0000950170-21-004086 | 10-Q | XBRL Viewer | 4D Molecular Therapeutics INC

Commitments and Contingencies (Tables)

9 Months Ended

Sep. 30, 2021

Commitments And Contingencies Disclosure [Abstract]

Summary of Future Minimum Commitments Under Lease Contracts

The following table summarizes the Company’s future minimum commitments under lease contracts as of September 30, 2021 (in thousands):


2021 (remaining three months)	$	756
2022		3,076
2023		3,157
2024		3,235
2025		3,262
Thereafter		11,820
Total	$	25,306

Research and Collaboration Arrangements (Tables)

9 Months Ended

Sep. 30, 2021

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,
	2021		2020			2021		2020
uniQure	$	64	$	255		$	724	$	504
Roche		1,290		7,215			17,190		14,174
CFF		12		(49	)		32		(89	)
Total revenue	$	1,366	$	7,421		$	17,946	$	14,589

Summary of Deferred Revenue

Deferred revenue is summarized as follows (in thousands):


	September 30, 2021		December 31, 2020
uniQure	$	3,672	$	4,396
Roche		—		14,318
CFF		1,066		1,098
Total deferred revenue	$	4,738	$	19,812

Fair Value Measurements - Additional Information (Details) - USD ($)	9 Months Ended	12 Months Ended
	Sep. 30, 2021	Dec. 31, 2020
Fair Value Disclosures [Abstract]
Assets, Level 1 to Level 2 transfers	$ 0	$ 0
Assets, Level 2 to Level 1 Transfers	0	0
Assets, Transfers into Level 3	0	0
Assets, Transfers out of Level 3	0	0
Liabilities, Level 1 to Level 2 transfers	0	0
Liabilities, Level 2 to Level 1 transfers	0	0
Liabilities, Transfers into Level 3	0	0
Liabilities, Transfers out of Level 3	$ 0	$ 0

Summary of Significant Accounting Policies (Tables)

9 Months Ended

Sep. 30, 2021

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,
	2021	2020		2021	2020
Customer A	94%	97%		96%	97%
Customer B	*	*		*	*
Total	94%	97%		96%	97%

* Less than 10%

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


	September 30, 2021	December 31, 2020
Customer A	100%	74%
Customer B	*	26%
Total	100%	100%

* Less than 10%

Schedule of Revenues by Geographic Region

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


	Three Months Ended September 30,					Nine Months Ended September 30,
	2021		2020			2021		2020
Netherlands	$	64	$	255		$	724	$	504
Switzerland		1,290		7,215			17,190		14,174
United States		12		(49	)		32		(89	)
Total revenue	$	1,366	$	7,421		$	17,946	$	14,589

Condensed Balance Sheets (Unaudited) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Current assets:
Cash and cash equivalents	$ 90,989	$ 276,726
Marketable securities	70,686	0
Accounts receivable	791	1,486
Prepaid expenses and other current assets (includes $526 and $169 at September 30, 2021 and December 31, 2020, respectively, attributable to related parties)	3,267	4,444
Total current assets	165,733	282,656
Marketable securities, long term	65,494	0
Property and equipment, net	8,789	5,073
Other assets	602	602
Total assets	240,618	288,331
Current liabilities
Accounts payable (includes $0 and $173 at September 30, 2021 and December 31, 2020, respectively, attributable to related parties)	3,713	1,787
Accrued and other current liabilities	5,932	8,371
Deferred revenue	1,728	6,586
Total current liabilities	11,373	16,744
Deferred revenue, net of current portion	3,010	13,226
Derivative liability	214	122
Other liabilities	1,887	1,852
Total liabilities	16,484	31,944
Commitments and contingencies (Note 9)
Stockholders’ equity
Preferred stock, $0.0001 par value, 10,000,000 shares authorized as of September 30, 2021 and December 31, 2020; no shares issued and outstanding as of September 30, 2021 and December 31, 2020	0	0
Common stock, $0.0001 par value, 300,000,000 shares authorized at September 30, 2021 and December 31, 2020; 27,165,577 and 26,681,983 shares issued and outstanding at September 30, 2021 and December 31, 2020, respectively	3	3
Additional paid-in-capital	406,239	392,063
Accumulated other comprehensive loss	(195)	0
Accumulated deficit	(181,913)	(135,679)
Total stockholders’ equity	224,134	256,387
Total liabilities and stockholders’ equity	$ 240,618	$ 288,331

Research and Collaboration Arrangements

9 Months Ended

Sep. 30, 2021

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,
	2021		2020			2021		2020
uniQure	$	64	$	255		$	724	$	504
Roche		1,290		7,215			17,190		14,174
CFF		12		(49	)		32		(89	)
Total revenue	$	1,366	$	7,421		$	17,946	$	14,589

Deferred revenue is summarized as follows (in thousands):


	September 30, 2021		December 31, 2020
uniQure	$	3,672	$	4,396
Roche		—		14,318
CFF		1,066		1,098
Total deferred revenue	$	4,738	$	19,812

The total amount of revenue in the nine months ended September 30, 2021, which was included in deferred revenue at January 1, 2021, was $15.1 million. The total amount of revenue in the nine months ended September 30, 2020, which was included in deferred revenue at January 1, 2020, was $4.0 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 was three years and uniQure exercised a one-time option to extend the research term

for an additional year to January 2018. Once the Company’s research plan concluded, uniQure was solely responsible for the continued development, manufacturing and commercialization of the project variants as potential product candidates. 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 percentage rate range on future sales of the potential product candidates and sublicense consideration in the low teens to low thirties percentage rate range on any future sublicensing arrangements. 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 recognized on a ratable basis over the performance period of four years. The options to purchase uniQure shares were deemed to be a noncash component of the arrangement consideration. The estimated fair value of the uniQure options was $10.6 million and was recognized ratably as revenue over the performance period of four years. The associated compensation expense related to the stock options were 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 receive 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 percentage rate range 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 but separate 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 was determined to be $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. 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 recognized using the input method based on actual costs incurred as a percentage of total budgeted costs as the Company completes its performance obligation. Based on the current estimated timelines, the deferred revenue is expected to be recognized as revenue over approximately one to two years from September 30, 2021.

During the three months ended September 30, 2021 and 2020, the Company recognized revenue of $0.1 million and $0.3 million under both of the Amended uniQure Agreement and the Second uniQure Agreement, respectively. During the nine months ended September 30, 2021 and 2020, the Company recognized revenue of $0.7 million and $0.5 million, respectively, under both of the Amended uniQure Agreement and the Second uniQure Agreement. As of September 30, 2021 and December 31, 2020, deferred revenue relating to uniQure was $3.7 million and $4.4 million, respectively. There were no amounts due from uniQure under the uniQure Agreement, Amended uniQure Agreement or Second uniQure Agreement as of September 30, 2021 and December 31, 2020. As of September 30, 2021 and December 31, 2020, the aggregate amount of the transaction price allocated to the remaining performance obligation was $3.7 million and $4.4 million, respectively.

CRF

In November 2015, the Company entered into a research funding and collaboration agreement (the “CRF Agreement”) with the Choroideremia Research Foundation (“CRF”). The goal of the CRF Agreement is for CRF to contribute funding to help with the advancement of the Company’s choroideremia research program. The Company is responsible for all decision making and execution of any and all of the related activities to be completed in its sole discretion. The initial term of the CRF Research Plan is two years. The agreement includes contribution to CRF of up to $2.5 million upon certain development or approval milestones. The overall arrangement has automatic extensions of up to three additional years. As of September 30, 2021, no milestones have been achieved. Revenue was fully recognized for this agreement in the year ended December 31, 2017.

Roche

In November 2017, the Company entered into a collaboration and license agreement with F. Hoffmann-La Roche Ltd and Hoffmann-La Roche Inc. (together, “Roche”) to discover and develop products containing optimized next generation AAV Vectors focused on ophthalmological diseases and disorders excluding select criteria (the “2017 Roche Agreement”).

At the effective date of the 2017 Roche Agreement, choroideremia was designated a Roche product class and the 4D-110 product was licensed to Roche. The Company was responsible for conducting research and development services prior to pivotal clinical studies, and Roche was responsible for conducting subsequent development and commercialization activities. In addition, Roche agreed to pay for research and development services at the agreed upon full-time employee rate for work performed for

choroideremia under the 2017 Roche Agreement, and certain external costs, except for the costs associated with the manufacturing work for choroideremia.

Under the 2017 Roche Agreement, Roche had an option exercisable prior to pivotal clinical studies, to license 4D-125 (for the treatment of XLRP), which was not exercised.

Pursuant to the 2017 Roche Agreement, the Company received a non-refundable upfront payment of $21.0 million as consideration. In addition, the Company was entitled to contingent payments including (i) $1.0 million for each Roche nominated product beyond the first three, (ii) up to $30.0 million upon exercise of the option to convert a product the Company nominated and developed prior to pivotal clinical studies, (iii) development milestone payments of up to $223.0 million, of which $86.0 million related to choroideremia and the rest related to other licensed products; and (iv) sales-based milestones of up to $123.0 million in connection with licensed products. Through September 30, 2021, the Company received $10.0 million for development milestone payments that are non-refundable. The 2017 Roche Agreement also included provisions that entitled the Company to receive royalty payments ranging from the mid-single digits to the mid-teens for the net sales of the licensed products, in each case subject to the reductions in accordance with the terms of the agreement.

The Company identified a single combined performance obligation for the license, research services and participation in the JSC and concluded that Roche’s option did not represent a material right. The Company’s contract with Roche did not include a significant financing component. The Company further concluded that the transaction price should not include the variable consideration related to development milestones as they were considered to be constrained as it was probable that the inclusion of such variable consideration could result in a significant reversal of cumulative revenue in the future. The Company excluded any consideration related to sales-based milestones, including royalties, which would be recognized when the related sales occur. The transaction price and estimated period of performance are re-evaluated at each reporting period and adjusted as needed to reflect changes in the scope of the project, reimbursable expenses and other conditions affecting variable consideration.

In June 2021, the Company received from Roche notice of termination without cause of the 2017 Roche Agreement. The licenses granted by the Company to Roche under the 2017 Roche Agreement terminated in their entirety on September 16, 2021. Therefore, as of September 16, 2021, Roche no longer has a license for 4D-110 for the treatment of choroideremia or any other product class, and the Company is no longer entitled to receive any further milestones or royalties from Roche. In July 2021, the Company notified Roche of the Company’s election to continue development of choroideremia. As a result, in accordance with the terms of the agreement, all rights to 4D-110 data and intellectual property generated under the collaboration reverted to the Company. Under the 2017 Roche Agreement, if the Company is required to initiate a new Phase 1/2a Study for 4D-110, the Company would not owe any royalty to Roche on net sales of 4D-110. If, however, the Company moves directly to a pivotal trial of 4D-110, without first being required to conduct a new Phase 1/2a Study, the Company must pay Roche a mid-single digits percentage royalty on the net sales of 4D-110.

As a result of the termination of the 2017 Roche Agreement, an adjustment of $8.7 million was made to the transaction price to reflect the decrease in reimbursable costs for the nine months ended September 30, 2021. The decrease in the transaction price and total budgeted costs for the remaining performance obligation resulted in an $11.1 million increase in revenue recognized in the nine months ended September 30, 2021 related to performance obligation partially satisfied prior to January 1, 2021. For the nine months ended September 30, 2020, an adjustment of $15.1 million was made to the transaction price to reflect a $5.1 million increase in the scope of the project and expected reimbursable costs and the addition of $10.0 million of variable consideration as the uncertainty associated with two development milestones were resolved. For the nine months ended September 30, 2020, the change in the transaction price and total budgeted costs resulted in an increase of $7.1 million in revenue recognized related to performance obligations partially satisfied in periods prior to January 1, 2020, respectively.

During the three months ended September 30, 2021 and 2020, the Company recognized revenue of $1.3 million and $7.2 million, respectively. During the nine months ended September 30, 2021 and 2020, the Company recognized revenue of $17.2 million and $14.2 million, respectively. As of September 30, 2021 and December 31, 2020, deferred revenue relating to the 2017 Roche Agreement was $0.0 million

and $14.3 million, respectively. Accounts receivable from Roche under this agreement as of September 30, 2021 and December 31, 2020 was $0.8 million and $1.1 million, respectively. As of September 30, 2021 and December 31, 2020, the aggregate amount of the transaction price allocated to the remaining performance obligation was $0.0 million and $25.8 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 agreement was subsequently amended in September 2017, August 2018 and February 2021 (all four agreements are collectively referred to as the “CFF Agreement”). The total amount of the award under the CFF Agreement is $3.5 million. As of September 30, 2021 and December 31, 2020, the Company achieved milestones totaling $1.3 million under the CFF Agreement. The remaining award amount to be paid by CFF is based on the 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 rate, 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 date.

Revenue recognized during the three and nine months ended September 30, 2021 and September 30, 2020 was immaterial. As of September 30, 2021 and December 31, 2020, deferred revenue relating to the CFF Agreement was $1.1 million. Accounts receivable from CFF under the CFF Agreement as of September 30, 2021 and December 31, 2020 were $0.0 million and $0.4 million, respectively. As of September 30, 2021 and December 31, 2020, the aggregate amount of the transaction price allocated to the remaining performance obligation was $1.1 million. Based on current timelines, the deferred revenue is

expected to be recognized as revenue over the next three to four 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.

401(K) Plan - Additional Information (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
2014 Plan
Defined Contribution Plan Disclosure [Line Items]
Employer contribution	$ 0.1	$ 0.1	$ 0.4	$ 0.3

Common Stock Warrants - Additional Information (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended		3 Months Ended			9 Months Ended		12 Months Ended
	Apr. 30, 2021	May 31, 2018	Sep. 30, 2021	Jun. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020	Dec. 31, 2016	Dec. 31, 2018
Class Of Warrant Or Right [Line Items]
Number of warrants issued	40,000	23,669						30,000	45,000
Exercise price	$ 9.41	$ 3.19						$ 18.00	$ 1.14
Issuance of common stock warrant in connection with settlement		$ 100		$ 1,146
Proceeds from offering of securities in a private placement									$ 25,000
Warrant expiration year		2025						2027	2023
Warrant shares, expense recognized			0		0	0	0
Warrant vesting period								4 years
Warrant exercised month and year	2021-05
Additional Paid-in Capital
Class Of Warrant Or Right [Line Items]
Issuance of common stock warrant in connection with settlement	$ 1,100
Series B Financing
Class Of Warrant Or Right [Line Items]
Warrant shares exercisable										15,000
Private Placement
Class Of Warrant Or Right [Line Items]
Warrant shares exercisable									15,000
Initial Public Offering
Class Of Warrant Or Right [Line Items]
Warrant shares exercisable								30,000	30,000

Stock-based Compensation (Tables)

9 Months Ended

Sep. 30, 2021

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Summary of Stock Options Activity

The following table summarizes the stock options activity for the nine month period ended September 30, 2021:


	Number of Shares Available for Grant			Number of Shares Underlying Outstanding Options
Balances at December 31, 2020		2,606,546			3,194,113
Options authorized		1,334,099			—
Options granted		(1,424,350	)		1,424,350
Options exercised		—			(443,594	)
Options expired		354			(354	)
Options forfeited		91,976			(91,976	)
Balances at September 30, 2021		2,608,625			4,082,539
Options exercisable at September 30, 2021					1,379,288
Options vested and expected to vest at September 30, 2021					4,082,539

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

The Company estimates the fair value of stock options using the Black-Scholes model. The fair value of stock 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, 2021 and 2020.


	Three Months Ended September 30,		Nine Months Ended September 30,
	2021	2020	2021	2020
Expected term	6.0 - 6.1 years	5.5 - 6.3 years	5.8 - 6.3 years	5.5 - 6.3 years
Expected volatility	80.4% - 82.8%	82.8% - 83.8%	80.4% - 84.3%	82.1% - 83.8%
Risk-free interest rate	0.8% - 1.1%	0.4% - 0.4%	0.6% - 1.1%	0.4% - 0.7%
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,
	2021		2020		2021		2020
Research and development	$	1,893	$	593	$	4,901	$	1,897
General and administrative		2,030		554		5,167		1,370
Total stock-based compensation expense	$	3,923	$	1,147	$	10,068	$	3,267

Marketable Securities - Summary of Available for Sale Securities (Details) $ in Thousands	Sep. 30, 2021 USD ($)
Short-term Marketable Securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost Basis	$ 70,716
Gross Unrealized Gains	0
Gross Unrealized Losses	(30)
Fair Value	70,686
Long-term Marketable Securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost Basis	65,659
Gross Unrealized Gains	11
Gross Unrealized Losses	(176)
Fair Value	65,494
Commercial Paper \| Short-term Marketable Securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost Basis	32,440
Gross Unrealized Gains	0
Gross Unrealized Losses	(2)
Fair Value	32,438
Commercial Paper \| Long-term Marketable Securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost Basis	5,278
Gross Unrealized Gains	6
Gross Unrealized Losses	(10)
Fair Value	5,274
Corporate Bonds \| Short-term Marketable Securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost Basis	38,276
Gross Unrealized Gains	0
Gross Unrealized Losses	(28)
Fair Value	38,248
Corporate Bonds \| Long-term Marketable Securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost Basis	35,291
Gross Unrealized Gains	5
Gross Unrealized Losses	(157)
Fair Value	35,139
US Treasuries \| Long-term Marketable Securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost Basis	25,090
Gross Unrealized Gains	0
Gross Unrealized Losses	(9)
Fair Value	$ 25,081

Research and Collaboration Arrangements - Summary of Collaboration and License Revenue (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Total revenue	$ 1,366	$ 7,421	$ 17,946	$ 14,589
Collaboration and License Revenue \| uniQure
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Total revenue	64	255	724	504
Collaboration and License Revenue \| Roche
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Total revenue	1,290	7,215	17,190	14,174
Collaboration and License Revenue \| CFF
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Total revenue	$ 12	$ (49)	$ 32	$ (89)

- 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

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 22
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=120311839&loc=d3e8736-108599

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 323
-SubTopic 10
-Section 50
-Paragraph 3
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=114001798&loc=d3e33918-111571

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 28
-Subparagraph (f)
-URI http://asc.fasb.org/extlink&oid=75031198&loc=d3e14064-108612

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 32
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=120311839&loc=d3e8933-108599

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 32
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=120311839&loc=d3e8933-108599

Reference 6: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 22
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=120311839&loc=d3e8736-108599

Reference 7: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 40
-URI http://asc.fasb.org/extlink&oid=120311839&loc=d3e9031-108599

Reference 8: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 30
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=120311839&loc=d3e8906-108599

Reference 9: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 41
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=120311839&loc=d3e9038-108599

Reference 10: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 42
-URI http://asc.fasb.org/extlink&oid=120311839&loc=d3e9054-108599

Reference 11: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08(g)(1)(ii))
-URI http://asc.fasb.org/extlink&oid=120395691&loc=d3e23780-122690

Reference 12: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 235
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-05(b)(2))
-URI http://asc.fasb.org/extlink&oid=120399901&loc=d3e537907-122884

Reference 13: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03(1))
-URI http://asc.fasb.org/extlink&oid=120395209&loc=SL114868664-224227

+ Details

Accrued and Other Liabilities, Current and Noncurrent - Summary of Accrued and Other Current Liabilities (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Payables And Accruals [Abstract]
Payroll and related	$ 3,683	$ 3,437
Accrued clinical and preclinical study costs	636	869
Consulting and professional	1,193	3,574
Other accrued expenses	420	491
Total accrued and other current liabilities	$ 5,932	$ 8,371

Condensed Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020
Cash flows from operating activities
Net loss	$ (46,234)	$ (36,146)
Adjustments to reconcile net loss to net cash used in operating activities
Stock-based compensation expense	10,068	3,267
Vesting of common stock warrants	66	0
Change in fair value of derivative liability	92	16
Depreciation and amortization	1,133	1,072
Loss on disposition of property and equipment	2	0
Changes in operating assets and liabilities
Accounts receivable	695	82
Prepaid expenses and other current assets	1,177	(1,007)
Other assets	0	75
Accounts payable	623	(524)
Accrued and other liabilities	154	1,872
Deferred revenue	(15,074)	(2,117)
Net cash used in operating activities	(47,298)	(33,410)
Cash flows from investing activities
Purchase of marketable securities	(136,375)	0
Acquisition of property and equipment	(4,014)	(492)
Net cash used in investing activities	(140,389)	(492)
Cash flows from financing activities
Issuance of redeemable convertible preferred stock, net of issuance costs	0	72,468
Issuance of common stock upon exercise of stock options and warrants	2,896	557
Payments of offering costs	(946)	(20)
Net cash provided by financing activities	1,950	73,005
Net increase (decrease) in cash and cash equivalents	(185,737)	39,103
Cash and cash equivalents, beginning of period	276,726	49,652
Cash and cash equivalents, end of period	90,989	88,755
Supplemental disclosures of noncash investing and financing information
Issuance of common stock warrant to settle liability	1,146	0
Purchases of property and equipment in accounts payable and accrued and other liabilities	1,303	512
Unpaid offering costs	$ 0	$ 53

Condensed Statements of Comprehensive Loss (Unaudited) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Statement of Comprehensive Income [Abstract]
Net loss	$ (22,239)	$ (7,833)	$ (46,234)	$ (36,146)
Other comprehensive gain (loss):
Net unrealized loss on marketable securities	(195)	0	(195)	0
Total Comprehensive loss	$ (22,434)	$ (7,833)	$ (46,429)	$ (36,146)

- Definition

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

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 45
-Paragraph 1A
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=121641772&loc=SL7669619-108580

Reference 2: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 31
-URI http://asc.fasb.org/extlink&oid=120311839&loc=d3e8924-108599

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 250
-SubTopic 10
-Section 50
-Paragraph 11
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=109234566&loc=d3e22694-107794

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 220
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-04(22))
-URI http://asc.fasb.org/extlink&oid=120399700&loc=SL114874048-224260

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 250
-SubTopic 10
-Section 50
-Paragraph 11
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=109234566&loc=d3e22694-107794

Reference 6: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 32
-Subparagraph (f)
-URI http://asc.fasb.org/extlink&oid=120311839&loc=d3e8933-108599

Reference 7: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 205
-SubTopic 20
-Section 50
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=109222650&loc=SL51721683-107760

Reference 8: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 250
-SubTopic 10
-Section 50
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=109234566&loc=d3e22595-107794

Reference 9: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 45
-Paragraph 1B
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=121641772&loc=SL7669625-108580

Reference 10: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 22
-URI http://asc.fasb.org/extlink&oid=120311839&loc=d3e8736-108599

Reference 11: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 220
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-04(18))
-URI http://asc.fasb.org/extlink&oid=120400993&loc=SL114874131-224263

Reference 12: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03(20))
-URI http://asc.fasb.org/extlink&oid=120395209&loc=SL114868664-224227

Reference 13: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-URI http://asc.fasb.org/extlink&oid=121586364&loc=d3e3602-108585

Reference 14: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 250
-SubTopic 10
-Section 50
-Paragraph 1
-Subparagraph (b)(2)
-URI http://asc.fasb.org/extlink&oid=109234566&loc=d3e22499-107794

Reference 15: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 250
-SubTopic 10
-Section 50
-Paragraph 9
-URI http://asc.fasb.org/extlink&oid=109234566&loc=d3e22663-107794

Reference 16: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 28
-Subparagraph (f)
-URI http://asc.fasb.org/extlink&oid=75031198&loc=d3e14064-108612

Reference 17: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 30
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=120311839&loc=d3e8906-108599

Reference 18: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 250
-SubTopic 10
-Section 50
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=109234566&loc=d3e22583-107794

Reference 19: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08(g)(1)(ii))
-URI http://asc.fasb.org/extlink&oid=120395691&loc=d3e23780-122690

Reference 20: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 260
-SubTopic 10
-Section 45
-Paragraph 60B
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=121326447&loc=SL5780133-109256

Reference 21: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 323
-SubTopic 10
-Section 50
-Paragraph 3
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=114001798&loc=d3e33918-111571

Reference 22: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 32
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=120311839&loc=d3e8933-108599

Reference 23: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 250
-SubTopic 10
-Section 50
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=109234566&loc=d3e22658-107794

+ Details

Common Stock - Schedule of Common Share Reserved for Future Issuance (Details) - shares	Sep. 30, 2021	Dec. 31, 2020
Class Of Stock [Line Items]
Common stock reserved	7,308,989	6,151,665
2020 Equity Incentive Award Plan
Class Of Stock [Line Items]
Common stock reserved	2,608,625	2,606,546
Employee Stock Purchase Plan
Class Of Stock [Line Items]
Common stock reserved	519,156	252,337
Options Issued and Outstanding
Class Of Stock [Line Items]
Common stock reserved	4,082,539	3,194,113
Common Stock Warrants
Class Of Stock [Line Items]
Common stock reserved	98,669	98,669

Fair Value Measurements (Tables)

9 Months Ended

Sep. 30, 2021

Fair Value Disclosures [Abstract]

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

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, 2021 and December 31, 2020 (in thousands):


	Basis for Fair Value Measurements						Fair Value as of
	Level 1		Level 2		Level 3		September 30, 2021
Assets
Money market funds	$	63,457	$	—	$	—	$	63,457
Short-term marketable securities		—		70,686		—		70,686
Long-term marketable securities		—		65,494		—		65,494
Total	$	63,457	$	136,180	$	—	$	199,637
Liabilities
Derivative liability	$	—	$	—	$	214	$	214
Total	$	—	$	—	$	214	$	214


	Basis for Fair Value Measurements						Fair Value as of
	Level 1		Level 2		Level 3		December 31, 2020
Assets
Money market funds	$	276,726	$	—	$	—	$	276,726
Total	$	276,726	$	—	$	—	$	276,726
Liabilities
Derivative liability	$	—	$	—	$	122	$	122
Total	$	—	$	—	$	122	$	122

Summary of Changes in Fair Value of Level 3 Financial Instruments

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, 2021 and 2020 (in thousands):


	Derivative Liability
Balance as of December 31, 2020	$	122
Change in fair value included in other income (expense), net		92
Balance as of September 30, 2021	$	214


	Derivative Liability
Balance as of December 31, 2019	$	101
Change in fair value included in other income (expense), net		16
Balance as of September 30, 2020	$	117

Property and Equipment, Net - Additional Information (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Property Plant And Equipment [Abstract]
Depreciation expense	$ 0.4	$ 0.4	$ 1.1	$ 1.1

Net Loss Per Share Attributable to Common Stockholders (Tables)

9 Months Ended

Sep. 30, 2021

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 attributable to common stockholders (in thousands, except share and per share data):


	Three Months Ended September 30,						Nine Months Ended September 30,
	2021			2020			2021			2020
Numerator
Net loss attributable to common stockholders	$	(22,239	)	$	(7,833	)	$	(46,234	)	$	(36,146	)
Denominator
Weighted-average shares outstanding used in computing net loss per share attributable to common stockholders, basic and diluted		27,022,380			5,197,982			26,818,595			5,188,628
Net loss per share attributable to common stockholders—basic and diluted	$	(0.82	)	$	(1.51	)	$	(1.72	)	$	(6.97	)

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

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


	September 30,
	2021		2020
Redeemable convertible preferred stock		—		11,575,984
Options issued and outstanding		4,082,539		2,928,321
ESPP shares issuable and outstanding		97,155		—
Common stock warrants		98,669		68,669
Total		4,278,363		14,572,974

Stock-based Compensation - Additional Information (Details) - shares		1 Months Ended	3 Months Ended		9 Months Ended		12 Months Ended
	Mar. 25, 2021	Dec. 31, 2020	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2021
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of shares reserved for issuance		6,151,665	7,308,989		7,308,989
Number of shares available for grant		2,606,546	2,608,625		2,608,625
Number of options outstanding		3,194,113	4,082,539		4,082,539
Expected dividend yield			0.00%	0.00%	0.00%	0.00%
2020 Equity Incentive Award Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of shares reserved for issuance		2,606,546	2,608,625		2,608,625
Number of shares available for grant			2,608,625		2,608,625
2020 Equity Incentive Award Plan \| Maximum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Options exercisable period		10 years
2020 Equity Incentive Award Plan \| Forecast
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of shares authorized and reserved for issuance							1,334,099
Percentage of aggregate number of shares of common stock outstanding reserved for future issuance							5.00%
2020 Equity Incentive Award Plan \| Forecast \| Maximum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of shares available for grant							18,000,000
2015 Equity Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of shares available for grant			0		0
Options exercisable period					10 years
Number of options outstanding			2,704,166		2,704,166
Minimum percentage of outstanding shares held by individual					10.00%
Exercise price as percentage of fair market value, minimum					110.00%
2020 ESPP
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of shares authorized and reserved for issuance	266,819	252,337
Percentage of aggregate number of shares of common stock outstanding reserved for future issuance	1.00%
Employee purchase price of common stock as percentage of fair market value		85.00%
Common stock overlapping offering period		24 months
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 available for grant	15,000,000

Research and Collaboration Arrangements - Additional Information (Details) - USD ($)	1 Months Ended	3 Months Ended		9 Months Ended
	Jan. 31, 2014	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020	Aug. 31, 2019
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue recognized				$ 15,100,000	$ 4,000,000.0
Deferred revenue		$ 4,738,000		4,738,000		$ 19,812,000
uniQure
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue		3,672,000		3,672,000		4,396,000
Due from related parties		$ 0		$ 0		0
uniQure \| uniQure Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
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	$ 5,000,000.0
Deferred revenue, estimated performance period		2 years		2 years
uniQure \| uniQure Agreement \| Minimum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue, estimated performance period		1 year		1 year
uniQure \| Amended uniQure Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue recognized		$ 100,000	$ 300,000	$ 700,000	$ 500,000
Remaining performance obligation		$ 3,700,000		$ 3,700,000		$ 4,400,000
uniQure \| Amended uniQure Agreement \| Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contingent payments eliminated							$ 5,000,000.0

Derivative Liability - Additional Information (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2021	Dec. 31, 2020
Derivative [Line Items]
Change in control discount rate	14.00%	14.00%
Estimated fair value of derivative liability	$ 0.2	$ 0.1
Minimum [Member]
Derivative [Line Items]
Change of control payment	$ 0.0	$ 0.0
Change in control event percentage	4.80%	3.40%
Maximum
Derivative [Line Items]
Change of control payment	$ 10.6	$ 10.6
Change in control event percentage	17.20%	12.30%

The Company

9 Months Ended

Sep. 30, 2021

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 gene therapy company pioneering the development of product candidates using its targeted and evolved adeno-associated viruses (“AAV”) vectors.

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, no shares of redeemable convertible preferred stock were outstanding.

Follow On Public Offering

In November 2021, the Company completed its second underwritten public offering ("Follow-on 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 Follow-on Offering were $110.8 million, after deducting underwriting discounts and commissions and offering expenses. As part of the Follow-on Offering, the Company granted the underwriters a 30-day option to purchase up to an additional 712,500 shares of common stock at the Follow-on Offering price of $25.00 per share, less underwriting discounts and commissions.

Liquidity

The Company has incurred significant losses and negative cash flows from operations and had an accumulated deficit of $181.9 million as of September 30, 2021. The Company believes that its cash and cash equivalents and marketable securities as of September 30, 2021 along with the net proceeds of the Follow-on Offering 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. Refer to Note 17 for information regarding the Follow-on Offering completed by the Company in November 2021. 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.

Derivative Liability

9 Months Ended

Sep. 30, 2021

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, in the condensed statements of operations. 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 $0.0 million to $10.6 million at September 30, 2021 and December 31, 2020), 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, 2021 and 3.4% to 12.3% at December 31, 2020) and the discount rate (14% at September 30, 2021 and December 31, 2020). The Company determined the fair value of this derivative liability was $0.2 million and $0.1 million as of September 30, 2021 and December 31, 2020, respectively.

Net Loss Per Share Attributable to Common Stockholders

9 Months Ended

Sep. 30, 2021

Earnings Per Share [Abstract]

Net Loss Per Share Attributable to Common Stockholders

13. Net Loss Per Share Attributable to Common Stockholders

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


	Three Months Ended September 30,						Nine Months Ended September 30,
	2021			2020			2021			2020
Numerator
Net loss attributable to common stockholders	$	(22,239	)	$	(7,833	)	$	(46,234	)	$	(36,146	)
Denominator
Weighted-average shares outstanding used in computing net loss per share attributable to common stockholders, basic and diluted		27,022,380			5,197,982			26,818,595			5,188,628
Net loss per share attributable to common stockholders—basic and diluted	$	(0.82	)	$	(1.51	)	$	(1.72	)	$	(6.97	)


	September 30,
	2021		2020
Redeemable convertible preferred stock		—		11,575,984
Options issued and outstanding		4,082,539		2,928,321
ESPP shares issuable and outstanding		97,155		—
Common stock warrants		98,669		68,669
Total		4,278,363		14,572,974

Research and Collaboration Arrangements - Roche Agreement - Additional Information (Details) - USD ($) $ in Thousands	1 Months Ended	3 Months Ended		9 Months Ended
	Nov. 30, 2017	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Total revenue		$ 1,366	$ 7,421	$ 17,946	$ 14,589
Roche \| 2017 Roche Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Non-refundable upfront payments received	$ 21,000
Contingent payment for each nominated product beyond the first three	1,000
Maximum contingent payment upon exercise of the option to convert a product	30,000
Maximum development milestone payments	223,000
Development milestone payments relating to Choroideremia	86,000
Maximum sales-based milestone payments	$ 123,000
Non-refundable development milestone payments				10,000
Collaborative arrangement adjustment to transaction price				8,700	15,100
Collaborative arrangement adjustment to transaction price due to increase in project scope and expected reimbursable costs					5,100
Collaborative arrangement adjustment to transaction price due to variable consideration					10,000
Increase in revenue recognized related to performance obligations partially satisfied in the beginning of the current year				11,100	7,100
Total revenue		1,300	$ 7,200	17,200	$ 14,200
Deferred revenue		0		0		$ 14,300
Accounts receivable		800		800		1,100
Aggregate amount of transaction price allocated to remaining performance obligation		$ 0		$ 0		$ 25,800

Condensed Statements of Redeemable Convertible Preferred Stock and Stockholders' Equity (Deficit) (Unaudited) - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-in Capital	Additional Paid-in Capital Cumulative Effect, Period of Adoption, Adjustment	Accumulated Other Comprehensive Loss	Accumulated Deficit	Accumulated Deficit Cumulative Effect, Period of Adoption, Adjustment
Stockholders’ Equity (Deficit), beginning balances at Dec. 31, 2019	$ (72,970)	$ 1	$ 6,054	$ (39)		$ (79,025)	$ 39
Redeemable Convertible Preferred Stock, beginning balances (in shares) at Dec. 31, 2019	7,375,631
Redeemable Convertible Preferred Stock, beginning balances at Dec. 31, 2019	$ 102,980
Stockholders' Equity (Deficit), beginning balances (in shares) at Dec. 31, 2019		5,178,955
Accounting Standards Update [Extensible List]	us-gaap:AccountingStandardsUpdate201807Member
Issuance of common stock upon exercise of stock options	$ 5		5
Issuance of common stock upon exercise of stock options (in shares)		5,000
Stock-based compensation expense	1,083		1,083
Net loss	(13,160)					(13,160)
Stockholders’ Equity (Deficit), ending balances at Mar. 31, 2020	$ (85,042)	$ 1	7,103			(92,146)
Redeemable Convertible Preferred Stock, ending balances (in shares) at Mar. 31, 2020	7,375,631
Redeemable Convertible Preferred Stock, ending balances at Mar. 31, 2020	$ 102,980
Stockholders' Equity (Deficit), ending balances (in shares) at Mar. 31, 2020		5,183,955
Stockholders’ Equity (Deficit), beginning balances at Dec. 31, 2019	$ (72,970)	$ 1	6,054	$ (39)		(79,025)	$ 39
Redeemable Convertible Preferred Stock, beginning balances (in shares) at Dec. 31, 2019	7,375,631
Redeemable Convertible Preferred Stock, beginning balances at Dec. 31, 2019	$ 102,980
Stockholders' Equity (Deficit), beginning balances (in shares) at Dec. 31, 2019		5,178,955
Net loss	(36,146)
Stockholders’ Equity (Deficit), ending balances at Sep. 30, 2020	$ (105,292)	$ 1	9,839			(115,132)
Redeemable Convertible Preferred Stock, ending balances (in shares) at Sep. 30, 2020	11,575,984
Redeemable Convertible Preferred Stock, ending balances at Sep. 30, 2020	$ 175,448
Stockholders' Equity (Deficit), ending balances (in shares) at Sep. 30, 2020		5,257,742
Stockholders’ Equity (Deficit), beginning balances at Mar. 31, 2020	$ (85,042)	$ 1	7,103			(92,146)
Redeemable Convertible Preferred Stock, beginning balances (in shares) at Mar. 31, 2020	7,375,631
Redeemable Convertible Preferred Stock, beginning balances at Mar. 31, 2020	$ 102,980
Stockholders' Equity (Deficit), beginning balances (in shares) at Mar. 31, 2020		5,183,955
Issuance of redeemable convertible preferred stock, net of $3,138 of issuance cost	$ 72,468
Issuance of redeemable convertible preferred stock, net of $3,138 of issuance cost (in shares)	4,200,353
Stock-based compensation expense	$ 1,037		1,037
Net loss	(15,153)					(15,153)
Stockholders’ Equity (Deficit), ending balances at Jun. 30, 2020	$ (99,158)	$ 1	8,140			(107,299)
Redeemable Convertible Preferred Stock, ending balances (in shares) at Jun. 30, 2020	11,575,984
Redeemable Convertible Preferred Stock, ending balances at Jun. 30, 2020	$ 175,448
Stockholders' Equity (Deficit), ending balances (in shares) at Jun. 30, 2020		5,183,955
Issuance of common stock upon exercise of stock options	552		552
Issuance of common stock upon exercise of stock options (in shares)		73,787
Stock-based compensation expense	1,147		1,147
Net loss	(7,833)					(7,833)
Stockholders’ Equity (Deficit), ending balances at Sep. 30, 2020	$ (105,292)	$ 1	9,839			(115,132)
Redeemable Convertible Preferred Stock, ending balances (in shares) at Sep. 30, 2020	11,575,984
Redeemable Convertible Preferred Stock, ending balances at Sep. 30, 2020	$ 175,448
Stockholders' Equity (Deficit), ending balances (in shares) at Sep. 30, 2020		5,257,742
Stockholders’ Equity (Deficit), beginning balances at Dec. 31, 2020	256,387	$ 3	392,063		$ 0	(135,679)
Stockholders' Equity (Deficit), beginning balances (in shares) at Dec. 31, 2020		26,681,983
Issuance of common stock upon exercise of stock options	99		99			0
Issuance of common stock upon exercise of stock options (in shares)		12,396
Stock-based compensation expense	2,329		2,329			0
Vesting of common stock warrants	22		22			0
Net loss	(16,406)				0	(16,406)
Stockholders’ Equity (Deficit), ending balances at Mar. 31, 2021	242,431	$ 3	394,513		0	(152,085)
Stockholders' Equity (Deficit), ending balances (in shares) at Mar. 31, 2021		26,694,379
Stockholders’ Equity (Deficit), beginning balances at Dec. 31, 2020	$ 256,387	$ 3	392,063		0	(135,679)
Stockholders' Equity (Deficit), beginning balances (in shares) at Dec. 31, 2020		26,681,983
Issuance of common stock upon exercise of stock options (in shares)	443,594
Net loss	$ (46,234)
Stockholders’ Equity (Deficit), ending balances at Sep. 30, 2021	224,134	$ 3	406,239		(195)	(181,913)
Stockholders' Equity (Deficit), ending balances (in shares) at Sep. 30, 2021		27,165,577
Stockholders’ Equity (Deficit), beginning balances at Mar. 31, 2021	242,431	$ 3	394,513		0	(152,085)
Stockholders' Equity (Deficit), beginning balances (in shares) at Mar. 31, 2021		26,694,379
Issuance of common stock upon exercise of stock options and warrants	820		820			0
Issuance of common stock upon exercise of stock options and warrants (in shares)		197,478
Stock-based compensation expense	3,816		3,816			0
Issuance of common stock warrant	1,146		1,146			0
Vesting of common stock warrants	22		22			0
Net loss	(7,589)				0	(7,589)
Stockholders’ Equity (Deficit), ending balances at Jun. 30, 2021	240,646	$ 3	400,317		0	(159,674)
Stockholders' Equity (Deficit), ending balances (in shares) at Jun. 30, 2021		26,891,857
Issuance of common stock upon exercise of stock options and warrants	1,977		1,977			0
Issuance of common stock upon exercise of stock options and warrants (in shares)		273,720
Stock-based compensation expense	3,923		3,923			0
Issuance of common stock warrant						0
Vesting of common stock warrants	22		22
Net unrealized loss on marketable securities	(195)				(195)
Net loss	(22,239)					(22,239)
Stockholders’ Equity (Deficit), ending balances at Sep. 30, 2021	$ 224,134	$ 3	$ 406,239		$ (195)	$ (181,913)
Stockholders' Equity (Deficit), ending balances (in shares) at Sep. 30, 2021		27,165,577

Common Stock - Additional Information (Details) - USD ($)	9 Months Ended	12 Months Ended
	Sep. 30, 2021	Dec. 31, 2020
Equity [Abstract]
Common stock, shares authorized	300,000,000	300,000,000
Common stock, par value	$ 0.0001	$ 0.0001
Common stock voting rights	one
Common stock dividend	$ 0	$ 0

Subsequent Events

9 Months Ended

Sep. 30, 2021

Subsequent Events [Abstract]

Subsequent Events

17. Subsequent Events

In accordance with the CFF Agreement, CFF committed to providing an additional $4.0 million of funding upon clearance of an Investigational New Drug application ("IND") or its equivalent to allow for human testing of 4D-710, the Company's product candidate to treat cystic fibrosis lung disease. The IND was cleared by the U.S. Food and Drug Administration in October 2021, 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.

In November 2021, the Company completed its Follow-on 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 Follow-on Offering were $110.8 million after deducting underwriting discounts and commissions and offering expenses. As part of the Follow-on Offering, the Company has granted the underwriters a 30-day option to purchase up to an additional 712,500 shares of common stock at the Follow-on Offering price of $25.00 per share, less underwriting discounts and commissions.

Research and Collaboration Arrangements - CFF - Additional Information (Details) - CFF $ in Millions	1 Months Ended	9 Months Ended	12 Months Ended
	Sep. 30, 2016 USD ($) Installment	Sep. 30, 2021 USD ($)	Dec. 31, 2020 USD ($)
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaborative arrangement total amount of award	$ 3.5
Collaborative arrangement achieved milestones		$ 1.3	$ 1.3
Expected payments percentage to equal actual award received	600.00%
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
Deferred revenue		1.1	1.1
Accounts receivable		0.0	0.4
Remaining performance obligation		$ 1.1	$ 1.1
Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Future sales-based milestone payments percentage	300.00%
Royalties percentage on actual award received	600.00%

Stock-based Compensation - Schedule of Assumptions Used in Black-Scholes Model to Estimate Fair Value of Stock Options (Details)	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected volatility, minimum	80.40%	82.80%	80.40%	82.10%
Expected volatility, maximum	82.80%	83.80%	84.30%	83.80%
Risk-free interest rate, minimum	0.80%	0.40%	0.60%	0.40%
Risk-free interest rate, maximum	1.10%	0.40%	1.10%	0.70%
Expected dividend yield	0.00%	0.00%	0.00%	0.00%
Minimum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected term	6 years	5 years 6 months	5 years 9 months 18 days	5 years 6 months
Maximum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected term	6 years 1 month 6 days	6 years 3 months 18 days	6 years 3 months 18 days	6 years 3 months 18 days

Marketable Securities (Tables)

9 Months Ended

Sep. 30, 2021

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, 2021 (in thousands):


	Amortized Cost Basis		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value as of September 30, 2021
Short-term marketable securities:
Commercial paper	$	32,440	$	—	$	(2	)	$	32,438
Corporate bonds		38,276		—		(28	)		38,248
Total short-term marketable securities	$	70,716	$	—	$	(30	)	$	70,686
Long-term marketable securities:
Commercial paper	$	5,278	$	6	$	(10	)	$	5,274
Corporate bonds		35,291		5		(157	)		35,139
US Treasuries		25,090		—		(9	)		25,081
Total long-term marketable securities	$	65,659	$	11	$	(176	)	$	65,494

Property and Equipment, Net - Schedule of Property and Equipment, Net (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Property Plant And Equipment [Line Items]
Total property and equipment	$ 13,759	$ 8,917
Less: Accumulated depreciation and amortization	(4,970)	(3,844)
Property and equipment, net	8,789	5,073
Machinery and Equipment
Property Plant And Equipment [Line Items]
Total property and equipment	5,860	4,911
Leasehold Improvements
Property Plant And Equipment [Line Items]
Total property and equipment	2,527	2,527
Furniture and Fixtures
Property Plant And Equipment [Line Items]
Total property and equipment	464	473
Office Equipment
Property Plant And Equipment [Line Items]
Total property and equipment	177	101
Computer Equipment and Software
Property Plant And Equipment [Line Items]
Total property and equipment	398	366
Construction in Progress
Property Plant And Equipment [Line Items]
Total property and equipment	$ 4,333	$ 539

Research and Collaboration Arrangements - CRF Agreement - Additional Information (Details) - CRF Agreement - CRF - USD ($)	1 Months Ended	9 Months Ended
	Nov. 30, 2015	Sep. 30, 2021
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaborative arrangement, initial research term	2 years
Collaborative arrangement contribution upon development milestone	$ 2,500,000
Collaborative arrangement additional research term	3 years
Collaborative arrangement achieved milestones		$ 0

Stock-based Compensation - Summary of Stock Options Activity (Details)	9 Months Ended
	Sep. 30, 2021 shares
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Number of Shares Available for Grant,Balances at December 31, 2020	2,606,546
Number of Shares Available for Grant, Options authorized	1,334,099
Number of Shares Available for Grant, Options granted	(1,424,350)
Number of Shares Available for Grant, Options expired	354
Number of Shares Available for Grant, Options forfeited	91,976
Number of Shares Available for Grant, Balances at September 30, 2021	2,608,625
Number of Shares Underlying Outstanding Options, Balances at December 31, 2020	3,194,113
Number of Shares Underlying Outstanding Options, granted	1,424,350
Number of Shares Underlying Outstanding Options, exercised	(443,594)
Number of Shares Underlying Outstanding Options, expired	(354)
Number of Shares Underlying Outstanding Options, forfeited	(91,976)
Number of Shares Underlying Outstanding Options, Balances at September 30, 2021	4,082,539
Number of Shares Underlying Outstanding Options, Options exercisable at September 30, 2021	1,379,288
Number of Shares Underlying Outstanding Options, Options vested and expected to vest at September 30, 2021	4,082,539

Research and Collaboration Arrangements - CFF - Additional Information (Details 1) - CFF - Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date: 2021-10-01	Sep. 30, 2021
Minimum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue, estimated performance period	3 years
Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue, estimated performance period	4 years

Condensed Balance Sheets (Unaudited) (Parenthetical) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Statement Of Financial Position [Abstract]
Prepaid expenses to related parties	$ 526	$ 169
Accounts payable to related parties	$ 0	$ 173
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	27,165,577	26,681,983
Common stock, shares, outstanding	27,165,577	26,681,983

Summary of Significant Accounting Policies (Policies)

9 Months Ended

Sep. 30, 2021

Accounting Policies [Abstract]

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 footnote 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, 2020 filed with the SEC.

The accompanying financial information for the three and nine months ended September 30, 2021 and 2020 are 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, 2021 and December 31, 2020 and its results of operations for the three and nine months ended September 30, 2021 and 2020 and cash flows for the nine months ended September 30, 2021 and 2020. 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 Judgements

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

Due to the coronavirus (“COVID-19”) pandemic, 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, 2021. These estimates may change, as new events occur and additional information is obtained, as well as other factors related to the COVID-19 pandemic 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.

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, 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,
	2021	2020		2021	2020
Customer A	94%	97%		96%	97%
Customer B	*	*		*	*
Total	94%	97%		96%	97%

* Less than 10%

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


	September 30, 2021	December 31, 2020
Customer A	100%	74%
Customer B	*	26%
Total	100%	100%

* Less than 10%

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


	Three Months Ended September 30,					Nine Months Ended September 30,
	2021		2020			2021		2020
Netherlands	$	64	$	255		$	724	$	504
Switzerland		1,290		7,215			17,190		14,174
United States		12		(49	)		32		(89	)
Total revenue	$	1,366	$	7,421		$	17,946	$	14,589

Cash and Cash Equivalents

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

Marketable Securities

Marketable securities consist of commercial paper, corporate bonds and U.S. Treasuries and are included in current and non-current assets. The Company classifies its marketable securities as available-for-sale and carries them at fair value on its condensed balance sheet. Fair value is estimated using independent pricing sources based on quoted prices in active markets for similar securities. Unrealized gains and losses on the marketable securities are reported as a component of stockholders' equity (deficit) in accumulated other comprehensive loss. Realized gains and losses are included in interest income in the condensed statement of operations.

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

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).

The extent of the impact of the COVID-19 pandemic on the Company’s business will depend upon the duration and spread of the outbreak and the extent and severity of its impact on the Company’s clinical trial activities, research activities and suppliers, all of which are uncertain and cannot be predicted. The extent to which the COVID-19 pandemic may materially impact the Company’s financial condition, liquidity or results of operations is uncertain.

Fair Value Measurements

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

The hierarchy below lists three levels of fair value based on the extent to which inputs used in measuring fair value are observable in the market. The Company categorizes each of their fair value measurements in one of these three levels based on the lowest level input that is significant to the fair value measurement in its entirety. These levels are:



Level 1 — Observable inputs, such as quoted prices in active markets for identical assets and liabilities.



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



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

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

Common Stock Warrants

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

Revenue Recognition

The Company determines revenue recognition for arrangements within the scope of ASC 606 by performing the following five steps: (i) identification of the promised goods or services in the contract; (ii) determination of whether the promised goods or services are performance obligations including whether they are distinct in the context of the contract; (iii) measurement of the transaction price, including the constraint on variable consideration; (iv) allocation of the transaction price to the performance obligations

based on estimated selling prices; and (v) recognition of revenue when (or as) the Company satisfies each performance obligation.

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

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

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

Embedded Derivative

Embedded derivatives that are required to be bifurcated from the underlying host instrument are accounted for and valued as a separate financial instrument. An embedded derivative exists in the award agreement with the Cystic Fibrosis Foundation (“CFF”). As described in Note 14, the embedded derivative has been bifurcated and is classified as a liability on the condensed balance sheet and is separately accounted for at its fair value. The derivative liability is subject to remeasurement to fair value each reporting period. Changes in the fair value of the derivative liability are recognized as a component of other income (expense), net within the condensed statements of operations.

Net Loss Per Share Attributable to Common Stockholders

The Company calculates basic and diluted net loss per share to common stockholders in conformity with the two-class method required for companies with participating securities. The Company considers all series of redeemable convertible preferred stock to be participating securities as the holders are entitled to receive non-cumulative dividends on a pari passu basis in the event the dividend is paid on common shares. Under the two-class method, the net loss attributable to common stockholders is not allocated to the redeemable convertible preferred stock as the holders of redeemable convertible preferred stock do not have a contractual obligation to share in losses.

Basic net loss per share attributable to common stockholders is calculated by dividing the net loss attributable to common stockholders by the weighted-average number of shares of common stock outstanding during the period, less shares subject to repurchase. Diluted net loss per share attributable to common stockholders is computed by giving effect to all potentially dilutive common shares outstanding for the period. For purposes of this calculation, redeemable convertible preferred shares, stock options to acquire shares of common stock, common stock warrants, and unvested common stock subject to repurchase, are considered potentially dilutive common shares, but have been excluded from the calculation of diluted net loss per share attributable to common stockholders as their effect is antidilutive.

Recent Accounting Pronouncements

New Accounting Pronouncements Recently Adopted

In November 2018, the Financial Accounting Standards Board (“FASB”) issued Accounting Standard Update (“ASU”) No. 2018-18, Collaborative Arrangements (Topic 808): Clarifying the Interaction Between Topic 808 and Topic 606. This ASU clarifies that certain transactions between participants in a collaborative arrangement should be accounted for under ASC 606, Revenue from Contracts with Customers, when the counterparty is a customer. This ASU also precludes an entity from presenting consideration received from a transaction as revenue from contracts with customers if the counterparty is not a customer for that transaction. This ASU is effective for fiscal years beginning after December 15, 2020, and interim periods within fiscal years beginning after December 15, 2021. Early adoption is permitted for entities that have adopted ASC 606, Revenue from Contracts with Customers. The Company adopted this ASU on January 1, 2021. The adoption of this ASU did not result in a material impact on the Company’s unaudited condensed financial statements and related disclosures.

In December 2019, the FASB issued ASU No. 2019-12, Income Taxes (Topic 740): Simplifying the Accounting for Income Taxes. This ASU affects general principles within Topic 740 and is meant to simplify the accounting for income taxes by removing certain exceptions to the general framework. The ASU adds guidance to reduce complexity in certain areas, including recognizing a franchise (or similar) tax that is partially based on income as an income-based tax and incremental amounts incurred as a non-income-based tax and recognizing deferred taxes for tax goodwill. ASU 2019-12 also created an exception to the incremental approach for intraperiod tax allocation when there is a loss from continuing operations and income or a gain from other items (for example, other comprehensive income). The amendments in ASU 2019-12 are effective for the Company for fiscal years beginning after December 15, 2021, and interim periods within fiscal years beginning after December 15, 2022. Early adoption of the amendments is permitted. The Company adopted ASU 2019-12 effective January 1, 2021 on a prospective basis. The adoption of this ASU did not result in a material impact on the Company's unaudited condensed financial statements and related disclosures.

New Accounting Pronouncements Not Yet Adopted

As an “emerging growth company,” the Jumpstart Our Business Startups Act (“JOBS Act”) allows the Company to delay adoption of new or revised accounting pronouncements applicable to public companies until such pronouncements are made applicable to private companies. The Company has elected to use this extended transition period under the JOBS Act. The adoption dates discussed below reflect this election.

In February 2016, the FASB issued ASU No. 2016-02, Leases (Topic 842), which for operating leases requires the lessee to recognize a right-of-use asset and a lease liability, initially measured at the present value of lease payments, in its balance sheet. A modified retrospective transition approach is required for leases existing at, or entered into after, the beginning of the earliest comparative period presented in the financial statements, including a number of optional practical expedients that entities may elect to apply. ASU 2016-02 is effective for annual periods beginning after December 15, 2021 and interim periods within annual periods beginning after December 15, 2022. Early adoption is permitted. The Company is planning to adopt early ASU 2016-02 in the fourth quarter of 2021 and expects that the recognition of the right-of-use assets and lease liabilities will have a material impact on its balance sheet.

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. Early adoption is permitted. The Company is in the process of evaluating the impact of this new guidance on its financial statements.

Related Party Transactions	9 Months Ended
Related Party Transactions	Sep. 30, 2021
Related Party Transactions [Abstract]
Related Party Transactions	15. Related Party Transactions During the three months ended September 30, 2021 and 2020, the Company paid $21,000 and $25,000, respectively, to David Schaffer, Ph.D., the co-founder, director and Chief Scientific Advisor of the Company for consulting services. During the nine months ended September 30, 2021 and 2020, the Company paid $64,000 and $37,500, respectively, to David Schaffer, Ph.D, for consulting services. In April 2019, the Company entered into two sponsored research agreements (“SRAs”) with the UC Regents to conduct research in a research facility on the U.C. Berkeley campus, under the direction of Dr. Schaffer. The SRAs have a three year term ending in May 2022. Under the SRAs, the Company has an option to license (on a royalty-bearing basis) all intellectual property generated under the SRAs. Any patent prosecution costs incurred under the SRAs will also be borne by the Company. The Company can terminate the SRAs for convenience and without cause with 60 days’ notice. The total amount the Company is committed to pay to the UC Regents under the SRAs is $1.5 million, which was fully paid as of March 31, 2021. In March 2021, the Company entered into another sponsored research agreement with the UC Regents to conduct research in laboratories on the U.C. Berkeley campus that are under the direction of Dr. Schaffer and another U.C. Berkeley professor covering investigations into how machine learning approaches may enhance AAV capsid engineering (the “Machine Learning SRA”). Pursuant to the Machine Learning SRA, the Company has committed to pay the UC Regents a total of $1.4 million, of which $0.4 million was paid as of September 30, 2021. The Machine Learning SRA has a three-year term ending in 2024. Any patent prosecution costs incurred under the SRAs will also be borne by the Company. The Company can terminate the SRAs for convenience and without cause with 60 days’ notice. While the Machine Learning SRA is between the Company and the UC Regents, the payments under the SRA will be used to fund the lab under the direction of Dr. Schaffer. As of September 30, 2021 and December 31, 2020, accounts payable related to the SRAs and Machine Learning SRA was $0.0 million and $0.2 million, respectively.

Marketable Securities - Additional Information (Details)	3 Months Ended	9 Months Ended
	Sep. 30, 2021 USD ($) Security	Sep. 30, 2021 USD ($) Security	Dec. 31, 2020 Security
Debt Securities, Available-for-sale [Line Items]
Number of marketable securities \| Security			0
Realized gains on marketable securities		$ 0
Realized losses on marketable securities		$ 0
Number of marketable securities unrealized loss position more than one year \| Security	0	0
Other-than-temporary impairment on investment in debt securities available-for-sale	$ 0	$ 0
Maximum Maturity Period of Marketable Securities		2 years

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, 2021	Dec. 31, 2020
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets	$ 199,637	$ 276,726
Liabilities	214	122
Derivative Liability
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Liabilities	214	122
Short-term Marketable Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets	70,686
Long-term Marketable Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets	65,494
Money Market Funds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets	63,457	276,726
Level 1
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets	63,457	276,726
Level 1 \| Money Market Funds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets	63,457	276,726
Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets	136,180
Level 2 \| Short-term Marketable Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets	70,686
Level 2 \| Long-term Marketable Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets	65,494
Level 3
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Liabilities	214	122
Level 3 \| Derivative Liability
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Liabilities	$ 214	$ 122

Summary of Significant Accounting Policies - Schedule of Concentration of Risk (Details) (Parenthetical) - Maximum	9 Months Ended
	Sep. 30, 2021
Revenue \| Customer Concentration Risk
Concentration Risk [Line Items]
Concentration risk, percentage	10.00%
Accounts Receivable \| Credit Concentration Risk
Concentration Risk [Line Items]
Concentration risk, percentage	10.00%

Summary of Significant Accounting Policies

9 Months Ended

Sep. 30, 2021

Accounting Policies [Abstract]

Summary of Significant Accounting Policies

2. Summary of Significant Accounting Policies

Basis of Presentation

Use of Estimates and Judgements

Segment Information

The Company operates and manages its business as one reportable and operating segment.

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,
	2021	2020		2021	2020
Customer A	94%	97%		96%	97%
Customer B	*	*		*	*
Total	94%	97%		96%	97%

* Less than 10%

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


	September 30, 2021	December 31, 2020
Customer A	100%	74%
Customer B	*	26%
Total	100%	100%

* Less than 10%

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


	Three Months Ended September 30,					Nine Months Ended September 30,
	2021		2020			2021		2020
Netherlands	$	64	$	255		$	724	$	504
Switzerland		1,290		7,215			17,190		14,174
United States		12		(49	)		32		(89	)
Total revenue	$	1,366	$	7,421		$	17,946	$	14,589

Cash and Cash Equivalents

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

Marketable Securities

Other Risks and Uncertainties

Fair Value Measurements



Level 1 — Observable inputs, such as quoted prices in active markets for identical assets and liabilities.



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

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

Common Stock Warrants

Revenue Recognition

based on estimated selling prices; and (v) recognition of revenue when (or as) the Company satisfies each performance obligation.

Embedded Derivative

Net Loss Per Share Attributable to Common Stockholders

Recent Accounting Pronouncements

New Accounting Pronouncements Recently Adopted

New Accounting Pronouncements Not Yet Adopted

Commitments and Contingencies - Additional Information (Details) - USD ($) $ in Millions	1 Months Ended						3 Months Ended		9 Months Ended
	Feb. 28, 2021	Dec. 31, 2019	May 31, 2019	Nov. 30, 2018	Oct. 31, 2018	Jan. 31, 2016	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Commitment And Contingencies [Line Items]
Lease extended term date			Dec. 31, 2029			Mar. 31, 2023
Tenant improvement allowance	$ 2.3	$ 0.4		$ 0.2
Lease extended term period					2026-09
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.
Lease existence of option to renew term					true
Lease renewal term					5 years
Annual rent							$ 0.8	$ 0.8	$ 2.3	$ 2.3
Deferred rent							1.7		1.7		$ 1.6
Other Assets [Member]
Commitment And Contingencies [Line Items]
Security deposit							$ 0.6		$ 0.6		$ 0.6

Commitments and Contingencies

9 Months Ended

Sep. 30, 2021

Commitments And Contingencies Disclosure [Abstract]

Commitments and Contingencies

9. Commitments and Contingencies

Operating Lease Commitments

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 to March 31, 2023. 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 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 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. 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 and laboratory space. 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.

The Company recognizes rent expense on a straight-line basis over the lease term with the difference between the rent payments and the straight-line rent expense recorded as deferred rent. Rent expense for each of the three months ended September 30, 2021 and 2020 was $0.8 million. Rent expense for each of the nine months ended September 30, 2021 and 2020 was $2.3 million. Deferred rent as of September 30, 2021 and December 31, 2020 was $1.7 million and $1.6 million, respectively. In conjunction with the lease agreements and amendments, the Company paid security deposits of $0.6 million, which is included in other assets within the condensed balance sheets as of September 30, 2021 and December 31, 2020.

The following table summarizes the Company’s future minimum commitments under lease contracts as of September 30, 2021 (in thousands):


2021 (remaining three months)	$	756
2022		3,076
2023		3,157
2024		3,235
2025		3,262
Thereafter		11,820
Total	$	25,306

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 fair value of its estimated exposure 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, 2021. See Note 12 for details of a settlement with a former employee in April 2021.

License Arrangements - Additional Information (Details) - UC Agreements - USD ($)	1 Months Ended		3 Months Ended		9 Months Ended
	Jul. 31, 2021	Jan. 31, 2016	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Jan. 31, 2019	Dec. 31, 2018
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	$ 100,000						$ 50,000
Non-refundable license fee payable annually	10,000						$ 5,000
Incurred expenses			$ 100,000	$ 0.0	$ 200,000	$ 100,000
Maximum
Research And Development Arrangement Contract To Perform For Others [Line Items]
Development milestones	3,900,000				3,100,000
Minimum [Member]
Research And Development Arrangement Contract To Perform For Others [Line Items]
Annual royalty	$ 100,000				$ 100,000

Research and Collaboration Arrangements - Summary of Deferred Revenue (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Total deferred revenue	$ 4,738	$ 19,812
uniQure
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Total deferred revenue	3,672	4,396
Roche
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Total deferred revenue		14,318
CFF
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Total deferred revenue	$ 1,066	$ 1,098

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, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total stock-based compensation expense	$ 3,923	$ 1,147	$ 10,068	$ 3,267
Research and Development
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total stock-based compensation expense	1,893	593	4,901	1,897
General and Administrative
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total stock-based compensation expense	$ 2,030	$ 554	$ 5,167	$ 1,370

Related Party Transactions - Additional Information (Details)	1 Months Ended		3 Months Ended		9 Months Ended
	Mar. 31, 2021 USD ($)	Apr. 30, 2019 USD ($) Agreement	Sep. 30, 2021 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2021 USD ($)	Sep. 30, 2020 USD ($)	Dec. 31, 2020 USD ($)
UC Regents \| SRAs
Related Party Transaction [Line Items]
Number of sponsored research agreements entered \| Agreement		2
Agreement term		3 years
Amount due to related party		$ 1,500,000
UC Regents \| SRAs and Machine Learning SRA
Related Party Transaction [Line Items]
Accounts payable, related party			$ 0.0		$ 0.0		$ 200,000
UC Regents \| Machine Learning SRA
Related Party Transaction [Line Items]
Agreement term	3 years
Amount due to related party	$ 1,400,000
Payment to related party					400,000
David Schaffer, PhD, \| Consulting Services and Services as Board of Director
Related Party Transaction [Line Items]
Related party expenses			$ 21,000	$ 25,000	$ 64,000	$ 37,500

Stock-based Compensation

9 Months Ended

Sep. 30, 2021

Disclosure Of Compensation Related Costs Sharebased Payments [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 our common stock may be issued upon the exercise of incentive stock options. In 2021, an additional 1,334,099 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, 2021, there were 2,608,625 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.

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, 2021, options to purchase 2,704,166 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 Company's board of directors. If an individual owns stock representing more than 10% of the Company’s outstanding shares, the exercise price of each share shall be at least 110% of the fair market value on the date of grant.

No additional grants will be made under the 2015 Plan, and all outstanding grants under the 2015 Plan that are repurchased, forfeited, expire or are cancelled will become available for grant under the 2020 Plan in accordance with its terms.

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 our common stock were initially reserved for employee purchases of our common stock under terms and provisions established by the Company's board of directors and approved by our 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 of our common stock may be issued under the plan. On March 25, 2021, an additional 266,819 shares of common stock became available for future issuance under the 2020 ESPP, as a result of the operation of this automatic annual increase provision.

Under the 2020 ESPP our 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 comprised of four six-month purchase periods. The initial offering period under the ESPP will be 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 month period ended September 30, 2021:


	Number of Shares Available for Grant			Number of Shares Underlying Outstanding Options
Balances at December 31, 2020		2,606,546			3,194,113
Options authorized		1,334,099			—
Options granted		(1,424,350	)		1,424,350
Options exercised		—			(443,594	)
Options expired		354			(354	)
Options forfeited		91,976			(91,976	)
Balances at September 30, 2021		2,608,625			4,082,539
Options exercisable at September 30, 2021					1,379,288
Options vested and expected to vest at September 30, 2021					4,082,539

fair value of the Company’s stock options was estimated using the following assumptions for the three and nine months ended September 30, 2021 and 2020.


	Three Months Ended September 30,		Nine Months Ended September 30,
	2021	2020	2021	2020
Expected term	6.0 - 6.1 years	5.5 - 6.3 years	5.8 - 6.3 years	5.5 - 6.3 years
Expected volatility	80.4% - 82.8%	82.8% - 83.8%	80.4% - 84.3%	82.1% - 83.8%
Risk-free interest rate	0.8% - 1.1%	0.4% - 0.4%	0.6% - 1.1%	0.4% - 0.7%
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 estimated based on a study of publicly traded peer companies as the Company does not have sufficient trading history for its common stock. The Company selects the peer group based on similarities in industry, stage of development, size and financial leverage with the Company’s principal business operations. For each grant, the Company measured historical volatility over a period equivalent to the expected term.

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

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

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,
	2021		2020		2021		2020
Research and development	$	1,893	$	593	$	4,901	$	1,897
General and administrative		2,030		554		5,167		1,370
Total stock-based compensation expense	$	3,923	$	1,147	$	10,068	$	3,267

Condensed Statements of Operations (Unaudited) (Parenthetical) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Income Statement [Abstract]
Research and development expense, related party	$ 263	$ 173	$ 672	$ 397

401(K) Plan

9 Months Ended

Sep. 30, 2021

Compensation And Retirement Disclosure [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 Plan for eligible participants, and recorded contribution expenses of $0.1 million for each of the three month periods ended September 30, 2021 and 2020. For the nine months ended September 30, 2021 and 2020, the Company recorded contribution expenses of $0.4 million and $0.3 million, respectively.

Document and Entity Information - shares	9 Months Ended
Document and Entity Information - shares	Sep. 30, 2021	Nov. 10, 2021
Cover [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Sep. 30, 2021
Document Fiscal Year Focus	2021
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		32,138,814
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

License Arrangements

9 Months Ended

Sep. 30, 2021

License Arrangements [Abstract]

License Arrangements

8. License Arrangements

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 percentage rate range on any future sublicensing arrangements the Company may enter into with third-party licensees.

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.

During the three months ended September 30, 2021 and 2020, the Company incurred $0.1 million and $0.0 million, respectively, under the provisions of the outstanding license arrangements. During the nine months ended September 30, 2021 and 2020, the Company incurred $0.2 million and $0.1 million, respectively, of expense under the provisions of the outstanding license arrangements.

Common Stock

9 Months Ended

Sep. 30, 2021

Equity [Abstract]

Common Stock

10. Common Stock

As of each of September 30, 2021 and December 31, 2020, the Company’s certificate of incorporation authorized the Company to issue 300,000,000 shares of common stock, at a 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, 2021 and December 31, 2020, the Company has reserved common stock, on an as-converted basis, for future issuance as follows:


	September 30, 2021		December 31, 2020
Issuance of common stock under the 2020 Equity Incentive Award Plan		2,608,625		2,606,546
Issuance of common stock under the Employee Stock Purchase Plan		519,156		252,337
Exercise of options issued and outstanding		4,082,539		3,194,113
Exercise of common stock warrants		98,669		98,669
Total common stock reserved for future issuance		7,308,989		6,151,665

Condensed Statements of Redeemable Convertible Preferred Stock and Stockholders' Equity (Deficit) (Unaudited) (Parenthetical)	3 Months Ended
	Jun. 30, 2020 USD ($)
Redeemable Convertible Preferred Stock
Issuance cost for issuance of preferred stock	$ 3,138

Common Stock (Tables)

9 Months Ended

Sep. 30, 2021

Equity [Abstract]

Schedule of Common Share Reserved for Future Issuance

As of September 30, 2021 and December 31, 2020, the Company has reserved common stock, on an as-converted basis, for future issuance as follows:


	September 30, 2021		December 31, 2020
Issuance of common stock under the 2020 Equity Incentive Award Plan		2,608,625		2,606,546
Issuance of common stock under the Employee Stock Purchase Plan		519,156		252,337
Exercise of options issued and outstanding		4,082,539		3,194,113
Exercise of common stock warrants		98,669		98,669
Total common stock reserved for future issuance		7,308,989		6,151,665

Property and Equipment, Net

9 Months Ended

Sep. 30, 2021

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, 2021			December 31, 2020
Machinery and equipment	$	5,860		$	4,911
Leasehold improvements		2,527			2,527
Furniture and fixtures		464			473
Office equipment		177			101
Computer equipment and software		398			366
Construction in progress		4,333			539
Total property and equipment		13,759			8,917
Less: Accumulated depreciation and amortization		(4,970	)		(3,844	)
Property and equipment, net	$	8,789		$	5,073

All property and equipment are maintained in the United States. Depreciation expense was $0.4 million for each of the three months ended September 30, 2021 and 2020. Depreciation expense was $1.1 million for each of the nine months ended September 30, 2021 and 2020.

Fair Value Measurements

9 Months Ended

Sep. 30, 2021

Fair Value Disclosures [Abstract]

Fair Value Measurements

3. Fair Value Measurements


	Basis for Fair Value Measurements						Fair Value as of
	Level 1		Level 2		Level 3		September 30, 2021
Assets
Money market funds	$	63,457	$	—	$	—	$	63,457
Short-term marketable securities		—		70,686		—		70,686
Long-term marketable securities		—		65,494		—		65,494
Total	$	63,457	$	136,180	$	—	$	199,637
Liabilities
Derivative liability	$	—	$	—	$	214	$	214
Total	$	—	$	—	$	214	$	214


	Basis for Fair Value Measurements						Fair Value as of
	Level 1		Level 2		Level 3		December 31, 2020
Assets
Money market funds	$	276,726	$	—	$	—	$	276,726
Total	$	276,726	$	—	$	—	$	276,726
Liabilities
Derivative liability	$	—	$	—	$	122	$	122
Total	$	—	$	—	$	122	$	122

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.


	Derivative Liability
Balance as of December 31, 2020	$	122
Change in fair value included in other income (expense), net		92
Balance as of September 30, 2021	$	214


	Derivative Liability
Balance as of December 31, 2019	$	101
Change in fair value included in other income (expense), net		16
Balance as of September 30, 2020	$	117

- 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

Marketable Securities

9 Months Ended

Sep. 30, 2021

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, 2021 (in thousands):


	Amortized Cost Basis		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value as of September 30, 2021
Short-term marketable securities:
Commercial paper	$	32,440	$	—	$	(2	)	$	32,438
Corporate bonds		38,276		—		(28	)		38,248
Total short-term marketable securities	$	70,716	$	—	$	(30	)	$	70,686
Long-term marketable securities:
Commercial paper	$	5,278	$	6	$	(10	)	$	5,274
Corporate bonds		35,291		5		(157	)		35,139
US Treasuries		25,090		—		(9	)		25,081
Total long-term marketable securities	$	65,659	$	11	$	(176	)	$	65,494

All marketable securities held as of September 30, 2021, had contractual maturities of less than two years. There have been no realized gains or losses on marketable securities for the periods presented. None of the Company's investments in marketable securities has been in an unrealized loss position for more than one year. The Company determined that it has the ability and intent to hold all marketable securities that have been in a continuous loss position until maturity or recovery, thus there has been no recognition of any other-than-temporary impairment in the three and nine months ended September 30, 2021.

As of December 31, 2020, the Company had no marketable securities.

Accrued and Other Liabilities, Current and Noncurrent - Summary of Other Liabilities, Noncurrent (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Payables And Accruals [Abstract]
Deferred rent, noncurrent	$ 1,343	$ 1,237
Other payable, noncurrent	544	615
Total other liabilities, noncurrent	$ 1,887	$ 1,852

Condensed Statements of Operations (Unaudited) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Revenue:
Collaboration and license revenue	$ 1,366	$ 7,421	$ 17,946	$ 14,340
Collaboration and license revenue, related parties	0	0	0	249
Total revenue	1,366	7,421	17,946	14,589
Operating expenses:
Research and development (includes $263 and $173 for the three months ended September 30, 2021 and 2020 and $672 and $397 for the nine months ended September 30, 2021 and 2020, respectively, attributable to related parties)	15,840	11,555	43,832	40,433
General and administrative	8,187	3,682	20,683	10,398
Total operating expenses	24,027	15,237	64,515	50,831
Loss from operations	(22,661)	(7,816)	(46,569)	(36,242)
Other income (expense):
Interest income	435	4	449	148
Other expense, net	(13)	(21)	(114)	(52)
Total other income (expense)	422	(17)	335	96
Net loss	$ (22,239)	$ (7,833)	$ (46,234)	$ (36,146)
Net loss per share attributable to common stockholders, basic and diluted	$ (0.82)	$ (1.51)	$ (1.72)	$ (6.97)
Weighted-average shares outstanding used in computing net loss per share attributable to common stockholders, basic and diluted	27,022,380	5,197,982	26,818,595	5,188,628

Commitments and Contingencies - Summary of Future Minimum Commitments Under Lease Contracts (Details) $ in Thousands	Sep. 30, 2021 USD ($)
Commitments And Contingencies Disclosure [Abstract]
2021 (remaining three months)	$ 756
2022	3,076
2023	3,157
2024	3,235
2025	3,262
2026 and beyond	11,820
Total	$ 25,306

Accrued and Other Liabilities, Current and Noncurrent (Tables)

9 Months Ended

Sep. 30, 2021

Payables And Accruals [Abstract]

Summary of Accrued and Other Current Liabilities

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


	September 30, 2021		December 31, 2020
Payroll and related	$	3,683	$	3,437
Accrued clinical and preclinical study costs		636		869
Consulting and professional		1,193		3,574
Other accrued expenses		420		491
Total accrued and other current liabilities	$	5,932	$	8,371

Summary of Other Liabilities, Noncurrent

Other liabilities, noncurrent consisted of the following (in thousands):


	September 30, 2021		December 31, 2020
Deferred rent	$	1,343	$	1,237
Other payable		544		615
Total other liabilities, noncurrent	$	1,887	$	1,852

Common Stock Warrants

9 Months Ended

Sep. 30, 2021

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. As the services had been completed at the date the warrant had been issued, the fair value of the warrant was determined at the issuance date. 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 December 2020. No expense was recognized during the three months and nine months ended September 30, 2021 and 2020 with respect to the warrant shares.

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 and recorded $0.1 million within additional paid-in-capital upon issuance of the warrant. The warrant expires in 2025.

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 model.

In April 2021, the Company issued a warrant for 40,000 shares of the Company’s common stock to a former employee with an exercise price of $9.41 per share as a result of a settlement agreement with such former employee. The warrant was exercised in May 2021. The Company previously recorded an accrual within accrued and other current liabilities within the condensed balance sheet at December 31, 2020 to accrue for the settlement. Upon the issuance of the warrant in April 2021 in connection with the settlement, $1.1 million was reclassified from accrued and other current liabilities to additional paid-in capital within the condensed balance sheet as of September 30, 2021.

For the three months and nine months ended September 30, 2021, expense related to the vested portion of warrant shares was immaterial and recorded within operating expenses in the condensed statements of operations and within additional paid-in-capital on the condensed balance sheet.

Summary of Significant Accounting Policies - Schedule of Revenues by Geographic Region (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Disaggregation Of Revenue [Line Items]
Revenues	$ 1,366	$ 7,421	$ 17,946	$ 14,589
Netherlands
Disaggregation Of Revenue [Line Items]
Revenues	64	255	724	504
Switzerland
Disaggregation Of Revenue [Line Items]
Revenues	1,290	7,215	17,190	14,174
United States
Disaggregation Of Revenue [Line Items]
Revenues	$ 12	$ (49)	$ 32	$ (89)

- Definition

+ References

+ Details

The Company - Additional Information (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended
	Nov. 30, 2021	Dec. 31, 2020	Sep. 30, 2021	Jan. 01, 2021	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019
Organization And Nature Of Business [Line Items]
Outstanding shares of redeemable convertible preferred stock				0	11,575,984	11,575,984	7,375,631	7,375,631
Accumulated deficit		$ 135,679	$ 181,913
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		$ 23.00
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 \| Follow-On Offering \| Forecast
Organization And Nature Of Business [Line Items]
Number of shares priced under public offering	4,750,000
Stock sold and issued price per share	$ 25.00
Aggregate net proceeds, after deducting underwriting discounts and commissions and offering costs	$ 110,800
Common Stock \| Exercise of Overallotment Option
Organization And Nature Of Business [Line Items]
Number of shares sold and issued		1,260,000
Common Stock \| Exercise of Overallotment Option \| Forecast
Organization And Nature Of Business [Line Items]
Number of shares priced under public offering	712,500
Number of days for option to purchase additional shares	30 days

Property and Equipment, Net (Tables)

9 Months Ended

Sep. 30, 2021

Property Plant And Equipment [Abstract]

Schedule of Property and Equipment, Net

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


	September 30, 2021			December 31, 2020
Machinery and equipment	$	5,860		$	4,911
Leasehold improvements		2,527			2,527
Furniture and fixtures		464			473
Office equipment		177			101
Computer equipment and software		398			366
Construction in progress		4,333			539
Total property and equipment		13,759			8,917
Less: Accumulated depreciation and amortization		(4,970	)		(3,844	)
Property and equipment, net	$	8,789		$	5,073

Accrued and Other Liabilities, Current and Noncurrent

9 Months Ended

Sep. 30, 2021

Payables And Accruals [Abstract]

Accrued and Other Liabilities, Current and Noncurrent

6. Accrued and Other Liabilities, Current and Noncurrent

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


	September 30, 2021		December 31, 2020
Payroll and related	$	3,683	$	3,437
Accrued clinical and preclinical study costs		636		869
Consulting and professional		1,193		3,574
Other accrued expenses		420		491
Total accrued and other current liabilities	$	5,932	$	8,371

Other liabilities, noncurrent consisted of the following (in thousands):


	September 30, 2021		December 31, 2020
Deferred rent	$	1,343	$	1,237
Other payable		544		615
Total other liabilities, noncurrent	$	1,887	$	1,852

Subsequent Events - Additional Information (Details) - USD ($) $ / shares in Units, $ in Millions	1 Months Ended
	Nov. 30, 2021	Oct. 31, 2021	Dec. 31, 2020	Apr. 30, 2020
Funding Agreement Product \| CFF
Subsequent Event [Line Items]
Additional funding upon acceptance of investigational New Drug application				$ 4.0
Subsequent Event \| Funding Agreement Product \| CFF
Subsequent Event [Line Items]
Additional investment in cash		$ 4.0
Subsequent Event \| Funding Agreement Product \| CFF \| Common Stock
Subsequent Event [Line Items]
Purchase shares of common stock		125,715
Overallotment Option to Purchase Additional Shares \| Common Stock
Subsequent Event [Line Items]
Purchase shares of common stock			1,260,000
Overallotment Option to Purchase Additional Shares \| Forecast \| Common Stock
Subsequent Event [Line Items]
Number of shares priced under public offering	712,500
Number of days for option to purchase additional shares	30 days
Follow-On Offering \| Forecast \| Common Stock
Subsequent Event [Line Items]
Number of shares priced under public offering	4,750,000
Stock issued price per share	$ 25.00
Aggregate net proceeds, after deducting underwriting discounts and commissions and offering costs	$ 110.8

Summary of Significant Accounting Policies - Schedule of Concentration of Risk (Details)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Revenue \| Customer Concentration Risk \| Customer A
Concentration Risk [Line Items]
Concentration risk, percentage	94.00%	97.00%	96.00%	97.00%
Revenue \| Customer Concentration Risk \| Customer
Concentration Risk [Line Items]
Concentration risk, percentage	94.00%	97.00%	96.00%	97.00%
Accounts Receivable \| Credit Concentration Risk \| Customer A
Concentration Risk [Line Items]
Concentration risk, percentage			100.00%		74.00%
Accounts Receivable \| Credit Concentration Risk \| Customer B
Concentration Risk [Line Items]
Concentration risk, percentage					26.00%
Accounts Receivable \| Credit Concentration Risk \| Customer
Concentration Risk [Line Items]
Concentration risk, percentage			100.00%		100.00%

Net Loss Per Share Attributable to Common Stockholders - Outstanding Potentially Dilutive Securities Excluded From Computation of Diluted Net Loss Per Share (Details) - shares	9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive securities	4,278,363	14,572,974
Redeemable Convertible Preferred Stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive securities	0	11,575,984
Options Issued and Outstanding
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive securities	4,082,539	2,928,321
ESPP Shares Issuable and Outstanding
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive securities	97,155	0
Common Stock Warrants
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive securities	98,669	68,669

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, 2021	Sep. 30, 2020
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Beginning Balance	$ 122	$ 101
Change in fair value included in other income (expense), net	$ 92	$ 16
Fair Value, Recurring Basis, Unobservable Input Reconciliation, Liability, Gain (Loss), Statement of Income [Extensible List]	us-gaap:OtherNonoperatingIncomeExpense	us-gaap:OtherNonoperatingIncomeExpense
Ending Balance	$ 214	$ 117

Summary of Significant Accounting Policies - Additional Information (Details)	9 Months Ended
	Sep. 30, 2021 Financial_Institution Segment shares	Dec. 31, 2020 shares
Summary Of Significant Accounting Policies [Line Items]
Number of reportable segments	1
Number of operating segments	1
Number of financial institutions cash is held \| Financial_Institution	2
Preferred stock, shares outstanding \| shares	0	0
Accounting Standard Update ("ASU") 2018-18
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, 2021
Change in accounting principle accounting standards update, immaterial effect	true
Accounting Standard Update ("ASU") 2019-12
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, 2021
Change in accounting principle accounting standards update, immaterial effect	true

Net Loss Per Share Attributable to Common Stockholders - Computation of Basic and Diluted Net Loss Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Numerator
Net loss attributable to common stockholders	$ (22,239)	$ (7,833)	$ (46,234)	$ (36,146)
Denominator
Weighted-average shares outstanding used in computing net loss per share attributable to common stockholders, basic and diluted	27,022,380	5,197,982	26,818,595	5,188,628
Net loss per share attributable to common stockholders, basic and diluted	$ (0.82)	$ (1.51)	$ (1.72)	$ (6.97)

- Definition

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

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 250
-SubTopic 10
-Section 50
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=109234566&loc=d3e22595-107794

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 260
-SubTopic 10
-Section 45
-Paragraph 10
-URI http://asc.fasb.org/extlink&oid=121326447&loc=d3e1448-109256

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 250
-SubTopic 10
-Section 50
-Paragraph 11
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=109234566&loc=d3e22694-107794

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 260
-SubTopic 10
-Section 45
-Paragraph 11
-URI http://asc.fasb.org/extlink&oid=121326447&loc=d3e1377-109256

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 260
-SubTopic 10
-Section 50
-Paragraph 1
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=6371337&loc=d3e3550-109257

Reference 6: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 22
-URI http://asc.fasb.org/extlink&oid=120311839&loc=d3e8736-108599

Reference 7: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 250
-SubTopic 10
-Section 50
-Paragraph 11
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=109234566&loc=d3e22694-107794

Reference 8: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 260
-SubTopic 10
-Section 45
-Paragraph 60B
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=121326447&loc=SL5780133-109256

Reference 9: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 32
-Subparagraph (f)
-URI http://asc.fasb.org/extlink&oid=120311839&loc=d3e8933-108599

Reference 10: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 31
-URI http://asc.fasb.org/extlink&oid=120311839&loc=d3e8924-108599

Reference 11: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 30
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=120311839&loc=d3e8906-108599

Reference 12: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 250
-SubTopic 10
-Section 50
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=109234566&loc=d3e22583-107794

Reference 13: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 32
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=120311839&loc=d3e8933-108599

+ 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: 2021-10-01
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue, estimated performance period	4 years