


2023 (remaining three months)	$	773
2024		3,149
2025		3,244
2026		3,135
2027		2,810
Thereafter		5,875
Total future minimum lease payments		18,986
Less: Amount representing interest		(3,829	)
Present value of future minimum lease payments		15,157
Less: Current portion of operating lease liabilities		3,126
Long-term portion of operating lease liabilities	$	12,031

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 September 30, 2023.

Common Stock - Additional Information (Details) - USD ($)	1 Months Ended		9 Months Ended	12 Months Ended
	Oct. 31, 2021	Apr. 30, 2020	Sep. 30, 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

Related Party Transactions - Additional Information (Details) - UC Regents	1 Months Ended		3 Months Ended		9 Months Ended
	Mar. 31, 2021 USD ($)	Apr. 30, 2019 USD ($) Agreement	Sep. 30, 2023 USD ($)	Sep. 30, 2022 USD ($)	Sep. 30, 2023 USD ($)	Sep. 30, 2022 USD ($)	Dec. 31, 2022 USD ($)	Mar. 31, 2022 USD ($)
SRAs
Related Party Transaction [Line Items]
Number of sponsored research agreements entered \| Agreement		2
Agreement term		3 years
Payment to related party								$ 1,400,000
Amount due to related party		$ 1,400,000
SRAs and Machine Learning SRA
Related Party Transaction [Line Items]
Accounts payable, related party			$ 0		$ 0		$ 0
Related party expenses			$ 0	$ 0	$ 0	$ 300,000
Machine Learning SRA
Related Party Transaction [Line Items]
Agreement term	3 years
Payment to related party								$ 400,000
Amount due to related party	$ 1,400,000
Agreement terminated					2022-03

Stock-based Compensation

9 Months Ended

Sep. 30, 2023

Share-Based Payment Arrangement [Abstract]

Stock-based Compensation

11. Stock-based Compensation

2020 Equity 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 September 30, 2023 there were 2,857,922 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 September 30, 2023, there were 323,418 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 September 30, 2023, options to purchase 1,829,240 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.

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 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.

Stock Options

The following table summarizes the stock options activity for the nine months ended September 30, 2023:


	Number of Shares Available for Grant			Number of Shares Underlying Outstanding Options
Balances at December 31, 2022		2,112,039			5,778,929
Options authorized		1,631,331			—
Options granted		(1,226,579	)		1,226,579
Options exercised		—			(319,576	)
Options expired		60,746			(60,746	)
Options forfeited		603,803			(603,803	)
Balances at September 30, 2023		3,181,340			6,021,383
Options exercisable at September 30, 2023					3,146,872
Options vested and expected to vest at September 30, 2023					6,021,383

The Company estimates the fair value of 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 three and nine months ended September 30, 2023 and 2022:


	Three Months Ended September 30,		Nine Months Ended September 30,
	2023	2022	2023	2022
Expected term	6.0 - 6.1 years	5.9 - 6.1 years	6.0 - 6.1 years	5.9 - 6.1 years
Expected volatility	80.6% - 82.2%	81.3% - 81.9%	80.6% - 84.4%	80.0% - 81.9%
Risk-free interest rate	4.1% - 4.4%	2.9% - 3.9%	3.5% - 4.4%	1.5% - 3.9%
Expected dividend yield	—%	—%	—%	—%

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 this 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 is based on a mix of the Company's historical volatility and the historical volatility of comparable companies with similar attributes to the Company, including industry, stage of life cycle, size and financial leverage. 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.

Stock-Based Compensation Expense

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


	Three Months Ended September 30,				Nine Months Ended September 30,
	2023		2022		2023		2022
Research and development	$	2,314	$	2,438	$	7,463	$	6,931
General and administrative		2,248		2,082		6,289		5,844
Total stock-based compensation expense	$	4,562	$	4,520	$	13,752	$	12,775

Commitments and Contingencies (Tables)

9 Months Ended

Sep. 30, 2023

Commitments and Contingencies Disclosure [Abstract]

Summary of Components of Lease Expense


	Three Months Ended September 30,				Nine Months Ended September 30,
	2023		2022		2023		2022
Operating lease cost	$	698	$	697	$	2,095	$	2,090
Variable lease cost		311		260		933		751
Total	$	1,009	$	957	$	3,028	$	2,841

Summary of Supplemental Information Related to Leases

The following table summarizes supplemental information related to operating leases:


	September 30, 2023			December 31, 2022
Weighted-average remaining lease term (in years)		6.0			6.7
Weighted-average discount rate:		7.8	%		7.8	%

Summary of Future Minimum Commitments Under Lease Contracts

The following table summarizes the maturities of lease liabilities as of September 30, 2023 (in thousands):


2023 (remaining three months)	$	773
2024		3,149
2025		3,244
2026		3,135
2027		2,810
Thereafter		5,875
Total future minimum lease payments		18,986
Less: Amount representing interest		(3,829	)
Present value of future minimum lease payments		15,157
Less: Current portion of operating lease liabilities		3,126
Long-term portion of operating lease liabilities	$	12,031

Summary of Significant Accounting Policies (Policies)

9 Months Ended

Sep. 30, 2023

Accounting Policies [Abstract]

Basis of Presentation

Use of Estimates and Judgements

Use of Estimates and Judgments

Segment Information

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:


	Three Months Ended September 30,		Nine Months Ended September 30,
	2023	2022	2023	2022
Customer A	*	*	*	*
Customer B	*	99%	*	98%
Customer C	99%	*	96%	*
Total	99%	99%	96%	98%

* Less than 10%

The Company did not have accounts receivable from its partners in collaboration and license agreements as of September 30, 2023 and December 31, 2022.

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


	Three Months Ended September 30,				Nine Months Ended September 30,
	2023		2022		2023		2022
Netherlands	$	41	$	497	$	566	$	1,849
United States		20,163		3		20,176		33
Total revenue	$	20,204	$	500	$	20,742	$	1,882

Acquisitions

Other Risks and Uncertainties

of its employees, consultants and other third parties (including for clinical trials and some aspects of research and preclinical testing).

Recently Adopted Accounting Pronouncements

- Definition

Disclosure of accounting policy for segment reporting.

+ References + Details

Summary of Significant Accounting Policies (Tables)

9 Months Ended

Sep. 30, 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:


	Three Months Ended September 30,		Nine Months Ended September 30,
	2023	2022	2023	2022
Customer A	*	*	*	*
Customer B	*	99%	*	98%
Customer C	99%	*	96%	*
Total	99%	99%	96%	98%

* Less than 10%

The Company did not have accounts receivable from its partners in collaboration and license agreements as of September 30, 2023 and December 31, 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):


	Three Months Ended September 30,				Nine Months Ended September 30,
	2023		2022		2023		2022
Netherlands	$	41	$	497	$	566	$	1,849
United States		20,163		3		20,176		33
Total revenue	$	20,204	$	500	$	20,742	$	1,882

Research and Collaboration Arrangements

9 Months Ended

Sep. 30, 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):


	Three Months Ended September 30,				Nine Months Ended September 30,
	2023		2022		2023		2022
uniQure	$	41	$	497	$	566	$	1,849
CFF		155		3		168		33
Astellas		20,008		—		20,008		—
Total revenue	$	20,204	$	500	$	20,742	$	1,882

Deferred revenue is summarized as follows (in thousands):


	September 30, 2023		December 31, 2022
uniQure	$	—	$	566
CFF		1,226		1,394
Total deferred revenue	$	1,226	$	1,960

The total amount of revenue in the nine months ended September 30, 2023, which was included in deferred revenue at January 1, 2023, was $0.7 million. The total amount of revenue in the nine months ended September 30, 2022, which was included in deferred revenue at January 1, 2022, was $1.9 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.

The Company recognized an immaterial amount and $0.6 million during the three and nine months ended September 30, 2023, respectively, under both of the Amended uniQure Agreement and the Second uniQure Agreement. During the three and nine months ended September 30, 2022, the Company recognized $0.5 million and $1.8 million, respectively, under both of the Amended uniQure Agreement and the Second uniQure Agreement. During the nine months ended September 30, 2022, the decrease in total budgeted costs for the remaining performance obligation resulted in a $1.8 million increase in revenue recognized in the nine months ended September 30, 2022 related to the performance obligation partially satisfied prior to January 1, 2022. As of September 30, 2023 and December 31, 2022, deferred revenue relating to uniQure was zero and $0.6 million, respectively. There were no amounts due from uniQure under the uniQure Agreement, Amended uniQure Agreement or Second uniQure Agreement as of September 30, 2023 and December 31, 2022. As of September 30, 2023 and December 31, 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 September 30, 2023 and December 31, 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 three and nine months ended September 30, 2023 was $0.2 million and $0.2 million, respectively, while revenue recognized during the three and nine months ended September 30, 2022 was immaterial. As of September 30, 2023 and December 31, 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 September 30, 2023 and December 31, 2022. As of September 30, 2023 and December 31, 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 14 for further discussion of the embedded derivative.

Condensed Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022
Cash flows from operating activities
Net loss	$ (68,554)	$ (80,115)
Adjustments to reconcile net loss to net cash used in operating activities
Stock-based compensation expense	13,752	12,775
Vesting of common stock warrant in return for services	66	66
Change in fair value of derivative liability	157	(1)
Depreciation and amortization	3,102	1,389
Amortization of right-of-use assets	1,134	1,103
Net amortization (accretion) of premium (discount) on marketable securities	(765)	1,290
Changes in operating assets and liabilities
Accounts receivable	0	47
Prepaid expenses and other current assets	(1,622)	3,693
Other assets	(69)	(451)
Accounts payable	1,984	(3,151)
Accrued and other liabilities	1,374	662
Deferred revenue	(734)	(1,882)
Operating lease liabilities	(967)	104
Net cash used in operating activities	(51,142)	(64,471)
Cash flows from investing activities
Purchase of marketable securities	(20,156)	(253,821)
Maturities of marketable securities	144,777	282,993
Acquisition of property and equipment	(2,222)	(10,443)
Net cash provided by investing activities	122,399	18,729
Cash flows from financing activities
Issuance of common stock upon exercise of stock options and warrants	3,069	568
Issuance of common stock - 2020 ESPP	658	471
Net cash provided by financing activities	152,070	1,039
Net increase (decrease) in cash and cash equivalents	223,327	(44,703)
Cash and cash equivalents, beginning of period	52,351	153,001
Cash and cash equivalents, end of period	275,678	108,298
Supplemental disclosures of noncash investing and financing information
Deferred ATM costs reclassification	468	0
Purchases of property and equipment in accounts payable and accrued and other liabilities	345	297
ATM Offering Program
Cash flows from financing activities
Issuance of common stock for the offering program, net of issuance costs	19,135	0
Public offering
Cash flows from financing activities
Issuance of common stock for the offering program, net of issuance costs	$ 129,208	$ 0

Research and Collaboration Arrangements - Summary of Deferred Revenue (Details) - USD ($)	Sep. 30, 2023	Dec. 31, 2022
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Total deferred revenue	$ 1,226,000	$ 1,960,000
uniQure
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Total deferred revenue	0	566,000
CFF
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Total deferred revenue	$ 1,226,000	$ 1,394,000

Fair Value Measurements - Summary of Changes in Fair Value of Level 3 Financial Instruments (Details) - Derivative Liability - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2023	Sep. 30, 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	$ (1)
Fair Value, Recurring Basis, Unobservable Input Reconciliation, Liability, Gain (Loss), Statement of Income [Extensible List]	Other income (expense), net	Other income (expense), net
Ending Balance	$ 369	$ 213

Summary of Significant Accounting Policies - Schedule of Revenues by Geographic Region (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Disaggregation Of Revenue [Line Items]
Total revenue	$ 20,204	$ 500	$ 20,742	$ 1,882
Netherlands
Disaggregation Of Revenue [Line Items]
Total revenue	41	497	566	1,849
United States
Disaggregation Of Revenue [Line Items]
Total revenue	$ 20,163	$ 3	$ 20,176	$ 33

- 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

- 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

+ Details

Property and Equipment, Net - Additional Information (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Property, Plant and Equipment [Abstract]
Depreciation expense	$ 1.1	$ 0.5	$ 3.1	$ 1.4

Accrued and Other Current Liabilities - Summary of Accrued and Other Current Liabilities (Details) - USD ($) $ in Thousands	Sep. 30, 2023	Dec. 31, 2022
Payables and Accruals [Abstract]
Payroll and related expenses	$ 6,223	$ 5,689
Accrued clinical and preclinical study costs	1,776	1,355
Consulting and professional	2,032	1,444
Other accrued expenses	163	382
Total accrued and other current liabilities	$ 10,194	$ 8,870

Condensed Balance Sheets (Unaudited) - USD ($) $ in Thousands	Sep. 30, 2023	Dec. 31, 2022
Current assets:
Cash and cash equivalents	$ 275,678	$ 52,351
Marketable securities	43,986	161,203
Prepaid expenses and other current assets	8,579	6,957
Total current assets	328,243	220,511
Marketable securities, long-term	0	4,908
Property and equipment, net	20,738	22,262
Operating lease right-of-use assets, net	11,950	13,085
Other assets	682	1,080
Total assets	361,613	261,846
Current liabilities
Accounts payable	4,713	3,322
Accrued and other current liabilities	10,194	8,870
Deferred revenue	130	884
Operating lease liabilities, current portion	3,126	2,655
Total current liabilities	18,163	15,731
Deferred revenue, net of current portion	1,096	1,076
Derivative liability	369	212
Operating lease liabilities, long-term portion	12,031	13,469
Other liabilities	20	21
Total liabilities	31,679	30,509
Commitments and contingencies (Note 9)
Stockholders’ equity
Preferred stock, $0.0001 par value, 10,000,000 shares authorized at September 30, 2023 and December 31, 2022; no shares issued and outstanding at September 30, 2023 and December 31, 2022	0	0
Common stock, $0.0001 par value, 300,000,000 shares authorized at September 30, 2023 and December 31, 2022; 42,752,452 and 32,626,627 shares issued and outstanding at September 30, 2023 and December 31, 2022, respectively	4	3
Additional paid-in-capital	712,440	547,020
Accumulated other comprehensive gain (loss)	534	(1,196)
Accumulated deficit	(383,044)	(314,490)
Total stockholders’ equity	329,934	231,337
Total liabilities and stockholders’ equity	$ 361,613	$ 261,846

Fair Value Measurements - Additional Information (Details) - USD ($)	Sep. 30, 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

Net Loss Per Share, Basic and Diluted

9 Months Ended

Sep. 30, 2023

Earnings Per Share [Abstract]

Net Loss Per Share, Basic and Diluted

13. 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):


	Three Months Ended September 30,						Nine Months Ended September 30,
	2023			2022			2023			2022
Numerator
Net loss	$	(10,256	)	$	(25,691	)	$	(68,554	)	$	(80,115	)
Denominator
Weighted-average shares outstanding used in computing net loss per share, basic and diluted		42,256,629			32,385,791			37,884,363			32,305,074
Net loss per share, basic and diluted	$	(0.24	)	$	(0.79	)	$	(1.81	)	$	(2.48	)

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:


	September 30,
	2023		2022
Options issued and outstanding		6,021,383		6,035,456
ESPP shares expected to be issued		269,102		—
Common stock warrants		53,669		53,669
Total		6,344,154		6,089,125

Stock-based Compensation - Additional Information (Details)	9 Months Ended
	Sep. 30, 2023 Period shares	Jan. 01, 2023 shares	Dec. 31, 2022 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	9,637,028		8,357,910
Number of shares available for grant	3,181,340		2,112,039
Number of options outstanding	6,021,383		5,778,929
Expected dividend yield	0.00%
2020 Equity Incentive Award Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of shares reserved for issuance	3,181,340		2,112,039		2,606,546
Percentage of common stock outstanding increased annually for future issuance	5.00%
Number of shares available for grant	2,857,922	1,631,331
2020 Equity Incentive Award Plan \| Maximum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of shares authorized	18,000,000
Options exercisable period	10 years
2015 Equity Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of shares available for grant	323,418
Options exercisable period	10 years
Number of options outstanding	1,829,240
Number of additional shares available for grant	0
2020 ESPP
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of shares reserved for issuance	380,636		413,273		252,337
Number of shares available for grant				50,000
Increase in shares available for issuance percentage	1.00%
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

Condensed Statements of Operations (Unaudited) (Parenthetical) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Income Statement [Abstract]
Research and development expense, related party	$ 0	$ 0	$ 0	$ 134

Common Stock Warrants

9 Months Ended

Sep. 30, 2023

Equity [Abstract]

Common Stock Warrants

12. 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. All 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.

There was no material expense related to the above warrant shares during each of the three and nine months ended September 30, 2023 and 2022.

Common Stock - Schedule of Common Share Reserved for Future Issuance (Details) - shares	Sep. 30, 2023	Dec. 31, 2022	Dec. 31, 2020
Class Of Stock [Line Items]
Common stock reserved	9,637,028	8,357,910
2020 Equity Incentive Award Plan
Class Of Stock [Line Items]
Common stock reserved	3,181,340	2,112,039	2,606,546
2020 Employee Stock Purchase Plan
Class Of Stock [Line Items]
Common stock reserved	380,636	413,273	252,337
Options Issued and Outstanding
Class Of Stock [Line Items]
Common stock reserved	6,021,383	5,778,929
Common Stock Warrants
Class Of Stock [Line Items]
Common stock reserved	53,669	53,669

- 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

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Topic 505
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 13
-Subparagraph (d)
-Publisher FASB
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Reference 2: http://www.xbrl.org/2003/role/recommendedDisclosureRef
-Topic 272
-SubTopic 10
-Name Accounting Standards Codification
-Section 45
-Paragraph 3
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147483014/272-10-45-3

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 272
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Reference 4: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 235
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-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08(d))
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-URI https://asc.fasb.org//1943274/2147480678/235-10-S99-1

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Topic 505
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 13
-Subparagraph (a)
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-URI https://asc.fasb.org//1943274/2147481112/505-10-50-13

Reference 6: http://www.xbrl.org/2003/role/disclosureRef
-Topic 505
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 13
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481112/505-10-50-13

Reference 7: http://www.xbrl.org/2003/role/disclosureRef
-Topic 505
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 13
-Subparagraph (e)
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-URI https://asc.fasb.org//1943274/2147481112/505-10-50-13

Reference 8: http://www.xbrl.org/2003/role/disclosureRef
-Topic 505
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-Subparagraph (h)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481112/505-10-50-13

Reference 9: http://www.xbrl.org/2003/role/disclosureRef
-Topic 505
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-Name Accounting Standards Codification
-Section 50
-Paragraph 14
-Subparagraph (b)
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-URI https://asc.fasb.org//1943274/2147481112/505-10-50-14

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-Topic 505
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-Topic 210
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-Section S99
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-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479886/946-10-S99-3

+ Details

Research and Collaboration Arrangements - Summary of Collaboration and License Revenue (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Total revenue	$ 20,204	$ 500	$ 20,742	$ 1,882
Collaboration and License Revenue
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Total revenue	20,204	500	20,742	1,882
Collaboration and License Revenue \| uniQure
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Total revenue	41	497	566	1,849
Collaboration and License Revenue \| CFF
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Total revenue	155	3	168	33
Collaboration and License Revenue \| Astellas
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Total revenue	$ 20,008	$ 0	$ 20,008	$ 0

- Definition

+ References

+ Details

Derivative Liability

9 Months Ended

Sep. 30, 2023

Derivative Instruments and Hedging Activities Disclosure [Abstract]

Derivative Liability

14. 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 September 30, 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 September 30, 2023 and December 31, 2022) and the discount rate (15.0% at September 30, 2023 and December 31, 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 September 30, 2023 and December 31, 2022, respectively.

Property and Equipment, Net (Tables)

9 Months Ended

Sep. 30, 2023

Property, Plant and Equipment [Abstract]

Schedule of Property and Equipment, Net

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


	September 30, 2023			December 31, 2022
Machinery and equipment	$	11,995		$	11,191
Leasehold improvements		16,902			16,902
Furniture and fixtures		566			566
Office equipment		240			239
Computer equipment and software		836			616
Construction in progress		1,030			476
Total property and equipment		31,569			29,990
Less: Accumulated depreciation and amortization		(10,831	)		(7,728	)
Property and equipment, net	$	20,738		$	22,262

Commitments and Contingencies -Summary of Supplemental Information Related to Leases - (Details)	Sep. 30, 2023	Dec. 31, 2022
Commitments and Contingencies Disclosure [Abstract]
Weighted-average remaining lease term (in years)	6 years	6 years 8 months 12 days
Weighted-average discount rate	7.80%	7.80%

Commitments and Contingencies - Additional Information (Details) - USD ($) $ in Thousands	1 Months Ended					9 Months Ended
	Feb. 28, 2021	Dec. 31, 2019	Nov. 30, 2018	Oct. 31, 2018	May 31, 2015	Sep. 30, 2023	Dec. 31, 2022	Jan. 31, 2016
Commitment And Contingencies [Line Items]
Operating lease right-of-use ("ROU") assets						$ 11,950	$ 13,085
Operating lease liability						15,157
5980 Horton Street Building Lease
Commitment And Contingencies [Line Items]
Lease extended term date					Mar. 31, 2023
Operating lease right-of-use ("ROU") assets						1,300	1,600
Operating lease liability						$ 1,500	1,900
Operating lease, discount rate						7.50%
Lease extended term period				2026-09
Tenant improvement allowance			$ 200
Lease renewal term					3 years			5 years
5858 Horton Street Building Lease and Expansion
Commitment And Contingencies [Line Items]
Operating lease right-of-use ("ROU") assets						$ 10,700	11,500
Operating lease liability						$ 13,600	$ 14,300
Operating lease, discount rate						7.80%
Tenant improvement allowances						$ 2,300
Tenant improvement allowance	$ 2,300	$ 400
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

Common Stock

9 Months Ended

Sep. 30, 2023

Equity [Abstract]

Common Stock

10. Common Stock

As of September 30, 2023 and December 31, 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. To date, no dividends on common stock have been declared by the board of directors.

As of September 30, 2023 and December 31, 2022, the Company has reserved common stock, on an as-converted basis, for future issuance as follows:


	September 30, 2023		December 31, 2022
Issuance of common stock under the 2020 Equity Incentive Award Plan		3,181,340		2,112,039
Issuance of common stock under the 2020 Employee Stock Purchase Plan		380,636		413,273
Exercise of options issued and outstanding		6,021,383		5,778,929
Exercise of common stock warrants		53,669		53,669
Total common stock reserved for future issuance		9,637,028		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 September 30, 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.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 505
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-Name Accounting Standards Codification
-Section 50
-Paragraph 13
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481112/505-10-50-13

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 505
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-Section 50
-Paragraph 13
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-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481112/505-10-50-13

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 505
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-Publisher FASB
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Reference 4: http://www.xbrl.org/2003/role/disclosureRef
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Reference 5: http://www.xbrl.org/2003/role/disclosureRef
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-Name Accounting Standards Codification
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Reference 6: http://www.xbrl.org/2003/role/disclosureRef
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Reference 7: http://www.xbrl.org/2003/role/disclosureRef
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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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Reference 11: http://www.xbrl.org/2003/role/disclosureRef
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-URI https://asc.fasb.org//1943274/2147481112/505-10-50-18

+ Details

401(K) Plan - Additional Information (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
2014 401(k) Plan
Defined Contribution Plan Disclosure [Line Items]
Employer contribution	$ 0.2	$ 0.2	$ 0.9	$ 0.7

Fair Value Measurements (Tables)

9 Months Ended

Sep. 30, 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		September 30, 2023
Assets
Money market funds	$	250,041	$	—	$	—	$	250,041
Commercial paper		—		24,687		—		24,687
Corporate bonds		—		6,279		—		6,279
U.S. Treasuries		—		4,969		—		4,969
U.S. government sponsored agencies		—		8,051		—		8,051
Total	$	250,041	$	43,986	$	—	$	294,027
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 Financial Instruments


	Derivative Liability
Balance as of December 31, 2022	$	212
Change in fair value included in other income (expense), net		157
Balance as of September 30, 2023	$	369


	Derivative Liability
Balance as of December 31, 2021	$	214
Change in fair value included in other income (expense), net		(1	)
Balance as of September 30, 2022	$	213

Marketable Securities - Summary of Aggregate fair values of marketable securities with unrealized losses and gains (Details) - USD ($) $ in Thousands	Sep. 30, 2023	Dec. 31, 2022
Debt Securities, Available-for-Sale [Line Items]
Aggregate fair value of securities held in a continuous unrealized loss position for less than twelve months	$ 12,783	$ 97,472
Aggregate fair value of marketable securities in a continuous loss position for more than twelve months	11,232	53,716
Aggregate fair value of marketable securities in unrealized gain position	19,971	14,923
Marketable Securities, Total	$ 43,986	$ 166,111

- 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

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 320
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481800/320-10-50-2

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+ Details

Marketable Securities (Tables)

9 Months Ended

Sep. 30, 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 September 30, 2023 and December 31, 2022 (in thousands):


	Amortized Cost Basis		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value as of September 30, 2023
Short-term marketable securities:
Commercial paper	$	24,452	$	312	$	(77	)	$	24,687
Corporate bonds		6,302		—		(23	)		6,279
U.S. treasuries		4,827		142		—			4,969
U.S. government sponsored agencies		7,871		195		(15	)		8,051
Total short-term marketable securities	$	43,452	$	649	$	(115	)	$	43,986


	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 September 30, 2023 and December 31, 2022 (in thousands):


	September 30,		December 31,
	2023		2022
Aggregate fair value of marketable securities in a continuous loss position for less than twelve months	$	12,783	$	97,472
Aggregate fair value of marketable securities in a continuous loss position for more than twelve months		11,232		53,716
Aggregate fair value of marketable securities in unrealized gain position		19,971		14,923
Total marketable securities	$	43,986	$	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).

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 320
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-Name Accounting Standards Codification
-Section 50
-Paragraph 3
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481800/320-10-50-3

Reference 3: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 320
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481800/320-10-50-2

Reference 4: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 320
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (c)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481800/320-10-50-2

Reference 5: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 320
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481800/320-10-50-2

Reference 6: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 320
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (aa)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481800/320-10-50-2

Reference 7: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 320
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (aaa)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481800/320-10-50-2

Reference 8: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 320
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (d)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481800/320-10-50-2

Reference 9: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 320
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 3
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481800/320-10-50-3

Reference 10: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 320
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 3
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481800/320-10-50-3

Reference 11: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 320
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 3
-Subparagraph (c)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481800/320-10-50-3

Reference 12: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 320
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 3
-Subparagraph (d)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481800/320-10-50-3

+ Details

Research and Collaboration Arrangements - Additional Information (Details 1)	Jan. 31, 2014
uniQure \| uniQure Agreement \| Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date: 2023-10-01
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue, estimated performance period	4 years

The Company - Additional Information (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended
	May 31, 2023	Nov. 30, 2021	Dec. 31, 2020	Sep. 30, 2023	Dec. 31, 2022	Jan. 01, 2021
Organization And Nature Of Business [Line Items]
Outstanding shares of redeemable convertible preferred stock						0
Accumulated deficit				$ 383,044	$ 314,490
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 \| Excess 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

Commitments and Contingencies - Summary of Maturities of Lease Liabilities (Details) - USD ($) $ in Thousands	Sep. 30, 2023	Dec. 31, 2022
Commitments and Contingencies Disclosure [Abstract]
2023 (remaining three months)	$ 773
2024	3,149
2025	3,244
2026	3,135
2027	2,810
Thereafter	5,875
Total future minimum lease payments	18,986
Less: Amount representing interest	(3,829)
Present value of future minimum lease payments	15,157
Less: Current portion of operating lease liabilities	3,126	$ 2,655
Operating lease liabilities, long-term portion	$ 12,031	$ 13,469

Research and Collaboration Arrangements - CFF - Additional Information (Details)	1 Months Ended		3 Months Ended	9 Months Ended		12 Months Ended
	Aug. 31, 2023 USD ($)	Sep. 30, 2016 USD ($) Installment	Sep. 30, 2023 USD ($)	Sep. 30, 2023 USD ($)	Sep. 30, 2022 USD ($)	Dec. 31, 2022 USD ($)
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue recognized				$ 700,000	$ 1,900,000
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	1,200,000		1,400,000
Accounts receivable			0	0		0
Aggregate amount of transaction price allocated to remaining performance obligation			1,200,000	1,200,000		$ 1,400,000
Revenue recognized			$ 200,000	$ 200,000
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

Marketable Securities - Summary of Available for Sale Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2023	Dec. 31, 2022
Short-term Marketable Securities
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost Basis	$ 43,452	$ 162,240
Gross Unrealized Gains	649	13
Gross Unrealized Losses	(115)	(1,050)
Fair Value	43,986	161,203
Long-term Marketable Securities
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost Basis		5,067
Gross Unrealized Gains		0
Gross Unrealized Losses		(159)
Fair Value		4,908
Commercial Paper \| Short-term Marketable Securities
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost Basis	24,452	43,988
Gross Unrealized Gains	312	0
Gross Unrealized Losses	(77)	(24)
Fair Value	24,687	43,964
Commercial Paper \| Long-term Marketable Securities
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost Basis		5,067
Gross Unrealized Gains		0
Gross Unrealized Losses		(159)
Fair Value		4,908
Corporate Bonds \| Short-term Marketable Securities
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost Basis	6,302	47,565
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(23)	(553)
Fair Value	6,279	47,012
US Treasuries \| Short-term Marketable Securities
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost Basis	4,827	63,502
Gross Unrealized Gains	142	1
Gross Unrealized Losses	0	(444)
Fair Value	4,969	63,059
U.S. Government Sponsored Agencies \| Short-term Marketable Securities
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost Basis	7,871	7,185
Gross Unrealized Gains	195	12
Gross Unrealized Losses	(15)	(29)
Fair Value	$ 8,051	$ 7,168

401(K) Plan

9 Months Ended

Sep. 30, 2023

Retirement Benefits [Abstract]

401(K) Plan

16. 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 401(k) plan for all eligible participants and recorded contribution expenses of $0.2 million for each of the three months ended September 30, 2023 and 2022. For the nine months ended September 30, 2023 and 2022, the Company recorded contribution expenses of $0.9 million and $0.7 million, respectively.

Marketable Securities - Additional Information (Details) - USD ($)	9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	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	100,000
Aggregate unrealized loss	100,000
Allowance for credit loss	0		$ 0
Other impairment charges related to marketable securities	$ 0		$ 0
Maximum
Debt Securities, Available-for-Sale [Line Items]
Maturity period of marketable securities	1 year

- 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

Condensed Statements of Operations (Unaudited) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Revenue:
Collaboration and license revenue	$ 20,204	$ 500	$ 20,742	$ 1,882
Operating expenses:
Research and development (includes $0 for each of the three months ended September 30, 2023 and 2022, respectively, and $0 and $134 for the nine months ended September 30, 2023 and 2022, respectively, attributable to related parties)	25,066	18,940	71,068	58,753
General and administrative	9,112	8,055	25,889	24,441
Total operating expenses	34,178	26,995	96,957	83,194
Loss from operations	(13,974)	(26,495)	(76,215)	(81,312)
Other income (expense):
Interest income	3,876	795	7,831	1,235
Other income (expense), net	(158)	9	(170)	(38)
Total other income (expense), net	3,718	804	7,661	1,197
Net loss	$ (10,256)	$ (25,691)	$ (68,554)	$ (80,115)
Net loss per share, basic	$ (0.24)	$ (0.79)	$ (1.81)	$ (2.48)
Net loss per share, diluted	$ (0.24)	$ (0.79)	$ (1.81)	$ (2.48)
Weighted-average shares outstanding used in computing net loss per share, basic	42,256,629	32,385,791	37,884,363	32,305,074
Weighted-average shares outstanding used in computing net loss per share, diluted	42,256,629	32,385,791	37,884,363	32,305,074

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	Sep. 30, 2023	Dec. 31, 2022
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets	$ 294,027	$ 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	24,687	48,872
Corporate Bonds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets	6,279	47,012
US Treasuries
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets	4,969	63,059
U.S. Government Sponsored Agencies
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets	8,051	7,168
Money Market Funds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets	250,041	44,447
Level 1
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets	250,041	44,447
Level 1 \| Money Market Funds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets	250,041	44,447
Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets	43,986	166,111
Level 2 \| Commercial Paper
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets	24,687	48,872
Level 2 \| Corporate Bonds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets	6,279	47,012
Level 2 \| US Treasuries
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets	4,969	63,059
Level 2 \| U.S. Government Sponsored Agencies
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets	8,051	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

Accrued and Other Current Liabilities

9 Months Ended

Sep. 30, 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):


	September 30, 2023		December 31, 2022
Payroll and related expenses	$	6,223	$	5,689
Accrued clinical and preclinical study costs		1,776		1,355
Consulting and professional		2,032		1,444
Other accrued expenses		163		382
Total accrued and other current liabilities	$	10,194	$	8,870

Net Loss Per Share, Basic and Diluted (Tables)

9 Months Ended

Sep. 30, 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):


	Three Months Ended September 30,						Nine Months Ended September 30,
	2023			2022			2023			2022
Numerator
Net loss	$	(10,256	)	$	(25,691	)	$	(68,554	)	$	(80,115	)
Denominator
Weighted-average shares outstanding used in computing net loss per share, basic and diluted		42,256,629			32,385,791			37,884,363			32,305,074
Net loss per share, basic and diluted	$	(0.24	)	$	(0.79	)	$	(1.81	)	$	(2.48	)

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


	September 30,
	2023		2022
Options issued and outstanding		6,021,383		6,035,456
ESPP shares expected to be issued		269,102		—
Common stock warrants		53,669		53,669
Total		6,344,154		6,089,125

Stock-based Compensation - Summary of Stock Options Activity (Details) - shares	9 Months Ended
	Sep. 30, 2023
Share-Based Payment Arrangement [Abstract]
Number of Shares Available for Grant, Balances at December 31, 2022	2,112,039
Number of Shares Available for Grant, Options authorized	1,631,331
Number of Shares Available for Grant, Options granted	(1,226,579)
Number of Shares Available for Grant, Options expired	60,746
Number of Shares Available for Grant, Options forfeited	603,803
Number of Shares Available for Grant, Balances at June 30, 2023	3,181,340
Number of Shares Underlying Outstanding Options, Balances at December 31, 2022	5,778,929
Number of Shares Underlying Outstanding Options, granted	1,226,579
Number of Shares Underlying Outstanding Options, exercised	(319,576)
Number of Shares Underlying Outstanding Options, expired	(60,746)
Number of Shares Underlying Outstanding Options, forfeited	(603,803)
Number of Shares Underlying Outstanding Options, Balances at June 30, 2023	6,021,383
Number of Shares Underlying Outstanding Options, Options exercisable at March 31, 2023	3,146,872
Number of Shares Underlying Outstanding Options, Options vested and expected to vest at March 31, 2023	6,021,383

Common Stock Warrants - Additional Information (Details) - USD ($)	1 Months Ended				3 Months Ended		9 Months Ended		12 Months Ended
	Oct. 31, 2021	Apr. 30, 2021	Dec. 31, 2020	May 31, 2018	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 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 expense recognized					$ 0	$ 0	$ 0	$ 0
Warrant vesting period									4 years
Warrant exercised month and year	2021-10	2021-05
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										$ 25,000,000
Initial Public Offering
Class Of Warrant Or Right [Line Items]
Warrant shares become exercisable			30,000						30,000

Research and Collaboration Arrangements - Additional Information (Details) - USD ($)		1 Months Ended	3 Months Ended	9 Months Ended
	Jul. 05, 2023	Jan. 31, 2014	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022	Dec. 31, 2022	Aug. 31, 2019
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue recognized				$ 700,000	$ 1,900,000
Deferred revenue				1,226,000		$ 1,960,000
Astellas GeneTherapies Inc
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront payments received	$ 20,000,000
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				1,800,000
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			$ 500,000	600,000	$ 1,800,000
Remaining performance obligations				$ 0		$ 600,000