0000950170-24-094010 | 10-Q | XBRL Viewer | 4D Molecular Therapeutics INC

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Insider Trading Arrangements	3 Months Ended
Insider Trading Arrangements	Jun. 30, 2024
Trading Arrangements, by Individual
Rule 10b5-1 Arrangement Adopted	false
Non-Rule 10b5-1 Arrangement Adopted	false
Rule 10b5-1 Arrangement Terminated	false
Non-Rule 10b5-1 Arrangement Terminated	false

Commitments and Contingencies

6 Months Ended

Jun. 30, 2024

Commitments and Contingencies Disclosure [Abstract]

Commitments and Contingencies

9. Commitments and Contingencies

Operating Lease Commitments

5980 Horton Street Building Lease

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

The 5980 Horton Street Building Lease is considered an operating lease under ASC 842 as it does not meet the criteria of a finance lease. As of June 30, 2024, the operating lease right-of-use asset and operating lease liability were $1.0 million and $1.2 million, respectively. As of December 31, 2023, the operating lease right-of-use asset and operating lease liability were $1.2 million and $1.4 million, respectively. The discount rate used to determine the lease liability was 7.5%.

5858 Horton Street Building Lease and Expansion

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

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

The 5858 Horton Street Building Lease and 2019 Expansion are considered operating leases under ASC 842 as they do not meet the criteria of a finance lease. As of June 30, 2024, the operating lease right-of-use asset and operating lease liability was $9.7 million and $12.5 million, respectively. As of December 31, 2023, the operating lease right-of-use asset and operating lease liability was $10.3 million and $13.2 million, respectively. The discount rate used to determine the lease liability was 7.8%.

In July 2024, the lease term for the 5858 Horton Street Building Lease and 2019 Expansion was extended for a period of 12 months and will expire on December 31, 2030. Neither of the leases include a general option for the Company to terminate the leases.

In July 2024, the Company entered into a lease for additional office and laboratory space in Emeryville, California (the "2024 Expansion") with a projected lease commencement date of September 1, 2024. The lease consists of approximately 32,038 square feet of office and laboratory space and has a lease term through December 31, 2030. Future minimum lease payments under the 2024 Expansion from its projected commencement date through lease expiration are approximately $13.2 million. As of June 30, 2024, the Company has not recorded a right-of-use asset or lease liability on its condensed balance sheets as the Company has not yet obtained control of the additional office and laboratory space.

Emeryville, California Warehouse Lease

In January 2024, the Company entered into a new lease for warehouse space in Emeryville, California (the “Warehouse Lease Agreement”), with a projected lease commencement date of August 31, 2024. The lease consists of approximately 7,800 square feet of warehouse space and has a lease term of five years and four months. Future minimum lease payments under the Warehouse Lease Agreement from its projected commencement date through lease expiration in the fourth quarter of 2029 are approximately $1.5 million. As of June 30, 2024, the Company has not recorded a right-of-use asset or lease liability on its condensed balance sheets as the Company has not yet obtained control of the warehouse space.

The following table summarizes the components of lease expense for the three and six months ended June 30, 2024 and 2023, which are included in operating expenses in the Company’s condensed statements of operations (in thousands):


	Three Months Ended June 30,				Six Months Ended June 30,
	2024		2023		2024		2023
Operating lease cost	$	698	$	698	$	1,397	$	1,397
Variable lease cost		354		311		707		622
Total	$	1,052	$	1,009	$	2,104	$	2,019

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

The following table summarizes supplemental information related to operating leases:


	June 30, 2024			December 31, 2023
Weighted-average remaining lease term (in years):		5.2			5.7
Weighted-average discount rate:		7.8	%		7.8	%

The following table summarizes the maturities of lease liabilities as of June 30, 2024 (in thousands):


2024 (remaining six months)	$	1,588
2025		3,244
2026		3,135
2027		2,810
2028		2,894
Thereafter		2,982
Total future minimum lease payments		16,653
Less: Amount representing interest		(2,997	)
Present value of future minimum lease payments		13,656
Less: Current portion of operating lease liabilities		3,196
Long-term portion of operating lease liabilities	$	10,460

Indemnification Agreements

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

Legal Proceedings

From time to time, the Company may become involved in legal proceedings arising from the ordinary course of its business. If applicable, the Company records a legal liability when it believes that it is both probable that a liability may be imputed, and the amount of the liability can be reasonably estimated. Significant judgment by the Company is required to determine both probability and the estimated amount. There are no material legal proceedings outstanding at June 30, 2024.

License Arrangements - Additional Information (Details) - USD ($)			1 Months Ended			3 Months Ended		6 Months Ended
	Jul. 05, 2023	Apr. 21, 2023	Jul. 31, 2021	Jan. 31, 2019	Jan. 31, 2016	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023	Dec. 31, 2018
Astellas GeneTherapies Inc
Research And Development Arrangement Contract To Perform For Others [Line Items]
Upfront payments received	$ 20,000,000
Deferred revenue						$ 0		$ 0
Accounts receivable						0		0
Astellas GeneTherapies Inc \| Maximum
Research And Development Arrangement Contract To Perform For Others [Line Items]
Future option fees and milestones	$ 942,500,000
Aevitas Therapeutics, Inc. Agreement
Research And Development Arrangement Contract To Perform For Others [Line Items]
Upfront consideration payment		$ 0
Liability related to contingent consideration						0		0
Aevitas Therapeutics, Inc. Agreement \| Maximum
Research And Development Arrangement Contract To Perform For Others [Line Items]
Potential cash payment upon achievement of certain late-stage development, regulatory and sales milestones		144,000,000
License Agreement for sCFH with University of Pennsylvania \| Maximum
Research And Development Arrangement Contract To Perform For Others [Line Items]
Potential cash payment upon achievement of certain late-stage development, regulatory and sales milestones		$ 42,000,000
UC Regents and the Trustees of the University of Pennsylvania
Research And Development Arrangement Contract To Perform For Others [Line Items]
Amount paid non-refundable license fee			$ 100,000
Non-refundable license fee payable annually			10,000
Incurred expenses						$ 0	$ 0	$ 0	$ 0
UC Regents and the Trustees of the University of Pennsylvania \| Maximum
Research And Development Arrangement Contract To Perform For Others [Line Items]
Contingent payments development milestones			3,900,000
UC Regents and the Trustees of the University of Pennsylvania \| Minimum
Research And Development Arrangement Contract To Perform For Others [Line Items]
Annual royalty			$ 100,000
UC Agreements
Research And Development Arrangement Contract To Perform For Others [Line Items]
Non-refundable license fee payable upon execution										$ 5,000
Non-refundable license fee payable upon execution every year thereafter until sales of a licensed product and payment of royalties										5,000
Amount obligated to pay upon closing first qualified financing at option of UC Regents										$ 50,000
Percentage of equity interest upon closing of first qualified financing at option of UC Regents										3.00%
Percentage of equity interests issued as common stock					6.00%
Amount paid non-refundable license fee				$ 50,000
Non-refundable license fee payable annually				5,000
UC Agreements \| Maximum
Research And Development Arrangement Contract To Perform For Others [Line Items]
Contingent payments development milestones				3,100,000
UC Agreements \| Minimum
Research And Development Arrangement Contract To Perform For Others [Line Items]
Annual royalty				$ 100,000

Summary of Significant Accounting Policies - Schedule of Concentration of Risk (Details) (Parenthetical) - Revenue - Customer Concentration Risk	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Customer A
Concentration Risk [Line Items]
Concentration risk, percentage	0.00%	96.00%	0.00%	98.00%
Customer B
Concentration Risk [Line Items]
Concentration risk, percentage	100.00%		100.00%
Maximum \| Customer B
Concentration Risk [Line Items]
Concentration risk, percentage			10.00%

Condensed Balance Sheets (Unaudited) - USD ($) $ in Thousands	Jun. 30, 2024	Dec. 31, 2023
Current assets:
Cash and cash equivalents	$ 189,061	$ 249,108
Marketable securities	352,885	39,124
Prepaid expenses and other current assets	10,687	8,356
Total current assets	552,633	296,588
Marketable securities, long-term	35,715	10,954
Property and equipment, net	18,761	20,126
Operating lease right-of-use assets, net	10,688	11,539
Other assets	2,320	684
Total assets	620,117	339,891
Current liabilities
Accounts payable	3,431	3,515
Accrued and other current liabilities	12,776	12,014
Deferred revenue	0	273
Operating lease liabilities, current portion	3,196	3,149
Total current liabilities	19,403	18,951
Deferred revenue, net of current portion	1,318	972
Derivative liability	382	369
Operating lease liabilities, long-term portion	10,460	11,522
Other liabilities	214	248
Total liabilities	31,777	32,062
Commitments and contingencies (Note 9)
Stockholders’ equity
Preferred stock, $0.0001 par value, 10,000,000 shares authorized at June 30, 2024 and December 31, 2023; no shares issued and outstanding at June 30, 2024 and December 31, 2023	0	0
Common stock, $0.0001 par value, 300,000,000 shares authorized at June 30, 2024 and December 31, 2023; 51,914,275 and 43,075,218 shares issued and outstanding at June 30, 2024 and December 31, 2023, respectively	5	4
Additional paid-in-capital	1,071,309	723,136
Accumulated other comprehensive gain (loss)	(293)	16
Accumulated deficit	(482,681)	(415,327)
Total stockholders’ equity	588,340	307,829
Total liabilities and stockholders’ equity	$ 620,117	$ 339,891

Net Loss Per Share, Basic and Diluted - Additional Information (Details) - $ / shares	Feb. 29, 2024	Dec. 31, 2020	May 31, 2018
Class of Warrant or Right [Line Items]
Exercise price		$ 18	$ 3.19
Common Stock \| 2024 Follow On Public Offering
Class of Warrant or Right [Line Items]
Warrants to purchase shares of common stock	3,583,476
Common Stock \| Pre-funded Warrants \| 2024 Follow On Public Offering
Class of Warrant or Right [Line Items]
Warrants to purchase shares of common stock	3,583,476
Exercise price	$ 0.0001

Commitments and Contingencies - Summary of Supplemental Information Related to Leases - (Details)	Jun. 30, 2024	Dec. 31, 2023
Commitments and Contingencies Disclosure [Abstract]
Weighted-average remaining lease term (in years)	5 years 2 months 12 days	5 years 8 months 12 days
Weighted-average discount rate	7.80%	7.80%

Common Stock

6 Months Ended

Jun. 30, 2024

Equity [Abstract]

Common Stock

11. Common Stock

As of June 30, 2024 and December 31, 2023, the Company’s certificate of incorporation authorized the Company to issue 300,000,000 shares of common stock, at the par value of $0.0001 per share. The holder of each share of common stock is entitled to one vote per share.

Common stockholders are entitled to dividends, if and when declared by the board of directors. To date, no dividends on common stock have been declared by the board of directors.

As of June 30, 2024 and December 31, 2023, the Company has reserved common stock, on an as-converted basis, for future issuance as follows:


	June 30, 2024		December 31, 2023
Issuance of common stock under the 2020 Equity Incentive Award Plan		2,332,657		524,078
Issuance of common stock under the 2020 Employee Stock Purchase Plan		273,375		313,034
Exercise of options issued and outstanding and future vesting of restricted stock units		8,450,290		8,423,481
Exercise of common stock warrants		3,637,145		53,669
Total common stock reserved for future issuance		14,693,467		9,314,262

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

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

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

The Company was committed to providing an amount equal to the funding provided by CFF to be used solely to advance the Funding Agreement Product. As of June 30, 2024, the funding commitment has been fulfilled. Under the terms of the Funding Agreement, neither the $10.0 million investment in the Series C redeemable convertible preferred stock, which converted to common stock as of December 31, 2020, nor the $4.0 million of funding upon Acceptance are restricted as to withdrawal or usage.

- Definition

The entire disclosure for equity.

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+ Details

Stock-based Compensation - Summary of Restricted Stock Units (RSU) (Details) - Restricted Stock Units (RSU)	6 Months Ended
	Jun. 30, 2024 shares
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Number of Shares, Unvested Beginning Balance	181,181
Issued	129,650
Vested	(21,963)
Canceled/Forfeited	(30,751)
Number of Shares, Unvested Ending Balance	258,117

- Definition

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

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Reference 28: http://www.xbrl.org/2003/role/disclosureRef
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+ Details

The Company

6 Months Ended

Jun. 30, 2024

Organization, Consolidation and Presentation of Financial Statements [Abstract]

The Company

1. The Company

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

Initial Public Offering

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

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

2021 Follow On Public Offering

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

2023 Follow On Public Offering

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

2024 Follow On Public Offering

In February 2024, the Company completed an underwritten public offering (the “2024 Offering”) in which 6,586,015 shares of the Company’s common stock were sold at an offering price of $29.50 per share, as well as pre-funded warrants to purchase 3,583,476 shares of the Company’s common stock at an offering price of $29.4999 per underlying share pursuant to an effective Registration Statement on Form S-3. The net proceeds from the 2024 Offering were $281.2 million, after deducting underwriting discounts and commissions and other offering expenses. The Company also granted the underwriters the option to purchase up to 1,525,423 additional shares of common stock in connection with the offering.

In March 2024, the underwriters exercised their option to purchase 1,259,299 additional shares of common stock resulting in net proceeds of $34.9 million, after deducting commissions.

Liquidity

The Company has incurred significant losses and negative cash flows from operations and had an accumulated deficit of $482.7 million as of June 30, 2024. The Company believes that its cash and cash equivalents and marketable securities as of June 30, 2024 are sufficient for the Company to fund planned operations for at least one year from the issuance date of these unaudited condensed financial statements. The Company has historically financed its operations primarily through the sale of equity securities, and to

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

Common Stock Warrants

6 Months Ended

Jun. 30, 2024

Equity [Abstract]

Common Stock Warrants

13. Common Stock Warrants

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

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

In February 2024, the Company completed an underwritten public follow-on offering which included the sale of pre-funded warrants (the “Pre-funded Warrants”) to purchase 3,583,476 shares of the Company’s common stock at an offering price of $29.4999. The exercise price of each Pre-funded Warrant is $0.0001 per share, and each Pre-funded Warrant is exercisable from the date of issuance. The Pre-funded Warrants are classified as a component of stockholders’ equity within additional paid-in-capital. The Company valued the Pre-funded Warrants at issuance and recorded net proceeds of $99.4 million after deducting underwriters fees during the six months ended June 30, 2024 related to the sale of the Pre-funded Warrants.

There was no material expense related to the above warrant shares during each of the three and six months ended June 30, 2024 and 2023.

Stock-based Compensation - Additional Information (Details)	6 Months Ended
	Jun. 30, 2024 Period shares	Feb. 29, 2024 shares	Dec. 31, 2023 shares	Dec. 31, 2020 shares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of shares reserved for issuance	14,693,467		9,314,262
Number of options outstanding	8,192,173		8,240,154
Expected dividend yield	0.00%
2020 Equity Incentive Award Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of shares reserved for issuance	2,332,657		524,078	2,606,546
Percentage of common stock outstanding increased annually for future issuance	5.00%
Number of shares available for grant	1,997,052	2,153,533
2020 Equity Incentive Award Plan \| Restricted Stock Units (RSU)
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Vesting period	4 years
2020 Equity Incentive Award Plan \| Maximum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of shares authorized	18,000,000
Options exercisable period	10 years
2015 Equity Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of shares available for grant	335,605
Options exercisable period	10 years
Number of options outstanding	1,545,023
Number of additional shares available for grant	0
2020 ESPP
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of shares reserved for issuance	273,375		313,034	252,337
Number of shares available for grant		50,000
Increase in shares available for issuance percentage	1.00%
Employee purchase price of common stock as percentage of fair market value	85.00%
Common stock overlapping offering period	24 months
Number of duration purchase period \| Period	4
Percentage of maximum contributions of employee's eligible compensation	15.00%
2020 ESPP \| Maximum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of shares authorized	15,000,000

Condensed Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023
Cash flows from operating activities
Net loss	$ (67,354)	$ (58,298)
Adjustments to reconcile net loss to net cash used in operating activities
Stock-based compensation expense	12,048	9,190
Vesting of common stock warrant in return for services	44	44
Change in fair value of derivative liability	13	0
Depreciation and amortization	2,367	2,108
Amortization of right-of-use assets	851	732
Net amortization (accretion) of premium (discount) on marketable securities	(3,092)	(615)
Changes in operating assets and liabilities
Prepaid expenses and other current assets	(2,330)	(475)
Other assets	(1,636)	0
Accounts payable	(83)	614
Accrued and other liabilities	785	(2,239)
Deferred revenue	72	(538)
Operating lease liabilities	(1,016)	(494)
Net cash used in operating activities	(59,331)	(49,971)
Cash flows from investing activities
Purchase of marketable securities	(374,084)	(20,156)
Maturities of marketable securities	38,346	114,391
Acquisition of property and equipment	(1,059)	(1,817)
Net cash provided by (used in) investing activities	(336,797)	92,418
Cash flows from financing activities
Issuance of Common Stock upon Exercise of Stock Options and Warrants	3,963	2,029
Issuance of common stock - 2020 ESPP	707	658
Net cash provided by financing activities	336,081	141,513
Net increase (decrease) in cash and cash equivalents	(60,047)	183,960
Cash and cash equivalents, beginning of period	249,108	52,351
Cash and cash equivalents, end of period	189,061	236,311
Supplemental disclosures of noncash investing and financing information
Unpaid offering costs	0	183
Purchases of property and equipment in accounts payable and accrued and other liabilities	177	312
Public offering
Cash flows from financing activities
Issuance of common stock for the offering program, net of issuance costs	316,148	129,720
Payments Of Stock Issuance Costs	0	(329)
ATM Offering Program
Cash flows from financing activities
Issuance of common stock for the offering program, net of issuance costs	15,263	9,647
Payments Of Stock Issuance Costs	$ 0	$ (212)

Research and Collaboration Arrangements - Additional Information (Details) - USD ($)		1 Months Ended	3 Months Ended		6 Months Ended
	Jul. 05, 2023	Jan. 31, 2014	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023	Dec. 31, 2023	Aug. 31, 2019
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue recognized					$ 0	$ 500,000
Deferred revenue			$ 1,318,000		1,318,000		$ 1,245,000
Astellas GeneTherapies Inc
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront payments received	$ 20,000,000
uniQure
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Accounts receivable			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
uniQure \| Amended uniQure Agreement \| Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contingent payments eliminated								$ 5,000,000
uniQure \| Amended uniQure Agreement and Second uniQure Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue recognized			0	$ 200,000	0	$ 500,000
Deferred revenue			0		0		0
Remaining performance obligations			0		0		$ 0
Arbor Biotechnologies Inc \| Arbor Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Shared product designated by JSC			0		0
Research and development expenses			$ 0		$ 0

Fair Value Measurements

6 Months Ended

Jun. 30, 2024

Fair Value Disclosures [Abstract]

Fair Value Measurements

3. Fair Value Measurements

The following tables represent the Company’s fair value hierarchy for financial assets and financial liabilities measured at fair value on a recurring basis as of June 30, 2024 and December 31, 2023 (in thousands):


	Basis for Fair Value Measurements						Fair Value as of
	Level 1		Level 2		Level 3		June 30, 2024
Assets
Money market funds	$	146,308	$	—	$	—	$	146,308
Certificates of deposit		—		69,677		—		69,677
Commercial paper		—		96,220		—		96,220
Corporate bonds		—		203,733		—		203,733
U.S. Treasuries		—		18,970		—		18,970
Total	$	146,308	$	388,600	$	—	$	534,908
Liabilities
Derivative liability	$	—	$	—	$	382	$	382
Total	$	—	$	—	$	382	$	382


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

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 Cystic Fibrosis Foundation (CFF), the probability of a change of control event, the product status at time of a change of control event and the discount rate. See Note 15 for further discussion on the embedded derivative.

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

The following tables set forth a summary of the changes in the fair value of the Company’s Level 3 financial instrument during the six months ended June 30, 2024 and 2023 (in thousands):


	Derivative Liability
Balance as of December 31, 2023	$	369
Change in fair value included in other income (expense), net		13
Balance as of June 30, 2024	$	382


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

- 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

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Topic 820
-SubTopic 10
-Name Accounting Standards Codification
-Section 55
-Paragraph 107
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482078/820-10-55-107

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-Topic 820
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Reference 3: http://www.xbrl.org/2003/role/disclosureRef
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Reference 4: http://www.xbrl.org/2003/role/disclosureRef
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Reference 6: http://www.xbrl.org/2003/role/disclosureRef
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+ Details

Related Party Transactions - Additional Information (Details) - Reignite Agreement $ in Thousands	1 Months Ended	3 Months Ended	6 Months Ended
	Mar. 31, 2024 USD ($) Product	Jun. 30, 2024 USD ($)	Jun. 30, 2024 USD ($)
Related Party Transaction [Line Items]
Research and development expenses		$ 400	$ 400
Option exercise fee		0	0
Option exercise fees per selected capsid	$ 1,000
Amount due to related party		$ 0	$ 0
Maximum
Related Party Transaction [Line Items]
Related party expenses	1,500
Equipment costs	$ 60
Number of capsids \| Product	3
Option exercise fee	$ 3,000

Commitments and Contingencies- Summary of Components of Lease Expense - (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Commitments and Contingencies Disclosure [Abstract]
Operating lease cost	$ 698	$ 698	$ 1,397	$ 1,397
Variable lease cost	354	311	707	622
Total	$ 1,052	$ 1,009	$ 2,104	$ 2,019

Property and Equipment, Net - Schedule of Property and Equipment, Net (Details) - USD ($) $ in Thousands	Jun. 30, 2024	Dec. 31, 2023
Property Plant And Equipment [Line Items]
Total property and equipment	$ 33,061	$ 32,058
Less: Accumulated depreciation and amortization	(14,300)	(11,932)
Property and equipment, net	18,761	20,126
Machinery and Equipment
Property Plant And Equipment [Line Items]
Total property and equipment	13,766	12,996
Leasehold Improvements
Property Plant And Equipment [Line Items]
Total property and equipment	17,325	16,902
Furniture and Fixtures
Property Plant And Equipment [Line Items]
Total property and equipment	566	566
Office Equipment
Property Plant And Equipment [Line Items]
Total property and equipment	239	239
Computer Equipment and Software
Property Plant And Equipment [Line Items]
Total property and equipment	866	805
Construction in Progress
Property Plant And Equipment [Line Items]
Total property and equipment	$ 299	$ 550

Commitments and Contingencies - Summary of Maturities of Lease Liabilities (Details) - USD ($) $ in Thousands	Jun. 30, 2024	Dec. 31, 2023
Commitments and Contingencies Disclosure [Abstract]
2024 (remaining six months)	$ 1,588
2025	3,244
2026	3,135
2027	2,810
2028	2,894
Thereafter	2,982
Total future minimum lease payments	16,653
Less: Amount representing interest	(2,997)
Present value of future minimum lease payments	13,656
Less: Current portion of operating lease liabilities	3,196	$ 3,149
Long-term portion of operating lease liabilities	$ 10,460	$ 11,522

Property and Equipment, Net

6 Months Ended

Jun. 30, 2024

Property, Plant and Equipment [Abstract]

Property and Equipment, Net

5. Property and Equipment, Net

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


	June 30, 2024			December 31, 2023
Machinery and equipment	$	13,766		$	12,996
Leasehold improvements		17,325			16,902
Furniture and fixtures		566			566
Office equipment		239			239
Computer equipment and software		866			805
Construction in progress		299			550
Total property and equipment		33,061			32,058
Less: Accumulated depreciation and amortization		(14,300	)		(11,932	)
Property and equipment, net	$	18,761		$	20,126

All property and equipment are maintained in the United States. Depreciation expense was $1.2 million and $1.1 million for the three months ended June 30, 2024 and 2023, respectively, and $2.4 million and $2.1 million for the six months ended June 30, 2024 and 2023, respectively.

Document and Entity Information - shares	6 Months Ended
Document and Entity Information - shares	Jun. 30, 2024	Aug. 02, 2024
Document Information [Line Items]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Jun. 30, 2024
Document Fiscal Year Focus	2024
Document Fiscal Period Focus	Q2
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 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
Pre-funded Warrants [Member]
Document Information [Line Items]
Entity Common Stock, Shares Outstanding		3,583,476
Common Stock [Member]
Document Information [Line Items]
Entity Common Stock, Shares Outstanding		51,969,590

Marketable Securities - Summary of Available for Sale Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2024	Dec. 31, 2023
Short-term Marketable Securities
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost Basis	$ 353,151	$ 39,144
Gross Unrealized Gains	39	13
Gross Unrealized Losses	(305)	(33)
Fair Value	352,885	39,124
Long-term Marketable Securities
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost Basis	35,741	10,918
Gross Unrealized Gains	20	36
Gross Unrealized Losses	(46)	0
Fair Value	35,715	10,954
Commercial Paper \| Short-term Marketable Securities
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost Basis	95,009	17,603
Gross Unrealized Gains	8	1
Gross Unrealized Losses	(68)	(26)
Fair Value	94,949	17,578
Commercial Paper \| Long-term Marketable Securities
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost Basis	1,275	661
Gross Unrealized Gains	0	4
Gross Unrealized Losses	(4)	0
Fair Value	1,271	665
Corporate Bonds \| Short-term Marketable Securities
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost Basis	169,491	18,641
Gross Unrealized Gains	10	12
Gross Unrealized Losses	(212)	(3)
Fair Value	169,289	18,650
Corporate Bonds \| Long-term Marketable Securities
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost Basis	34,466	10,257
Gross Unrealized Gains	20	32
Gross Unrealized Losses	(42)	0
Fair Value	34,444	10,289
U.S. Treasuries \| Short-term Marketable Securities
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost Basis	18,977
Gross Unrealized Gains	0
Gross Unrealized Losses	(7)
Fair Value	18,970
U.S. Government Sponsored Agencies \| Short-term Marketable Securities
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost Basis		2,900
Gross Unrealized Gains		0
Gross Unrealized Losses		(4)
Fair Value		$ 2,896
Certificates of Deposit \| Short-term Marketable Securities
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost Basis	69,674
Gross Unrealized Gains	21
Gross Unrealized Losses	(18)
Fair Value	$ 69,677

Research and Collaboration Arrangements - Summary of Collaboration and License Revenue (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Total revenue	$ 5	$ 239	$ 33	$ 538
Collaboration and License Revenue
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Total revenue	5	239	33	538
Collaboration and License Revenue \| uniQure
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Total revenue	0	230	0	525
Collaboration and License Revenue \| Cystic Fibrosis Foundation
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Total revenue	$ 5	$ 9	$ 33	$ 13

- 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).

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+ Details

Stock-based Compensation - Schedule of Assumptions Used in Black-Scholes Valuation Model to Estimate Fair Value of Stock Options (Details)	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected volatility, minimum	84.90%	81.10%	81.40%	80.90%
Expected volatility, maximum	85.40%	82.20%	85.40%	84.40%
Risk-free interest rate, minimum	4.40%	3.50%	3.90%	3.50%
Risk-free interest rate, maximum	4.50%	4.00%	4.50%	4.00%
Expected dividend yield			0.00%
Minimum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected term	6 years	6 years	6 years	6 years
Maximum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected term	6 years 1 month 6 days	6 years 1 month 6 days	6 years 1 month 6 days	6 years 1 month 6 days

- Definition

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

+ References

+ Details

Condensed Statements of Comprehensive Loss (Unaudited) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Statement of Comprehensive Income [Abstract]
Net loss	$ (34,953)	$ (29,616)	$ (67,354)	$ (58,298)
Other comprehensive loss:
Net unrealized gain (loss) on marketable securities	(184)	615	(309)	1,541
Total comprehensive loss	$ (35,137)	$ (29,001)	$ (67,663)	$ (56,757)

- Definition

Amount after tax of increase (decrease) in equity from transactions and other events and circumstances from net income and other comprehensive income, attributable to parent entity. Excludes changes in equity resulting from investments by owners and distributions to owners.

+ References + Details

- Definition

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

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-Topic 220
-SubTopic 10
-Name Accounting Standards Codification
-Section 45
-Paragraph 1B
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482790/220-10-45-1B

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-Topic 942
-SubTopic 220
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-04(22))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147478524/942-220-S99-1

+ Details

Common Stock - Schedule of Common Share Reserved for Future Issuance (Details) - shares	Jun. 30, 2024	Dec. 31, 2023	Dec. 31, 2020
Class Of Stock [Line Items]
Common stock reserved	14,693,467	9,314,262
2020 Equity Incentive Award Plan
Class Of Stock [Line Items]
Common stock reserved	2,332,657	524,078	2,606,546
2020 Employee Stock Purchase Plan
Class Of Stock [Line Items]
Common stock reserved	273,375	313,034	252,337
Exercise of Options Issued and Outstanding and Future Vesting of Restricted Stock Units
Class Of Stock [Line Items]
Common stock reserved	8,450,290	8,423,481
Common Stock Warrants
Class Of Stock [Line Items]
Common stock reserved	3,637,145	53,669

- Definition

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

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
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+ Details

401(K) Plan

6 Months Ended

Jun. 30, 2024

Retirement Benefits [Abstract]

401(K) Plan

17. 401(k) Plan

In 2014, the Company adopted a 401(k) plan for all employees who have met certain eligibility requirements. The 401(k) plan allows employees to make pre-tax and post-tax contributions up to the maximum allowable amount set by the Internal Revenue Service. The Company made contributions to the 401(k) plan for all eligible participants and recorded contribution expenses of $0.4 million and $0.2 million for the three months ended June 30, 2024 and 2023, respectively. For the six months ended June 30, 2024 and 2023, the Company recorded contribution expenses of $0.8 million and $0.7 million, respectively.

Income Taxes

6 Months Ended

Jun. 30, 2024

Income Tax Disclosure [Abstract]

Income Taxes

10. Income Taxes

For the three and six months ended June 30, 2024 and 2023, the Company did not record a federal or state income tax provision due to its recurring net losses. In addition, the Company has taken a full valuation allowance against its net deferred tax assets as the Company believes it is not more likely than not that the benefit will be realized.

- Definition

The entire disclosure for income tax.

+ References

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-Topic 740
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+ Details

Marketable Securities (Tables)

6 Months Ended

Jun. 30, 2024

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 June 30, 2024 and December 31, 2023 (in thousands):


	Amortized Cost Basis		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value as of June 30, 2024
Short-term marketable securities:
Certificates of deposit	$	69,674	$	21	$	(18	)	$	69,677
Commercial paper		95,009		8		(68	)		94,949
Corporate bonds		169,491		10		(212	)		169,289
U.S. Treasuries		18,977		—		(7	)		18,970
Total short-term marketable securities	$	353,151	$	39	$	(305	)	$	352,885
Long-term marketable securities:
Commercial paper	$	1,275	$	—	$	(4	)	$	1,271
Corporate bonds		34,466		20		(42	)		34,444
Total long-term marketable securities	$	35,741	$	20	$	(46	)	$	35,715


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

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

Aggregate fair values of marketable securities with unrealized losses and gains were as follows as of June 30, 2024 and December 31, 2023 (in thousands):


	June 30,		December 31,
	2024		2023
Aggregate fair value of marketable securities in a continuous loss position for less than twelve months	$	304,777	$	20,494
Aggregate fair value of marketable securities in a continuous loss position for more than twelve months		—		4,986
Aggregate fair value of marketable securities in unrealized gain position		83,823		24,598
Total marketable securities	$	388,600	$	50,078

- Definition

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

+ References

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-Topic 320
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 9
-Subparagraph (a)
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+ Details

Stock-based Compensation - Summary of Stock Options Activity (Details)	6 Months Ended
	Jun. 30, 2024 shares
Share-Based Payment Arrangement [Abstract]
Number of Shares Underlying Outstanding Options, Balances at December 31, 2023	8,240,154
Number of Shares Underlying Outstanding Options, granted	434,500
Number of Shares Underlying Outstanding Options, exercised	(296,183)
Number of Shares Underlying Outstanding Options, expired	(8,837)
Number of Shares Underlying Outstanding Options, forfeited	(177,461)
Number of Shares Underlying Outstanding Options, Balances at June 30, 2024	8,192,173
Number of Shares Underlying Outstanding Options, Options exercisable at June 30, 2024	3,726,662
Number of Shares Underlying Outstanding Options, Options vested and expected to vest at June 30, 2024	8,192,173

Summary of Significant Accounting Policies (Policies)

6 Months Ended

Jun. 30, 2024

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 note disclosures normally included in annual financial statements prepared in accordance with U.S. GAAP have been condensed or omitted. Accordingly, the unaudited condensed financial statements should be read in conjunction with the audited financial statements and the related notes thereto included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2023 filed with the SEC.

The accompanying financial information for the three and six months ended June 30, 2024 and 2023 is unaudited. The unaudited condensed financial statements have been prepared on the same basis as the annual audited financial statements and, in the opinion of management, reflect all adjustments, which include only normal recurring adjustments, necessary to present fairly the Company’s financial position as of June 30, 2024 and December 31, 2023 and its results of operations and cash flows for the six months ended June 30, 2024 and 2023. 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

Use of Estimates and Judgments

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

Due to the war in Ukraine, conflicts in the Middle East, rising interest rates and inflation, and the lingering impacts of 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 June 30, 2024. While there was not a material impact to the Company’s unaudited condensed financial statements as of June 30, 2024, these estimates may change, as new events occur and additional information is obtained, as well as other factors that could result in material impacts to the unaudited condensed financial statements in future reporting periods.

Segment Information

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

Concentration of Credit Risk

Financial instruments that potentially subject the Company to a concentration of credit risk consist of cash and cash equivalents, marketable securities and accounts receivable. The Company’s cash is held at two financial institutions in the United States of America. The Company’s cash equivalents are invested in money market funds. The Company also invests in U.S. Treasuries, U.S. government sponsored agencies, commercial paper, corporate bonds and certificates of deposit. 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 June 30,		Six Months Ended June 30,
	2024	2023	2024	2023
Customer A	0%	96%	0%	98%
Customer B	100%	*	100%	*
Total	100%	96%	100%	98%

* Less than 10%

The Company did not have accounts receivable from its partners in collaboration and license agreements as of June 30, 2024 and December 31, 2023.

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


	Three Months Ended June 30,				Six Months Ended June 30,
	2024		2023		2024		2024
Netherlands	$	—	$	230	$	—	$	525
United States		5		9		33		13
Total revenue	$	5	$	239	$	33	$	538

Other Risks and Uncertainties

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

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

Recently Issued Accounting Pronouncements Not Yet Adopted

On December 14, 2023, the Financial Accounting Standards Board issued Accounting Standards Update (“ASU”) No. 2023-09, Improvements to Income Tax Disclosures. The final guidance adds clarifications related to the presentation of rate reconciliation for public business entities and definitions of specific categories in rate reconciliation. These amendments are effective for public business entities for annual periods beginning after December 15, 2024. For other entities, amendments are effective for annual periods beginning after December 15, 2025. The Company is not early adopting ASU 2023-09. The Company expects to make additional disclosures for income taxes upon adoption of this ASU.

- Definition

Disclosure of accounting policy for segment reporting.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 280
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 41
-Subparagraph (a)
-Publisher FASB
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+ Details

Net Loss Per Share, Basic and Diluted (Tables)

6 Months Ended

Jun. 30, 2024

Earnings Per Share [Abstract]

Computation of Basic and Diluted Net Loss Per Share

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


	Three Months Ended June 30,						Six Months Ended June 30,
	2024			2023			2024			2023
Numerator
Net loss	$	(34,953	)	$	(29,616	)	$	(67,354	)	$	(58,298	)
Denominator
Weighted-average shares outstanding used in computing net loss per share, basic and diluted		55,282,754			38,335,219			52,277,369			35,661,995
Net loss per share, basic and diluted	$	(0.63	)	$	(0.77	)	$	(1.29	)	$	(1.63	)

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 for the periods presented because including them would have been antidilutive:


	June 30,
	2024		2023
Options issued and outstanding		8,192,173		6,422,876
Restricted stock units subject to future vesting		258,117		—
2020 ESPP		244,622		269,102
Common stock warrants		53,669		53,669
Total		8,748,581		6,745,647

Summary of Significant Accounting Policies - Additional Information (Details) $ in Thousands	6 Months Ended
	Jun. 30, 2024 USD ($) Segment	Dec. 31, 2023 USD ($)
Summary Of Significant Accounting Policies [Line Items]
Number of reportable segments	1
Number of operating segments	1
Collaboration and License Agreements
Summary Of Significant Accounting Policies [Line Items]
Accounts receivable \| $	$ 0	$ 0

Accrued and Other Current Liabilities (Tables)

6 Months Ended

Jun. 30, 2024

Payables and Accruals [Abstract]

Summary of Accrued and Other Current Liabilities

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


	June 30, 2024		December 31, 2023
Payroll and related expenses	$	5,494	$	8,149
Accrued clinical and preclinical study costs		5,165		2,180
Consulting and professional		2,021		1,581
Other accrued expenses		96		104
Total accrued and other current liabilities	$	12,776	$	12,014

Stock-based Compensation - Summary of Stock-based Compensation Expense for Employees and Nonemployees by Function (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total stock-based compensation expense	$ 6,073	$ 4,969	$ 12,048	$ 9,190
Research and Development
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total stock-based compensation expense	3,196	2,678	6,231	5,148
General and Administrative
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total stock-based compensation expense	$ 2,877	$ 2,291	$ 5,817	$ 4,042

Research and Collaboration Arrangements

6 Months Ended

Jun. 30, 2024

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 June 30,				Six Months Ended June 30,
	2024		2023		2024		2023
uniQure	$	—	$	230	$	—	$	525
CFF		5		9		33		13
Total revenue	$	5	$	239	$	33	$	538

Deferred revenue is summarized as follows (in thousands):


	June 30, 2024		December 31, 2023
CFF	$	1,318	$	1,245
Total deferred revenue	$	1,318	$	1,245

The total amount of revenue in the six months ended June 30, 2024 which was included in deferred revenue at January 1, 2024 was zero. The total amount of revenue in the six months ended June 30, 2023 which was included in deferred revenue at January 1, 2023 was $0.5 million.

uniQure

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

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

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

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

In August 2019, the Company and uniQure entered into an Amended and Restated Collaboration and License Agreement (the “Amended uniQure Agreement”), which amended and restated the uniQure

Agreement, and a separate Collaboration and License Agreement (the “Second uniQure Agreement”). Under these agreements, the Company agreed to transfer incremental rights and services to uniQure in exchange for uniQure eliminating the uniQure Exclusivity Clause and transferring other rights back to the Company.

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

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

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

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

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

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

The Company determined the transaction price using the risk adjusted net present value analysis (“rNPV”) methodology to value the elimination of the uniQure exclusivity clause and other material rights received by the Company, including the potential royalties the Company would receive from uniQure. The rNPVs incorporate estimates and assumptions including the number of products the Company and uniQure would develop, the risk-adjusted probability of successfully developing a biopharmaceutical product, the probability that uniQure will develop a product, the research and development costs, the potential worldwide sales and associated commercialization costs, corporate tax rate, and discount rate.

During each of the three and six months ended June 30, 2024, the Company recognized no revenue under both of the Amended uniQure Agreement and the Second uniQure Agreement. The Company recognized $0.2 million and $0.5 million during the three and six months ended June 30, 2023, respectively, under both of the Amended uniQure Agreement and the Second uniQure Agreement. There were no amounts due from uniQure under the uniQure Agreement, Amended uniQure Agreement or Second uniQure Agreement as of June 30, 2024 and December 31, 2023. As of June 30, 2024 and December 31, 2023, deferred revenue relating to uniQure was zero and the aggregate amount of the transaction price allocated to the remaining performance obligation was zero.

Cystic Fibrosis Foundation (CFF)

In September 2016, the Company entered into an award agreement for the Optimized Adeno-Associated Virus for Lung Epithelia Gene Delivery Development Program with CFF, a non-profit organization dedicated to finding a cure for cystic fibrosis, an inherited disorder that causes disease in the pulmonary airways leading to morbidity and mortality. Under this agreement, CFF contributes funding to help advance the Company’s CF research program. The September 2016 grant award agreement was incorporated into a new grant award agreement with the CFF in September 2017 with the same objectives, which was subsequently amended in August 2018 and February 2021. In August 2023, the Company executed a third amendment to the agreement (the “August 2023 Amendment”), which modified the research plan, increased the aggregate milestone payments from $3.5 million to $6.3 million and extended the estimated project completion date. The aforementioned September 2017 agreement and three amendments are collectively referred to as the “CFF Agreement”. The August 2023 Amendment represents a contract modification to an existing contract under Topic 606, given the amendment did not include any additional goods or services, and the remaining research activities are not distinct from those previously provided. The August 2023 Amendment did not impact the transaction price, given the increased award amount relates to variable consideration for future milestones that are fully constrained. Accordingly, the contract modification did not result in a revenue adjustment. As of June 30, 2024 and December 31, 2023, the Company had achieved milestones totaling $1.8 million and $1.7 million, respectively, under the CFF Agreement. The remaining award amount will be paid by CFF based on achievement of certain development milestones by the Company.

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

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

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

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

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

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

Revenue recognized during each of the three and six months ended June 30, 2024 and 2023 was immaterial. As of June 30, 2024 and December 31, 2023, deferred revenue relating to the CFF Agreement was $1.3 million and $1.2 million, respectively. There were no accounts receivable from CFF under the CFF Agreement as of June 30, 2024 and December 31, 2023. As of June 30, 2024 and December 31, 2023, the aggregate amount of the transaction price allocated to the remaining performance obligation was $1.3 million and $1.2 million, respectively. Based on current timelines, the deferred revenue is expected to be recognized as revenue over the next one to three years as the Company performs research services through the completion of IND-enabling studies.

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

Arbor Biotechnologies, Inc.

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

The Company will account for its costs related to the co-development and co-commercialization of product candidates under the Arbor Agreement as research and development expenses in the periods in which they are incurred. As of June 30, 2024, no Shared Product has been designated by the JSC (each as defined in the Arbor Agreement) under the Arbor Agreement, and no research and development expenses were incurred related to the Arbor Agreement during the three and six months ended June 30, 2024.

Research and Collaboration Arrangements - Additional Information (Details 1)	Jan. 31, 2014
uniQure \| uniQure Agreement \| Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date: 2024-07-01
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue, estimated performance period	4 years

Net Loss Per Share, Basic and Diluted

6 Months Ended

Jun. 30, 2024

Earnings Per Share [Abstract]

Net Loss Per Share, Basic and Diluted

14. Net Loss Per Share, Basic and Diluted

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


	Three Months Ended June 30,						Six Months Ended June 30,
	2024			2023			2024			2023
Numerator
Net loss	$	(34,953	)	$	(29,616	)	$	(67,354	)	$	(58,298	)
Denominator
Weighted-average shares outstanding used in computing net loss per share, basic and diluted		55,282,754			38,335,219			52,277,369			35,661,995
Net loss per share, basic and diluted	$	(0.63	)	$	(0.77	)	$	(1.29	)	$	(1.63	)

In February 2024, the Company issued and sold Pre-funded Warrants to purchase 3,583,476 shares of common stock at a nominal exercise price of $0.0001 (see Note 13). The shares of common stock into which the Pre-funded Warrants may be exercised are considered outstanding for the purposes of computing earnings per share, because the shares may be issued for little or no consideration, they are fully vested and the Pre-funded Warrants are immediately exercisable upon their issuance date.


	June 30,
	2024		2023
Options issued and outstanding		8,192,173		6,422,876
Restricted stock units subject to future vesting		258,117		—
2020 ESPP		244,622		269,102
Common stock warrants		53,669		53,669
Total		8,748,581		6,745,647

Fair Value Measurements - Additional Information (Details) - USD ($)	Jun. 30, 2024	Dec. 31, 2023
Fair Value Disclosures [Abstract]
Assets, Level 1 and Level 2 transfers	$ 0	$ 0
Liabilities, Level 1 and Level 2 transfers	$ 0	$ 0

Net Loss Per Share, Basic and Diluted - Computation of Basic and Diluted Net Loss Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Numerator
Net loss	$ (34,953)	$ (29,616)	$ (67,354)	$ (58,298)
Denominator
Weighted-average shares outstanding used in computing net loss per share, basic	55,282,754	38,335,219	52,277,369	35,661,995
Weighted-average shares outstanding used in computing net loss per share, diluted	55,282,754	38,335,219	52,277,369	35,661,995
Net loss per share, basic	$ (0.63)	$ (0.77)	$ (1.29)	$ (1.63)
Net loss per share, diluted	$ (0.63)	$ (0.77)	$ (1.29)	$ (1.63)

- Definition

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

+ References

+ Details

- Definition

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

+ References

+ Details

- 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
-Topic 805
-SubTopic 60
-Name Accounting Standards Codification
-Section 65
-Paragraph 1
-Subparagraph (g)
-Publisher FASB
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Reference 2: http://www.xbrl.org/2003/role/disclosureRef
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-Section 45
-Paragraph 60B
-Subparagraph (c)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482689/260-10-45-60B

+ Details

Derivative Liability

6 Months Ended

Jun. 30, 2024

Derivative Instruments and Hedging Activities Disclosure [Abstract]

Derivative Liability

15. Derivative Liability

The Company identified an embedded derivative resulting from the change of control provision in the CFF Agreement. Embedded derivatives that are required to be bifurcated from the underlying host instrument are accounted for and valued as separate financial instruments. At the inception of the derivative in 2017, the Company recognized this derivative as a liability and revenue was reduced by the initial fair value of the derivative liability. The Company remeasures the derivative liability to fair value at each reporting period and records the change in fair value of the derivative liability as other income (expense), net. The Company uses a present value analysis with multiple scenarios, which incorporates assumptions and estimates to value the derivative instrument. The Company assesses these assumptions and estimates on a periodic basis as additional information impacting the assumptions is obtained. Estimates and assumptions impacting the fair value measurement include the change of control payment to CFF (range of zero to $18.9 million at June 30, 2024 and December 31, 2023), 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 June 30, 2024 and December 31, 2023) and the discount rate (15% at June 30, 2024 and December 31, 2023). The Company determined the estimated fair value of this liability as of the inception date of the CFF Agreement and concluded that the amount was immaterial. The Company determined the fair value of this derivative liability was $0.4 million as of June 30, 2024 and December 31, 2023.

Research and Collaboration Arrangements - CFF - Additional Information (Details) - CFF Agreement - Cystic Fibrosis Foundation	1 Months Ended		6 Months Ended	12 Months Ended
	Aug. 31, 2023 USD ($)	Sep. 30, 2016 USD ($) Installment	Jun. 30, 2024 USD ($)	Dec. 31, 2023 USD ($)
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaborative arrangement total amount of award	$ 6,300,000	$ 3,500,000
Collaborative arrangement achieved milestones			$ 1,800,000	$ 1,700,000
Expected payments multiples to equal actual award received		6
Number of installments to pay actual award \| Installment		3
Actual award payment period upon first commercial sale of product		4 years
Continuous period where the Company ceases to develop or commercialize any product		180 days
Collaborative arrangements special term		If at any time prior to the first commercial sale of a product developed as a result of the CFF Agreement, the Company ceases to use commercially reasonable efforts to develop or commercialize any product under the CFF Agreement for a continuous period of 180 consecutive days and fails to present a reasonable plan to resume commercially reasonable efforts, the Company will grant to CFF an irrevocable, exclusive worldwide interruption license under all of the Company’s interest in the research plan technology to exploit such product. Any third-party license granted by the Company shall be subject to such interruption license.
Deferred revenue			1,300,000	1,200,000
Accounts receivable			0	0
Aggregate amount of transaction price allocated to remaining performance obligation			$ 1,300,000	$ 1,200,000
Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Future sales-based milestone payments multiples		3
Royalties multiples on actual award received		6

The Company - Additional Information (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended
	Mar. 31, 2024	Feb. 29, 2024	May 31, 2023	Nov. 30, 2021	Dec. 31, 2020	Jun. 30, 2024	Dec. 31, 2023	Jan. 01, 2021
Organization And Nature Of Business [Line Items]
Outstanding shares of redeemable convertible preferred stock								0
Accumulated deficit						$ 482,681	$ 415,327
Common Stock \| Initial Public Offering
Organization And Nature Of Business [Line Items]
Number of shares sold and issued					9,660,000
Stock sold and issued price per share					$ 23
Aggregate net proceeds, after deducting underwriting discounts and commissions and offering costs					$ 204,700
Number shares issued upon conversion of redeemable convertible preferred shares					11,575,984
Common Stock \| Exercise of Overallotment Option
Organization And Nature Of Business [Line Items]
Number of shares sold and issued	1,259,299				1,260,000
Aggregate net proceeds, after deducting underwriting discounts and commissions and offering costs	$ 34,900
Common Stock \| 2021 Follow On Public Offering
Organization And Nature Of Business [Line Items]
Number of shares priced under public offering				4,750,000
Stock sold and issued price per share				$ 25
Aggregate net proceeds, after deducting underwriting discounts and commissions and offering costs				$ 111,100
Common Stock \| 2023 Follow On Public Offering
Organization And Nature Of Business [Line Items]
Number of shares priced under public offering			8,625,000
Stock sold and issued price per share			$ 16
Aggregate net proceeds, after deducting underwriting discounts and commissions and offering costs			$ 129,200
Common Stock \| 2024 Follow On Public Offering
Organization And Nature Of Business [Line Items]
Number of shares priced under public offering		6,586,015
Stock sold and issued price per share		$ 29.5
Aggregate net proceeds, after deducting underwriting discounts and commissions and offering costs		$ 281,200
Warrants to purchase shares of common stock		3,583,476
Common Stock \| 2024 Follow On Public Offering \| Pre-funded Warrants
Organization And Nature Of Business [Line Items]
Warrants to purchase shares of common stock		3,583,476
Common stock offering price		$ 29.4999
Common Stock \| Maximum \| Exercise of Overallotment Option
Organization And Nature Of Business [Line Items]
Number of shares sold and issued		1,525,423

Marketable Securities

6 Months Ended

Jun. 30, 2024

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 June 30, 2024 and December 31, 2023 (in thousands):


	Amortized Cost Basis		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value as of June 30, 2024
Short-term marketable securities:
Certificates of deposit	$	69,674	$	21	$	(18	)	$	69,677
Commercial paper		95,009		8		(68	)		94,949
Corporate bonds		169,491		10		(212	)		169,289
U.S. Treasuries		18,977		—		(7	)		18,970
Total short-term marketable securities	$	353,151	$	39	$	(305	)	$	352,885
Long-term marketable securities:
Commercial paper	$	1,275	$	—	$	(4	)	$	1,271
Corporate bonds		34,466		20		(42	)		34,444
Total long-term marketable securities	$	35,741	$	20	$	(46	)	$	35,715


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

All marketable securities held as of June 30, 2024 had contractual maturities of less than two years. There have been no material realized gains or losses on marketable securities for the periods presented.

Aggregate fair values of marketable securities with unrealized losses and gains were as follows as of June 30, 2024 and December 31, 2023 (in thousands):


	June 30,		December 31,
	2024		2023
Aggregate fair value of marketable securities in a continuous loss position for less than twelve months	$	304,777	$	20,494
Aggregate fair value of marketable securities in a continuous loss position for more than twelve months		—		4,986
Aggregate fair value of marketable securities in unrealized gain position		83,823		24,598
Total marketable securities	$	388,600	$	50,078

The Company manages credit risk associated with its investment portfolio through its investment policy, which limits purchases to high-quality issuers and also limits the amount of its portfolio that can be invested in a single issuer. The Company did not record an allowance for credit losses or other impairment charges related to its marketable securities for any period presented. The Company has determined that (i) it does not have the intent to sell any of these investments, and (ii) it is not more likely than not that it will be required to sell any of these investments before recovery of the entire amortized cost basis. The Company further considered the maximum unrealized loss amounts both at the individual instrument level, $13 thousand, as well as in aggregate, $350 thousand, as of June 30, 2024, as immaterial. These unrealized losses were not attributed to credit risk and were associated with changes in market conditions. The Company periodically reviews its marketable securities for indications of credit losses. The Company anticipates that it will recover the entire amortized cost basis of such securities, and therefore, no credit loss existed as of June 30, 2024.

Stock-based Compensation (Tables)

6 Months Ended

Jun. 30, 2024

Share-Based Payment Arrangement [Abstract]

Summary of Restricted Stock Units (RSU)

The Company has granted restricted stock unit (RSU) awards under the 2020 Equity Incentive Award Plan that vest over a period of four years. The following table summarizes the RSU activity for the six months ended June 30, 2024:


	Number of Shares
Unvested balance at December 31, 2023		181,181
Issued		129,650
Vested		(21,963	)
Canceled/Forfeited		(30,751	)
Unvested balance at June 30, 2024		258,117

Summary of Stock Options Activity

The following table summarizes the stock options activity for the six months ended June 30, 2024:


	Number of Shares Underlying Outstanding Options
Balances at December 31, 2023		8,240,154
Options granted		434,500
Options exercised		(296,183	)
Options expired		(8,837	)
Options forfeited		(177,461	)
Balances at June 30, 2024		8,192,173
Options exercisable at June 30, 2024		3,726,662
Options vested and expected to vest at June 30, 2024		8,192,173

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

The Company estimates the fair value of stock options using the Black-Scholes option pricing model. The fair value of employee and nonemployee options is recognized on a straight-line basis over the requisite service period of the awards. The fair value of the Company’s stock options was estimated using the following assumptions for the three and six months ended June 30, 2024 and 2023:


	Three Months Ended June 30,		Six Months Ended June 30,
	2024	2023	2024	2023
Expected term	6.0 - 6.1 years	6.0 - 6.1 years	6.0 - 6.1 years	6.0 - 6.1 years
Expected volatility	84.9% - 85.4%	81.1% - 82.2%	81.4% - 85.4%	80.9% - 84.4%
Risk-free interest rate	4.4% - 4.5%	3.5% - 4.0%	3.9% - 4.5%	3.5% - 4.0%
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 June 30,				Six Months Ended June 30,
	2024		2023		2024		2023
Research and development	$	3,196	$	2,678	$	6,231	$	5,148
General and administrative		2,877		2,291		5,817		4,042
Total stock-based compensation expense	$	6,073	$	4,969	$	12,048	$	9,190

Common Stock Warrants - Additional Information (Details) - USD ($)	1 Months Ended		3 Months Ended		6 Months Ended
	Dec. 31, 2020	May 31, 2018	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023	Feb. 29, 2024
Class Of Warrant Or Right [Line Items]
Number of warrant shares issued	30,000	23,669
Exercise price	$ 18	$ 3.19
Warrant expiration year	2027	2025
Warrant vesting period	4 years
Warrant expense			$ 0	$ 0	$ 0	$ 0
2024 Follow On Public Offering \| Common Stock
Class Of Warrant Or Right [Line Items]
Warrant shares become exercisable							3,583,476
2024 Follow On Public Offering \| Common Stock \| Pre-funded Warrants
Class Of Warrant Or Right [Line Items]
Exercise price							$ 0.0001
Warrant shares become exercisable							3,583,476
Common stock offering price							$ 29.4999
Issuance of pre-funded warrants					$ 99,400,000

Common Stock (Tables)

6 Months Ended

Jun. 30, 2024

Equity [Abstract]

Schedule of Common Shares Reserved for Future Issuance

As of June 30, 2024 and December 31, 2023, the Company has reserved common stock, on an as-converted basis, for future issuance as follows:


	June 30, 2024		December 31, 2023
Issuance of common stock under the 2020 Equity Incentive Award Plan		2,332,657		524,078
Issuance of common stock under the 2020 Employee Stock Purchase Plan		273,375		313,034
Exercise of options issued and outstanding and future vesting of restricted stock units		8,450,290		8,423,481
Exercise of common stock warrants		3,637,145		53,669
Total common stock reserved for future issuance		14,693,467		9,314,262

Marketable Securities - Summary of Aggregate fair values of marketable securities with unrealized losses and gains (Details) - USD ($) $ in Thousands	Jun. 30, 2024	Dec. 31, 2023
Debt Securities, Available-for-Sale [Line Items]
Aggregate fair value of securities held in a continuous unrealized loss position for less than twelve months	$ 304,777	$ 20,494
Aggregate fair value of marketable securities in a continuous loss position for more than twelve months	0	4,986
Aggregate fair value of marketable securities in unrealized gain position	83,823	24,598
Total marketable securities	$ 388,600	$ 50,078

- Definition

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

+ References

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

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

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 320
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (aa)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481800/320-10-50-2

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Topic 320
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (aaa)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481800/320-10-50-2

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Topic 320
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481800/320-10-50-2

Reference 6: http://www.xbrl.org/2003/role/disclosureRef
-Topic 320
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (c)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481800/320-10-50-2

Reference 7: http://www.xbrl.org/2003/role/disclosureRef
-Topic 320
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (d)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481800/320-10-50-2

Reference 8: http://www.xbrl.org/2003/role/disclosureRef
-Topic 320
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 3
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481800/320-10-50-3

Reference 9: http://www.xbrl.org/2003/role/disclosureRef
-Topic 320
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 3
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481800/320-10-50-3

Reference 10: http://www.xbrl.org/2003/role/disclosureRef
-Topic 320
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 3
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481800/320-10-50-3

Reference 11: http://www.xbrl.org/2003/role/disclosureRef
-Topic 320
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 3
-Subparagraph (c)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481800/320-10-50-3

Reference 12: http://www.xbrl.org/2003/role/disclosureRef
-Topic 320
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 3
-Subparagraph (d)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481800/320-10-50-3

+ Details

Condensed Balance Sheets (Unaudited) (Parenthetical) - $ / shares	Jun. 30, 2024	Dec. 31, 2023
Statement of Financial Position [Abstract]
Preferred stock, par value	$ 0.0001	$ 0.0001
Preferred stock, shares authorized	10,000,000	10,000,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value	$ 0.0001	$ 0.0001
Common stock, shares authorized	300,000,000	300,000,000
Common stock, shares, issued	51,914,275	43,075,218
Common stock, shares, outstanding	51,914,275	43,075,218

Condensed Statements of Stockholders' Equity (Unaudited) - USD ($) $ in Thousands	Total	ATM Offering Program	Public offering	February / March 2024 Public Offering Sale	Common Stock	Common Stock ATM Offering Program	Common Stock Public offering	Common Stock February / March 2024 Public Offering Sale	Additional Paid-in Capital	Additional Paid-in Capital ATM Offering Program	Additional Paid-in Capital Public offering	Additional Paid-in Capital February / March 2024 Public Offering Sale	Accumulated Other Comprehensive Loss	Accumulated Deficit
Stockholders' Equity, beginning balances at Dec. 31, 2022	$ 231,337				$ 3				$ 547,020				$ (1,196)	$ (314,490)
Stockholders' Equity, beginning balances (in shares) at Dec. 31, 2022					32,626,627
Issuance of common stock upon exercise of stock options	1,135								1,135
Issuance of common stock upon exercise of stock options (in shares)					122,207
Issuance of common stock, net of issuance costs		$ 9,647								$ 9,647
Issuance of common stock, net of issuance costs (in shares)						487,934
Stock-based compensation expense	4,221								4,221
Vesting of common stock warrants issued for services	22								22
Net unrealized gain (loss) on marketable securities	926												926
Net loss	(28,682)													(28,682)
Stockholders' Equity, ending balances at Mar. 31, 2023	218,606				$ 3				562,045				(270)	(343,172)
Stockholders' Equity, ending balances (in shares) at Mar. 31, 2023					33,236,768
Stockholders' Equity, beginning balances at Dec. 31, 2022	231,337				$ 3				547,020				(1,196)	(314,490)
Stockholders' Equity, beginning balances (in shares) at Dec. 31, 2022					32,626,627
Net unrealized gain (loss) on marketable securities	1,541
Net loss	(58,298)
Stockholders' Equity, ending balances at Jun. 30, 2023	325,144				$ 4				697,583				345	(372,788)
Stockholders' Equity, ending balances (in shares) at Jun. 30, 2023					42,016,622
Stockholders' Equity, beginning balances at Mar. 31, 2023	218,606				$ 3				562,045				(270)	(343,172)
Stockholders' Equity, beginning balances (in shares) at Mar. 31, 2023					33,236,768
Issuance of common stock upon exercise of stock options	894								894
Issuance of common stock upon exercise of stock options (in shares)					72,217
Issuance of common stock, net of issuance costs			$ 129,208				$ 1				$ 129,207
Issuance of common stock, net of issuance costs (in shares)							8,625,000
Issuance of common stock - 2020 ESPP	658								658
Issuance of common stock - 2020 ESPP (in shares)					82,637
ATM offering costs		(212)								(212)
Stock-based compensation expense	4,969								4,969
Vesting of common stock warrants issued for services	22								22
Net unrealized gain (loss) on marketable securities	615												615
Net loss	(29,616)													(29,616)
Stockholders' Equity, ending balances at Jun. 30, 2023	325,144				$ 4				697,583				345	(372,788)
Stockholders' Equity, ending balances (in shares) at Jun. 30, 2023					42,016,622
Stockholders' Equity, beginning balances at Dec. 31, 2023	307,829				$ 4				723,136				16	(415,327)
Stockholders' Equity, beginning balances (in shares) at Dec. 31, 2023					43,075,218
Issuance of common stock upon exercise of stock options and vesting of RSUs	3,113								3,113
Issuance of common stock upon exercise of stock options and vesting of RSUs (in shares)					236,856
Issuance of common stock, net of issuance costs				$ 316,149				$ 1				$ 316,148
Issuance of common stock, net of issuance costs (in shares)								7,845,314
Stock-based compensation expense	5,974								5,974
Vesting of common stock warrants issued for services	22								22
Net unrealized gain (loss) on marketable securities	(125)												(125)
Net loss	(32,401)													(32,401)
Stockholders' Equity, ending balances at Mar. 31, 2024	600,561				$ 5				1,048,393				(109)	(447,728)
Stockholders' Equity, ending balances (in shares) at Mar. 31, 2024					51,157,388
Stockholders' Equity, beginning balances at Dec. 31, 2023	$ 307,829				$ 4				723,136				16	(415,327)
Stockholders' Equity, beginning balances (in shares) at Dec. 31, 2023					43,075,218
Issuance of common stock upon exercise of stock options (in shares)	296,183
Net unrealized gain (loss) on marketable securities	$ (309)
Net loss	(67,354)
Stockholders' Equity, ending balances at Jun. 30, 2024	588,340				$ 5				1,071,309				(293)	(482,681)
Stockholders' Equity, ending balances (in shares) at Jun. 30, 2024					51,914,275
Stockholders' Equity, beginning balances at Mar. 31, 2024	600,561				$ 5				1,048,393				(109)	(447,728)
Stockholders' Equity, beginning balances (in shares) at Mar. 31, 2024					51,157,388
Issuance of common stock upon exercise of stock options and vesting of RSUs	850								850
Issuance of common stock upon exercise of stock options and vesting of RSUs (in shares)					81,290
Issuance of common stock, net of issuance costs		$ 15,263								$ 15,263
Issuance of common stock, net of issuance costs (in shares)						585,938
Issuance of common stock - 2020 ESPP	707								707
Issuance of common stock - 2020 ESPP (in shares)					89,659
Stock-based compensation expense	6,074								6,074
Vesting of common stock warrants issued for services	22								22
Net unrealized gain (loss) on marketable securities	(184)												(184)
Net loss	(34,953)													(34,953)
Stockholders' Equity, ending balances at Jun. 30, 2024	$ 588,340				$ 5				$ 1,071,309				$ (293)	$ (482,681)
Stockholders' Equity, ending balances (in shares) at Jun. 30, 2024					51,914,275

Summary of Significant Accounting Policies

6 Months Ended

Jun. 30, 2024

Accounting Policies [Abstract]

Summary of Significant Accounting Policies

2. Summary of Significant Accounting Policies

Basis of Presentation

Use of Estimates and Judgments

Segment Information

Concentration of Credit Risk

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


	Three Months Ended June 30,		Six Months Ended June 30,
	2024	2023	2024	2023
Customer A	0%	96%	0%	98%
Customer B	100%	*	100%	*
Total	100%	96%	100%	98%

* Less than 10%

The Company did not have accounts receivable from its partners in collaboration and license agreements as of June 30, 2024 and December 31, 2023.

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


	Three Months Ended June 30,				Six Months Ended June 30,
	2024		2023		2024		2024
Netherlands	$	—	$	230	$	—	$	525
United States		5		9		33		13
Total revenue	$	5	$	239	$	33	$	538

Other Risks and Uncertainties

Recently Issued Accounting Pronouncements Not Yet Adopted

Related Party Transactions

6 Months Ended

Jun. 30, 2024

Related Party Transactions

16. Related Party Transactions

In March 2024, the Company entered into a research and option agreement (the “Reignite Agreement”) with Reignite Therapeutics Inc. (“Reignite”), which was founded by David Kirn, M.D., Chief Executive Officer of the Company. Reignite has the expertise to develop high-capacity, helper-dependent adenovirus capsids which the Company plans to utilize as they expand their therapeutic capsid evolution platform. Reignite and the Company plan to collaborate on a one-to-two year program to develop these high capacity, helper-dependent adenovirus capsids. Under the Reignite Agreement, the Company shall have the final authority to amend and make updates to the plan and budget of the program. Further, the Company is responsible for the funding of the related research which includes the budgeted full-time employees and CRO costs, equipment costs not to exceed $60 thousand and all other costs, in total not to exceed $1.5 million in any year of the program. The Company will have an option to acquire up to three capsids resulting from the program. The Company shall pay to Reignite an option exercise fee of $1.0 million per selected capsid for which the Company has exercised its option. The maximum total amount payable is $3.0 million.

During the three and six months ended June 30, 2024, the Company paid Reignite $0.4 million for research and development expenses. No option exercise fees were incurred during the three and six months ended June 30, 2024. As of June 30, 2024, no other amounts were due to Reignite under the Reignite Agreement.

- Definition

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

+ References

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

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 946
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 5
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480990/946-20-50-5

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 946
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 6
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480990/946-20-50-6

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Topic 946
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-Name Accounting Standards Codification
-Section 50
-Paragraph 2
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-Publisher FASB
-URI https://asc.fasb.org/1943274/2147477968/946-235-50-2

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Topic 946
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-Subparagraph (e)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147477968/946-235-50-2

Reference 6: http://www.xbrl.org/2003/role/disclosureRef
-Topic 850
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-URI https://asc.fasb.org/1943274/2147483326/850-10-50-1

Reference 7: http://www.xbrl.org/2003/role/disclosureRef
-Topic 946
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-Name Accounting Standards Codification
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Reference 8: http://www.xbrl.org/2003/role/disclosureRef
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Reference 9: http://www.xbrl.org/2003/role/disclosureRef
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-URI https://asc.fasb.org/1943274/2147479134/946-220-S99-1

Reference 10: http://www.xbrl.org/2003/role/disclosureRef
-Topic 850
-Name Accounting Standards Codification
-Publisher FASB
-URI https://asc.fasb.org/850/tableOfContent

Reference 11: http://www.xbrl.org/2003/role/disclosureRef
-Topic 850
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 6
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147483326/850-10-50-6

Reference 12: http://www.xbrl.org/2003/role/disclosureRef
-Topic 850
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147483326/850-10-50-1

Reference 13: http://www.xbrl.org/2003/role/disclosureRef
-Topic 850
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147483326/850-10-50-1

+ Details

Research and Collaboration Arrangements (Tables)

6 Months Ended

Jun. 30, 2024

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 June 30,				Six Months Ended June 30,
	2024		2023		2024		2023
uniQure	$	—	$	230	$	—	$	525
CFF		5		9		33		13
Total revenue	$	5	$	239	$	33	$	538

Summary of Deferred Revenue

Deferred revenue is summarized as follows (in thousands):


	June 30, 2024		December 31, 2023
CFF	$	1,318	$	1,245
Total deferred revenue	$	1,318	$	1,245

Summary of Significant Accounting Policies - Schedule of Revenues by Geographic Region (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Disaggregation Of Revenue [Line Items]
Total revenue	$ 5	$ 239	$ 33	$ 538
Netherlands
Disaggregation Of Revenue [Line Items]
Total revenue	0	230	0	525
United States
Disaggregation Of Revenue [Line Items]
Total revenue	$ 5	$ 9	$ 33	$ 13

- Definition

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

+ References + Details

- Definition

+ References

+ Details

Accrued and Other Current Liabilities

6 Months Ended

Jun. 30, 2024

Payables and Accruals [Abstract]

Accrued and Other Current Liabilities

6. Accrued and Other Current Liabilities

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


	June 30, 2024		December 31, 2023
Payroll and related expenses	$	5,494	$	8,149
Accrued clinical and preclinical study costs		5,165		2,180
Consulting and professional		2,021		1,581
Other accrued expenses		96		104
Total accrued and other current liabilities	$	12,776	$	12,014

Research and Collaboration Arrangements - CFF - Additional Information (Details 1) - Cystic Fibrosis Foundation - Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date: 2024-07-01	Jun. 30, 2024
Minimum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue, estimated performance period	1 year
Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue, estimated performance period	3 years

Stock-based Compensation

6 Months Ended

Jun. 30, 2024

Share-Based Payment Arrangement [Abstract]

Stock-based Compensation

12. Stock-based Compensation

2020 Equity Incentive Award Plan

In December 2020, the Company adopted the 2020 Incentive Award Plan (“2020 Plan”), which became effective on December 10, 2020. The 2020 Plan initially reserved 2,606,546 shares of common stock for the issuance of stock options, stock appreciation rights, restricted stock awards, restricted stock units, performance bonus awards, performance stock units, dividend equivalents or other stock or cash based award granted to employees, directors and consultants of the Company. The number of shares reserved for future issuance under the 2020 Plan will increase annually on the first day of each fiscal year beginning in 2021 and ending in 2030 by the lesser of (i) 5% of the shares of common stock outstanding (on an as converted basis) on the last day of the immediately preceding fiscal year and (ii) such number of shares of common stock as determined by the Company's board of directors, provided, however, no more than 18,000,000 shares of the Company's common stock may be issued upon the exercise of incentive stock options. On February 29, 2024, an additional 2,153,533 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 June 30, 2024 there were 1,997,052 shares available for grant under the 2020 Plan.

Following the effectiveness of the 2020 Plan, the Company will not make any further grants under the 2015 Equity Incentive Plan (the “2015 Plan”). However, the 2015 Plan continues to govern the terms of options that remain outstanding under the 2015 Plan. As of June 30, 2024, there were 335,605 shares available for grant under the 2015 Plan.

2015 Equity Incentive Plan

The 2015 Plan provided for grants of stock options, stock appreciation rights, restricted stock and restricted stock unit awards to employees, directors and consultants of the Company. As of June 30, 2024, options to purchase 1,545,023 shares of common stock were outstanding under the 2015 Plan. All options are exercisable over a period not to exceed the contractual term of ten years from the date the stock options were issued and are granted at prices not less than the estimated fair market value of the Company’s common stock on the grant date as determined by the board of directors.

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

Employee Stock Purchase Plan

In December 2020, the Company adopted the 2020 Employee Stock Purchase Plan (the “2020 ESPP”). Under the 2020 ESPP, 252,337 shares of the Company's common stock were initially reserved for employee purchases of the Company's common stock under terms and provisions established by the Company's board of directors and approved by the Company's stockholders. The number of shares reserved for future issuance under the 2020 ESPP will increase annually on the first day of each fiscal year beginning in 2021 and ending in 2030 by the lesser of (i) 1% of the shares of common stock outstanding (on an as converted basis) on the last day of the immediately preceding fiscal year and (ii) such number of shares of common stock as determined by the Company's board of directors, provided, however, no more than 15,000,000 shares the Company's common stock may be issued under the 2020 ESPP. On February 29, 2024, an additional 50,000 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 the Company's employees may purchase common stock through payroll deductions at a price equal to 85% of the lower of the fair market value of the stock at the beginning of the offering period or at the end of each applicable purchase period. The 2020 ESPP provides for a series of overlapping 24-month offering periods comprising four six-month purchase periods. The initial offering period under the 2020 ESPP is longer than 24 months, commencing February 15, 2021 and ending on May 14, 2023. Contributions under the 2020 ESPP are limited to a maximum of 15% of an employee's eligible compensation.

Restricted Stock Units


	Number of Shares
Unvested balance at December 31, 2023		181,181
Issued		129,650
Vested		(21,963	)
Canceled/Forfeited		(30,751	)
Unvested balance at June 30, 2024		258,117

Stock Options

The following table summarizes the stock options activity for the six months ended June 30, 2024:


	Number of Shares Underlying Outstanding Options
Balances at December 31, 2023		8,240,154
Options granted		434,500
Options exercised		(296,183	)
Options expired		(8,837	)
Options forfeited		(177,461	)
Balances at June 30, 2024		8,192,173
Options exercisable at June 30, 2024		3,726,662
Options vested and expected to vest at June 30, 2024		8,192,173


	Three Months Ended June 30,		Six Months Ended June 30,
	2024	2023	2024	2023
Expected term	6.0 - 6.1 years	6.0 - 6.1 years	6.0 - 6.1 years	6.0 - 6.1 years
Expected volatility	84.9% - 85.4%	81.1% - 82.2%	81.4% - 85.4%	80.9% - 84.4%
Risk-free interest rate	4.4% - 4.5%	3.5% - 4.0%	3.9% - 4.5%	3.5% - 4.0%
Expected dividend yield	—%	—%	—%	—%

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

Expected Volatility. The expected volatility is based on a mix of the Company's historical volatility and the historical volatility of comparable companies with similar attributes to the Company, including industry, stage of life cycle, size and financial leverage. For each grant, the Company measured historical volatility over a period equivalent to the expected term.

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

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

Stock-Based Compensation Expense

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


	Three Months Ended June 30,				Six Months Ended June 30,
	2024		2023		2024		2023
Research and development	$	3,196	$	2,678	$	6,231	$	5,148
General and administrative		2,877		2,291		5,817		4,042
Total stock-based compensation expense	$	6,073	$	4,969	$	12,048	$	9,190

Accrued and Other Current Liabilities - Summary of Accrued and Other Current Liabilities (Details) - USD ($) $ in Thousands	Jun. 30, 2024	Dec. 31, 2023
Payables and Accruals [Abstract]
Payroll and related expenses	$ 5,494	$ 8,149
Accrued clinical and preclinical study costs	5,165	2,180
Consulting and professional	2,021	1,581
Other accrued expenses	96	104
Total accrued and other current liabilities	$ 12,776	$ 12,014

Derivative Liability - Additional Information (Details) - USD ($) $ in Millions	6 Months Ended	12 Months Ended
	Jun. 30, 2024	Dec. 31, 2023
Derivative [Line Items]
Change in control discount rate	15.00%	15.00%
Estimated fair value of derivative liability	$ 0.4	$ 0.4
Minimum
Derivative [Line Items]
Change of control payment	$ 0.0	$ 0.0
Change in control event percentage	4.80%	4.80%
Maximum
Derivative [Line Items]
Change of control payment	$ 18.9	$ 18.9
Change in control event percentage	17.20%	17.20%

Property and Equipment, Net (Tables)

6 Months Ended

Jun. 30, 2024

Property, Plant and Equipment [Abstract]

Schedule of Property and Equipment, Net

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


	June 30, 2024			December 31, 2023
Machinery and equipment	$	13,766		$	12,996
Leasehold improvements		17,325			16,902
Furniture and fixtures		566			566
Office equipment		239			239
Computer equipment and software		866			805
Construction in progress		299			550
Total property and equipment		33,061			32,058
Less: Accumulated depreciation and amortization		(14,300	)		(11,932	)
Property and equipment, net	$	18,761		$	20,126

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	Jun. 30, 2024	Dec. 31, 2023
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets	$ 534,908	$ 267,722
Liabilities	382	369
Derivative Liability
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Liabilities	382	369
Commercial Paper
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets	96,220	18,243
Corporate Bonds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets	203,733	28,939
U.S. Treasuries
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets	18,970
U.S. Government Sponsored Agencies
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets		2,896
Money Market Funds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets	146,308	217,644
Certificates of Deposit
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets	69,677
Level 1
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets	146,308	217,644
Level 1 \| Money Market Funds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets	146,308	217,644
Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets	388,600	50,078
Level 2 \| Commercial Paper
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets	96,220	18,243
Level 2 \| Corporate Bonds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets	203,733	28,939
Level 2 \| U.S. Treasuries
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets	18,970
Level 2 \| U.S. Government Sponsored Agencies
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets		2,896
Level 2 \| Certificates of Deposit
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets	69,677
Level 3
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Liabilities	382	369
Level 3 \| Derivative Liability
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Liabilities	$ 382	$ 369

Fair Value Measurements (Tables)

6 Months Ended

Jun. 30, 2024

Fair Value Disclosures [Abstract]

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


	Basis for Fair Value Measurements						Fair Value as of
	Level 1		Level 2		Level 3		June 30, 2024
Assets
Money market funds	$	146,308	$	—	$	—	$	146,308
Certificates of deposit		—		69,677		—		69,677
Commercial paper		—		96,220		—		96,220
Corporate bonds		—		203,733		—		203,733
U.S. Treasuries		—		18,970		—		18,970
Total	$	146,308	$	388,600	$	—	$	534,908
Liabilities
Derivative liability	$	—	$	—	$	382	$	382
Total	$	—	$	—	$	382	$	382


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

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 six months ended June 30, 2024 and 2023 (in thousands):


	Derivative Liability
Balance as of December 31, 2023	$	369
Change in fair value included in other income (expense), net		13
Balance as of June 30, 2024	$	382


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

Pay vs Performance Disclosure - USD ($) $ in Thousands	3 Months Ended				6 Months Ended
Pay vs Performance Disclosure - USD ($) $ in Thousands	Jun. 30, 2024	Mar. 31, 2024	Jun. 30, 2023	Mar. 31, 2023	Jun. 30, 2024	Jun. 30, 2023
Pay vs Performance Disclosure
Net Income (Loss)	$ (34,953)	$ (32,401)	$ (29,616)	$ (28,682)	$ (67,354)	$ (58,298)

- Definition

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

+ References

+ Details

Commitments and Contingencies (Tables)

6 Months Ended

Jun. 30, 2024

Commitments and Contingencies Disclosure [Abstract]

Summary of Components of Lease Expense


	Three Months Ended June 30,				Six Months Ended June 30,
	2024		2023		2024		2023
Operating lease cost	$	698	$	698	$	1,397	$	1,397
Variable lease cost		354		311		707		622
Total	$	1,052	$	1,009	$	2,104	$	2,019

Summary of Supplemental Information Related to Operating Leases

The following table summarizes supplemental information related to operating leases:


	June 30, 2024			December 31, 2023
Weighted-average remaining lease term (in years):		5.2			5.7
Weighted-average discount rate:		7.8	%		7.8	%

Summary of Maturities of Lease Liabilities

The following table summarizes the maturities of lease liabilities as of June 30, 2024 (in thousands):


2024 (remaining six months)	$	1,588
2025		3,244
2026		3,135
2027		2,810
2028		2,894
Thereafter		2,982
Total future minimum lease payments		16,653
Less: Amount representing interest		(2,997	)
Present value of future minimum lease payments		13,656
Less: Current portion of operating lease liabilities		3,196
Long-term portion of operating lease liabilities	$	10,460

License Arrangements

6 Months Ended

Jun. 30, 2024

License Arrangements [Abstract]

License Arrangements

8. License Arrangements

Astellas Gene Therapies, Inc.

On July 5, 2023, the Company entered into a licensing agreement (the “License Agreement”) with Astellas Gene Therapies, Inc. (“AGT”), pursuant to which the Company granted to AGT a license to utilize its intravitreal R100 vector (“4D Vector”) to develop and commercialize licensed compounds and licensed products for one genetic target implicated in rare monogenic ophthalmic disease(s), with options to add up to two additional targets implicated in rare monogenic ophthalmic diseases after paying additional option exercise fees. Under the terms of the License Agreement, the Company has provided its 4D vector technology to AGT to deliver AGT's genetic payloads for the treatment of rare monogenic diseases. AGT will conduct all subsequent research, development, manufacturing, and commercialization activities. As partial consideration for the rights and licenses granted to AGT by the Company under this Agreement, AGT paid the Company an upfront amount of $20 million, which was received in July 2023. The Company

may receive potential future option fees and milestones of up to $942.5 million. In addition, the Company is entitled to receive mid-single digit to double-digit, sub-teen royalties on net sales of all licensed products.

Under the License Agreement, the Company’s performance obligation is to grant and make available to AGT the Licensed IP and Licensed Know-How (each as defined in the License Agreement) with respect to the 4D Vector. In connection with the grant of the license, in July 2023, the Company delivered to AGT the Transferred Material (as defined in the License Agreement). As of June 30, 2024, there was no deferred revenue and no accounts receivable relating to the License Agreement.

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

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

As of June 30, 2024, the Company has not recorded a liability related to contingent consideration for future milestone and royalty payments to either Aevitas or the University of Pennsylvania, as the achievement of such milestones has not occurred and was not deemed probable and product sales have not commenced.

Regents of the University of California

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

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

In addition, the Company is obligated to make certain contingent payments including (i) development milestones up to $3.1 million, (ii) low single digit royalties on the net sales of its developed products that consists of a minimum annual royalty of up to $0.1 million per year for the term of the agreement beginning in the first calendar year after the year in which net sales first occurred, and (iii) sublicense consideration in the mid-teens to the mid-twenties-range on any future sublicensing arrangements the Company may enter into with third-party licensees.

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

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

The Company did not incur any expense under the provisions of the outstanding license agreements during each of the three and six months ended June 30, 2024 and 2023.

Common Stock - Additional Information (Details) - USD ($)	1 Months Ended		6 Months Ended	12 Months Ended
	Oct. 31, 2021	Apr. 30, 2020	Jun. 30, 2024	Dec. 31, 2023
Class of Stock [Line Items]
Common stock, shares authorized			300,000,000	300,000,000
Common stock, par value			$ 0.0001	$ 0.0001
Common stock voting rights			one vote per share
Common stock dividend			$ 0	$ 0
Funding Agreement \| Cystic Fibrosis Foundation
Class of Stock [Line Items]
Additional funding upon acceptance of investigational New Drug application	$ 4,000,000	$ 4,000,000
Additional investment in cash	$ 4,000,000
Funding Agreement \| Cystic Fibrosis Foundation \| Common Stock
Class of Stock [Line Items]
Number of shares sold and issued	125,715
Funding Agreement \| Cystic Fibrosis Foundation \| Series C Redeemable Convertible Preferred Stock
Class of Stock [Line Items]
Investment in redeemable convertible preferred stock financing		$ 10,000,000

Commitments and Contingencies - Additional Information (Details) $ in Thousands		1 Months Ended						6 Months Ended	12 Months Ended
	Jan. 01, 2024 USD ($) ft²	Jul. 31, 2024 USD ($) ft²	Feb. 28, 2021 USD ($)	Dec. 31, 2019 USD ($)	Nov. 30, 2018 USD ($)	Oct. 31, 2018	Jan. 31, 2016	Jun. 30, 2024 USD ($)	Dec. 31, 2023 USD ($)	May 31, 2015
Commitment And Contingencies [Line Items]
Operating lease right-of-use ("ROU") assets								$ 10,688	$ 11,539
Operating lease liability								13,656
Future minimum lease payments								16,653
5980 Horton Street Building Lease
Commitment And Contingencies [Line Items]
Lease extended term date							Mar. 31, 2023
Operating lease right-of-use ("ROU") assets								1,000	1,200
Operating lease liability								$ 1,200	1,400
Operating lease, discount rate								7.50%
Lease extended term period						2026-09
Tenant improvement allowance					$ 200
Lease renewal term							5 years			3 years
5858 Horton Street Building Lease and Expansion
Commitment And Contingencies [Line Items]
Tenant improvement allowance				$ 400
Lease initial term						87 months
Lease option to renew term description								The new lease has an initial term of 87 months beginning on the rent commencement date with the option to renew the lease for one additional term of five years at market rent.
Lease existence of option to renew term						true
Lease renewal term						5 years
5858 Horton Street Building Lease and 2019 Expansion
Commitment And Contingencies [Line Items]
Operating lease right-of-use ("ROU") assets								$ 9,700	10,300
Operating lease liability								$ 12,500	13,200
Operating lease, discount rate								7.80%
Tenant improvement allowance			$ 2,300						$ 2,300
5858 Horton Street Building Lease and 2019 Expansion \| Subsequent Event
Commitment And Contingencies [Line Items]
Lease extended term date		Dec. 31, 2030
Lease renewal term		12 months
5858 Horton Street Building Lease and 2024 Expansion
Commitment And Contingencies [Line Items]
Operating lease right-of-use ("ROU") assets								$ 0
Operating lease liability								0
5858 Horton Street Building Lease and 2024 Expansion \| Subsequent Event
Commitment And Contingencies [Line Items]
Lease extended term date		Dec. 31, 2030
Lease commencement date		Sep. 01, 2024
Area of office and warehouse space \| ft²		32,038
Future minimum lease payments		$ 13,200
Warehouse Lease Agreement
Commitment And Contingencies [Line Items]
Operating lease right-of-use ("ROU") assets								0
Operating lease liability								$ 0
Lease initial term	5 years 4 months
Lease commencement date	Aug. 31, 2024
Area of office and warehouse space \| ft²	7,800
Future minimum lease payments	$ 1,500

Marketable Securities - Additional Information (Details) - USD ($)	6 Months Ended	12 Months Ended
	Jun. 30, 2024	Dec. 31, 2023
Debt Securities, Available-for-Sale [Line Items]
Realized gains or losses on marketable securities	$ 0	$ 0
Maximum unrealized loss at individual instrument level	13,000
Aggregate unrealized loss	350,000
Allowance for credit loss	0	0
Other impairment charges related to marketable securities	$ 0	$ 0
Maximum
Debt Securities, Available-for-Sale [Line Items]
Maturity period of marketable securities	2 years

- Definition

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

+ References

+ Details

Summary of Significant Accounting Policies - Schedule of Concentration of Risk (Details) - Revenue - Customer Concentration Risk	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Customer A
Concentration Risk [Line Items]
Concentration risk, percentage	0.00%	96.00%	0.00%	98.00%
Customer B
Concentration Risk [Line Items]
Concentration risk, percentage	100.00%		100.00%
Customer
Concentration Risk [Line Items]
Concentration risk, percentage	100.00%	96.00%	100.00%	98.00%

401(K) Plan - Additional Information (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2024	Mar. 31, 2023	Jun. 30, 2024	Jun. 30, 2023
2014 401(k) Plan
Defined Contribution Plan Disclosure [Line Items]
Employer contribution	$ 0.4	$ 0.2	$ 0.8	$ 0.7

Summary of Significant Accounting Policies (Tables)

6 Months Ended

Jun. 30, 2024

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 June 30,		Six Months Ended June 30,
	2024	2023	2024	2023
Customer A	0%	96%	0%	98%
Customer B	100%	*	100%	*
Total	100%	96%	100%	98%

* Less than 10%

The Company did not have accounts receivable from its partners in collaboration and license agreements as of June 30, 2024 and December 31, 2023.

Schedule of Revenues by Geographic Region

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


	Three Months Ended June 30,				Six Months Ended June 30,
	2024		2023		2024		2024
Netherlands	$	—	$	230	$	—	$	525
United States		5		9		33		13
Total revenue	$	5	$	239	$	33	$	538

Condensed Statements of Operations (Unaudited) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Revenue:
Collaboration and license revenue	$ 5	$ 239	$ 33	$ 538
Operating expenses:
Research and development	31,860	23,584	59,727	46,002
General and administrative	10,601	8,791	20,898	16,777
Total operating expenses	42,461	32,375	80,625	62,779
Loss from operations	(42,456)	(32,136)	(80,592)	(62,241)
Other income (expense):
Interest income	7,548	2,512	13,293	3,955
Other income (expense), net	(45)	8	(55)	(12)
Total other income, net	7,503	2,520	13,238	3,943
Net loss	$ (34,953)	$ (29,616)	$ (67,354)	$ (58,298)
Net loss per share, basic	$ (0.63)	$ (0.77)	$ (1.29)	$ (1.63)
Net loss per share, diluted	$ (0.63)	$ (0.77)	$ (1.29)	$ (1.63)
Weighted-average shares outstanding used in computing net loss per share, basic	55,282,754	38,335,219	52,277,369	35,661,995
Weighted-average shares outstanding used in computing net loss per share, diluted	55,282,754	38,335,219	52,277,369	35,661,995

Fair Value Measurements - Summary of Changes in Fair Value of Level 3 Financial Instruments (Details) - Derivative Liability - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Beginning Balance	$ 369	$ 212
Change in fair value included in other income (expense), net	$ 13	$ 0
Fair Value, Recurring Basis, Unobservable Input Reconciliation, Liability, Gain (Loss), Statement of Income [Extensible List]	Other income (expense), net	Other income (expense), net
Ending Balance	$ 382	$ 212

Income Taxes - Additional Information (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Income Tax Disclosure [Abstract]
Provision for income tax	$ 0	$ 0	$ 0	$ 0

- Definition

Amount of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 270
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Subparagraph (i)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482964/270-10-50-1

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 280
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 32
-Subparagraph (ee)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482810/280-10-50-32

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 740
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 12
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482685/740-10-50-12

Reference 4: http://www.xbrl.org/2003/role/exampleRef
-Topic 740
-SubTopic 10
-Name Accounting Standards Codification
-Section 55
-Paragraph 231
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482663/740-10-55-231

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Topic 250
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 9
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147483443/250-10-50-9

Reference 6: http://www.xbrl.org/2003/role/disclosureRef
-Topic 740
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SAB Topic 6.I.7)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479360/740-10-S99-1

Reference 7: http://www.xbrl.org/2003/role/disclosureRef
-Topic 250
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 8
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147483443/250-10-50-8

Reference 8: http://www.xbrl.org/2003/role/disclosureRef
-Topic 740
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 10
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482685/740-10-50-10

Reference 9: http://www.xbrl.org/2003/role/disclosureRef
-Topic 280
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 22
-Subparagraph (h)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482810/280-10-50-22

Reference 10: http://www.xbrl.org/2003/role/disclosureRef
-Topic 944
-SubTopic 220
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-04(9))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147477250/944-220-S99-1

Reference 11: http://www.xbrl.org/2003/role/disclosureRef
-Name Accounting Standards Codification
-Section 45
-Paragraph 2
-Subparagraph (a)
-SubTopic 20
-Topic 740
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482659/740-20-45-2

Reference 12: http://www.xbrl.org/2003/role/disclosureRef
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08(h))
-SubTopic 10
-Topic 235
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480678/235-10-S99-1

+ Details

Research and Collaboration Arrangements - Summary of Deferred Revenue (Details) - USD ($) $ in Thousands	Jun. 30, 2024	Dec. 31, 2023
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Total deferred revenue	$ 1,318	$ 1,245
Cystic Fibrosis Foundation
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Total deferred revenue	$ 1,318	$ 1,245

Property and Equipment, Net - Additional Information (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Property, Plant and Equipment [Abstract]
Depreciation expense	$ 1.2	$ 1.1	$ 2.4	$ 2.1

Net Loss Per Share, Basic and Diluted - Outstanding Potentially Dilutive Securities Excluded From Computation of Diluted Net Loss Per Share (Details) - shares	6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive securities	8,748,581	6,745,647
Options Issued and Outstanding
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive securities	8,192,173	6,422,876
Restricted Stock Units Subject to Future Vesting
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive securities	258,117	0
2020 ESPP
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive securities	244,622	269,102
Common Stock Warrants
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive securities	53,669	53,669