Generation Bio (GBIO) 10-K 6 Mar 24 2023 FY Annual report Financial data

Cover Page

Cover Page - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Feb. 29, 2024	Jun. 30, 2023
Cover [Abstract]
Document Type	10-K
Document Annual Report	true
Document Period End Date	Dec. 31, 2023
Document Fiscal Year Focus	2023
Document Transition Report	false
Entity File Number	001-39319
Entity Registrant Name	GENERATION BIO CO.
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	81-4301284
Entity Address, Address Line One	301 Binney Street
Entity Address, City or Town	Cambridge
Entity Address, State or Province	MA
Entity Address, Postal Zip Code	02142
City Area Code	617
Local Phone Number	655-7500
Title of 12(b) Security	Common Stock, $0.0001 Par Value
Trading Symbol	GBIO
Security Exchange Name	NASDAQ
Entity Well-known Seasoned Issuer	No
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Non-accelerated Filer
Entity Small Business	true
Entity Emerging Growth Company	false
ICFR Auditor Attestation Flag	false
Entity Shell Company	false
Document Financial Statement Error Correction [Flag]	false
Entity Public Float			$ 299.4
Entity Common Stock, Shares Outstanding		66,477,111
Entity Central Index Key	0001733294
Current Fiscal Year End Date	--12-31
Document Fiscal Period Focus	FY
Amendment Flag	false
Auditor Name	Ernst & Young LLP
Auditor Firm ID	42
Auditor Location	Boston, Massachusetts

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Current assets:
Cash and cash equivalents	$ 66,446	$ 93,171
Marketable securities	197,918	185,920
Tenant receivable	3,960	395
Prepaid expenses and other current assets	4,294	7,530
Total current assets	272,618	287,016
Property and equipment, net	25,799	22,215
Operating lease right-of-use assets	69,852	59,208
Restricted cash	5,791	5,692
Deferred offering costs	433	434
Other long-term assets	265	1,699
Total assets	374,758	376,264
Current liabilities:
Accounts payable	2,346	662
Accrued expenses and other current liabilities	16,529	11,402
Deferred revenue	12,919	0
Operating lease liability	8,120	7,086
Total current liabilities	39,914	19,150
Deferred revenue, net of current portion	41,942	0
Operating lease liability, net of current portion	89,774	74,621
Total liabilities	171,630	93,771
Commitments and contingencies
Stockholders' equity:
Preferred stock, $0.0001 par value; 5,000,000 shares authorized and no shares issued or outstanding at December 31, 2023 and December 31, 2022	0	0
Common stock, $0.0001 par value; 150,000,000 shares authorized at December 31, 2023 and December 31, 2022; 66,205,550 and 59,505,437 shares issued and outstanding at December 31, 2023 and December 31, 2022, respectively	7	6
Additional paid-in capital	774,224	727,335
Accumulated other comprehensive income (loss)	274	(83)
Accumulated deficit	(571,377)	(444,765)
Total stockholders' equity	203,128	282,493
Total liabilities and stockholders' equity	$ 374,758	$ 376,264

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2023	Dec. 31, 2022
Statement of Financial Position [Abstract]
Preferred stock, par value per share	$ 0.0001	$ 0.0001
Preferred stock, shares authorized	5,000,000	5,000,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value per share	$ 0.0001	$ 0.0001
Common stock, shares authorized	150,000,000	150,000,000
Common stock, shares issued	66,205,550	59,505,437
Common stock, shares outstanding	66,205,550	59,505,437

Consolidated Statements of Oper

Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Revenues:
Collaboration revenue	$ 5,904	$ 0	$ 0
Operating expenses:
Research and development	93,617	96,718	85,247
General and administrative	50,850	44,464	33,854
Total operating expenses	144,467	141,182	119,101
Loss from operations	(138,563)	(141,182)	(119,101)
Other income:
Other income (expense) and interest income, net	11,951	4,543	(50)
Net loss	$ (126,612)	$ (136,639)	$ (119,151)
Net loss per share, basic	$ (1.96)	$ (2.35)	$ (2.12)
Net loss per share, diluted	$ (1.96)	$ (2.35)	$ (2.12)
Weighted average common shares outstanding, basic	64,483,520	58,114,893	56,295,409
Weighted average common shares outstanding, diluted	64,483,520	58,114,893	56,295,409
Comprehensive loss:
Net loss	$ (126,612)	$ (136,639)	$ (119,151)
Other comprehensive loss:
Unrealized gains (losses) on marketable securities	357	(83)	(9)
Comprehensive loss	$ (126,255)	$ (136,722)	$ (119,160)

Consolidated Statements of Stoc

Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Common Stock ModernaTX, Inc	Common Stock ATM Offering	Common Stock	Additional Paid-in Capital ModernaTX, Inc	Additional Paid-in Capital ATM Offering	Additional Paid-in Capital	Accumulated Other Comprehensive (Loss) Income	Accumulated Deficit	ModernaTX, Inc	ATM Offering	Total
Beginning balance at Dec. 31, 2020			$ 5			$ 456,974	$ 9	$ (188,975)			$ 268,013
Beginning balance, shares at Dec. 31, 2020			46,291,877
Issuance/Sale of common stock			$ 1			211,285					211,286
Issuance/Sale of common stock, shares			9,200,000
Issuance of common stock upon exercise of stock options						3,257					3,257
Issuance of common stock upon exercise of stock options, shares			767,185
Vesting of restricted common stock, shares			672,049
Issuance of common stock under other equity plans						493					493
Issuance of common stock under other equity plans, shares			38,507
Stock-based compensation expense						17,857					17,857
Unrealized gains (losses) on marketable securities							(9)				(9)
Net loss								(119,151)			(119,151)
Ending balance at Dec. 31, 2021			$ 6			689,866		(308,126)			381,746
Ending balance, shares at Dec. 31, 2021			56,969,618
Issuance/Sale of common stock					$ 12,338					$ 12,338
Issuance/Sale of common stock, shares		1,795,524
Issuance of common stock upon exercise of stock options						628					628
Issuance of common stock upon exercise of stock options, shares			150,699
Vesting of restricted common stock						(581)					(581)
Vesting of restricted common stock, shares			446,141
Issuance of common stock under other equity plans						632					632
Issuance of common stock under other equity plans, shares			143,455
Stock-based compensation expense						24,452					24,452
Unrealized gains (losses) on marketable securities							(83)				(83)
Net loss								(136,639)			(136,639)
Ending balance at Dec. 31, 2022			$ 6			727,335	(83)	(444,765)			282,493
Ending balance, shares at Dec. 31, 2022			59,505,437
Issuance/Sale of common stock	$ 1			$ 22,567					$ 22,568
Issuance/Sale of common stock, shares	5,859,375
Vesting of restricted common stock						(565)					(565)
Vesting of restricted common stock, shares			605,524
Issuance of common stock under other equity plans						546					546
Issuance of common stock under other equity plans, shares			235,214
Stock-based compensation expense						24,341					24,341
Unrealized gains (losses) on marketable securities							357				357
Net loss								(126,612)			(126,612)
Ending balance at Dec. 31, 2023			$ 7			$ 774,224	$ 274	$ (571,377)			$ 203,128
Ending balance, shares at Dec. 31, 2023			66,205,550

Consolidated Statements of St_2

Consolidated Statements of Stockholders' Equity (Parenthetical) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021
IPO
Stock issuance costs		$ 590
ATM Offering
Stock issuance costs	$ 404

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Cash flows from operating activities:
Net loss	$ (126,612)	$ (136,639)	$ (119,151)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	24,341	24,452	17,857
Depreciation and amortization expense	5,263	5,130	4,532
Amortization (accretion) of premium (discount) on marketable securities, net	(9,340)	(2,289)	529
Loss on sale of property and equipment	27	41	237
Loss on impairment of property and equipment	39	5,811	0
Changes in operating assets and liabilities:
Tenant receivable	(3,565)	(395)	0
Prepaid expenses and other current assets	3,156	(3,489)	1,367
Operating lease right-of-use assets	(10,644)	5,935	3,072
Other noncurrent assets	1,433	(1,295)	(152)
Accounts payable	1,087	(1,386)	1,756
Accrued expenses and other current liabilities	4,287	795	1,833
Deferred revenue	41,596	0	0
Operating lease liability	16,187	881	(3,701)
Net cash used in operating activities	(52,745)	(102,448)	(91,821)
Cash flows from investing activities:
Purchases of property and equipment	(7,397)	(8,801)	(5,958)
Proceeds from sale of property and equipment	0	0	105
Purchases of marketable securities	(405,301)	(323,714)	0
Maturities of marketable securities	403,000	140,000	198,900
Net cash (used in) provided by investing activities	(9,698)	(192,515)	193,047
Cash flows from financing activities:
Proceeds from public offering of common stock, net of underwriting discounts and commissions	0	0	211,876
Payment of share issuance costs	(164)	(50)	(955)
Proceeds from exercise of stock options and ESPP, net	546	1,260	3,750
Tax withholding payments related to net share settlements of restricted stock units	(565)	(581)	0
Net cash provided by financing activities	35,817	12,989	214,671
Net increase (decrease) in cash, cash equivalents and restricted cash	(26,626)	(281,974)	315,897
Cash, cash equivalents and restricted cash at beginning of period	98,863	380,837	64,940
Cash, cash equivalents and restricted cash at end of period	72,237	98,863	380,837
Supplemental disclosure of noncash investing and financing information:
Lease liabilities arising from obtaining right-of-use assets	0	0	34,806
Purchases of property and equipment included in accounts payable and accrued expenses	1,461	24	1,515
Unrealized gains (losses) on marketable securities	356	(83)	(9)
Offering costs included in accounts payable and accrued expenses	0	0	55
ModernaTX, Inc
Cash flows from financing activities:
Proceeds from issuance/sale of common stock	36,000	0	0
ATM Offering
Cash flows from financing activities:
Proceeds from issuance/sale of common stock	$ 0	$ 12,360	$ 0

Nature of the Business and Basi

Nature of the Business and Basis of Presentation	12 Months Ended
	Dec. 31, 2023
Accounting Policies [Abstract]
Nature of the Business and Basis of Presentation	1. Nature of the Business and Basis of Presentation Generation Bio Co., or Generation Bio, was incorporated on October 21, 2016 as Torus Therapeutics, Inc. and subsequently changed its name to Generation Bio Co. Generation Bio Co. and its consolidated subsidiary, or the company, we, our or us, are innovating non-viral genetic medicines to provide durable, redosable treatments for potentially hundreds of millions of patients living with rare and prevalent diseases. We are developing two distinct and complementary platforms that we believe will enable highly differentiated therapeutic applications. Our first platform is a potent, highly selective cell-targeted lipid nanoparticle, or ctLNP, delivery system for nucleic acids, which is designed to avoid off-target clearance by the liver and spleen, enabling ctLNPs to persist in systemic circulation and allowing for highly selective and potent ligand-driven targeting to specific tissues and cell types. The identification and optimization of new ligands to target new tissues and cell types is an efficient, flexible, and modular process, which we believe will allow us to rapidly expand our portfolio. Our second platform is our novel immune-quiet DNA, or iqDNA, which is a variant of our closed-ended DNA, or ceDNA, designed to enable long-lasting high levels of gene expression from non-integrating episomes, while avoiding innate immune sensors that have long prevented DNA from use in non-viral systems. Underpinning the iqDNA platform is our highly scalable capsid-free manufacturing process that uses our proprietary cell-free rapid enzymatic synthesis, or RES, to produce highly pure iqDNA at scale. We are headquartered in Cambridge, Massachusetts. We are 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, dependence on key personnel, protection of proprietary technology, compliance with government regulations, the ability to establish clinical- and commercial-scale manufacturing processes and the ability to secure additional capital to fund operations. Programs currently under development will require significant additional research and development efforts, including extensive preclinical and clinical testing and regulatory approval prior to commercialization of a product. These efforts require significant amounts of additional capital, adequate personnel and infrastructure and extensive compliance-reporting capabilities. Even if our development efforts are successful, it is uncertain when, if ever, we will realize significant revenue from product sales. In June 2020, we completed our initial public offering, or IPO, pursuant to which we issued and sold 12,105,263 shares of our common stock, including 1,578,947 shares pursuant to the full exercise of the underwriters’ option to purchase additional shares, resulting in net proceeds of $210.7 million, after deducting underwriting discounts and commissions and other offering expenses. Upon the closing of the IPO, all of our outstanding convertible preferred stock automatically converted into shares of common stock. In January 2021, we issued and sold 9,200,000 shares of our common stock, including 1,200,000 shares pursuant to the full exercise of the underwriters’ option to purchase additional shares, in a follow-on public offering, resulting in net proceeds of $211.3 million after deducting underwriting discounts and commissions and other offering expenses. The accompanying consolidated financial statements have been prepared on the basis of continuity of operations, realization of assets and the satisfaction of liabilities and commitments in the ordinary course of business. Since inception, we have funded our operations with proceeds from the sale of instruments convertible into convertible preferred stock (which converted into convertible preferred stock in 2017), sales of convertible preferred stock (which converted into common stock in 2020), and sales of common stock in underwritten public offerings, “at-the-market” offerings, and in a private placement, as well as payments pursuant to our collaboration with ModernaTX, Inc., or Moderna. We have incurred recurring losses, including net losses of $126.6 million for the year ended December 31, 2023. As of December 31, 2023, we had an accumulated deficit of $571.4 million. We expect to continue to generate operating losses in the foreseeable future. As of March 6, 2024, the issuance date of these consolidated financial statements, we expect that our cash, cash equivalents, and marketable securities will be sufficient to fund our operating expenses and capital expenditure requirements for at least 12 months. We will need to obtain additional funding through public or private equity offerings, debt financings, collaborations, strategic alliances and/or licensing arrangements. We may not be able to obtain financing on acceptable terms, or at all, and we may not be able to enter into additional collaborative or strategic alliances or licensing arrangements. The terms of any financing may adversely affect the holdings or the rights of our stockholders. Arrangements with collaborators or others may require us to relinquish rights to certain of our technologies or programs. If we are unable to obtain funding, we could be forced to delay, reduce or eliminate some or all of our research and development programs, pipeline expansion or commercialization efforts, which could adversely affect our business prospects. Although management will continue to pursue these plans, there is no assurance that we will be successful in obtaining sufficient funding on terms acceptable to us to fund continuing operations when needed or at all. The accompanying consolidated financial statements reflect the operations of Generation Bio and our wholly owned subsidiary, Generation Bio Securities Corporation. Intercompany balances and transactions have been eliminated in consolidation. The accompanying consolidated financial statements have been prepared in conformity with generally accepted accounting principles in the United States of America, or GAAP. Any reference in these notes to applicable guidance is meant to refer to the authoritative GAAP as found in the Accounting Standards Codification, or ASC, and Accounting Standards Update, or ASU, of the Financial Accounting Standards Board, or FASB.

Summary of Significant Accounti

Summary of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2023
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	2. Summary of Significant Accounting Policies Use of estimates The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities at the date of the financial statements and the reported amounts of expenses during the reporting periods. Significant estimates and assumptions reflected in these consolidated financial statements include, but are not limited to, the accrual of research and development expenses and stock-based compensation expense. We base our estimates on historical experience, known trends and other market-specific or other relevant factors that we believe to be reasonable under the circumstances. On an ongoing basis, management evaluates its estimates, as there are changes in circumstances, facts and experience. Changes in estimates are recorded in the period in which they become known. Actual results may differ from those estimates or assumptions. Concentrations of credit risk and of significant suppliers Financial instruments that potentially expose us to concentrations of credit risk consist primarily of cash, cash equivalents, and marketable securities. We believe that we are not exposed to significant credit risk due to the financial strength of the depository institutions in which our cash, cash equivalents, and marketable securities are held. We maintain our cash equivalents in money market funds that invest in U.S. treasury securities. We have adopted an investment policy that limits the amounts that we may invest in the securities of a single issuer with the exclusion of the U.S. government. We have not experienced any credit losses. We are dependent on a small number of third-party suppliers for our drug substance and drug product. In particular, we rely, and expect to continue to rely, on third-party suppliers for certain materials and components required for the production of any product candidates we may develop for our programs. These programs could be adversely affected by a significant interruption in the supply process. Cash equivalents We consider all highly liquid investments with a remaining maturity when purchased of three months or less to be cash equivalents. Restricted cash Amounts included in restricted cash represent amounts pledged as collateral for letters of credit required for security deposits on our leased facilities. These amounts are classified as restricted cash on our consolidated balance sheets. Marketable securities Our marketable securities, which consisted of debt securities as of December 31, 2023, are classified as available-for-sale and are reported at fair value. Unrealized gains and losses on available-for-sale debt securities are reported as a component of accumulated other comprehensive income (loss) in stockholders’ equity. We assess our available-for-sale debt securities under the available-for-sale debt security impairment model in Topic 326 as of each reporting date in order to determine if a portion of any decline in fair value below carrying value recognized on our available-for-sale debt securities is the result of a credit loss. We record credit losses in the consolidated statements of operations and comprehensive loss as credit loss expense, which is limited to the difference between the fair value and the amortized cost of the security. To date, we have not recorded any credit losses on our available-for-sale debt securities. Fair value measurements Certain of our assets and liabilities are carried at fair value under GAAP. Fair value is defined as the exchange price that would be received for an asset or paid to transfer a liability (an exit price) in the principal or most advantageous market for the asset or liability in an orderly transaction between market participants on the measurement date. Valuation techniques used to measure fair value must maximize the use of observable inputs and minimize the use of unobservable inputs. Financial assets and financial liabilities carried at fair value are to be classified and disclosed in one of the following three levels of the fair value hierarchy, of which the first two are considered observable and the last is considered unobservable: ● Level 1—Quoted prices in active markets for identical assets or liabilities. ● Level 2—Observable inputs (other than Level 1 quoted prices), such as quoted prices in active markets for similar assets or liabilities, quoted prices in markets that are not active for identical or similar assets or liabilities, or other inputs that are observable or can be corroborated by observable market data. ● Level 3—Unobservable inputs that are supported by little or no market activity and that are significant to determining the fair value of the assets or liabilities, including pricing models, discounted cash flow methodologies and similar techniques. Our cash equivalents and marketable securities are carried at fair value, determined according to the fair value hierarchy described above (see Note 3). The carrying values of our accounts payable and accrued expenses approximate their fair values due to the short-term nature of these liabilities. Property and equipment Property and equipment are stated at cost less accumulated depreciation and amortization. Depreciation and amortization expense is recognized using the straight-line method over the estimated useful life of each asset as follows: ​ ​ ​ ​ ​ Estimated Useful Life Laboratory equipment ​ 5 years Computer equipment and software 3 years Furniture and fixtures 5 years Leasehold improvements Shorter of remaining life of lease or useful life ​ Costs for capital assets not yet placed into service are capitalized as construction-in-progress and depreciated once placed into service. Upon retirement or sale, the cost of assets disposed of and the related accumulated depreciation and amortization are removed from the accounts and any resulting gain or loss is included in loss from operations. Expenditures for repairs and maintenance which do not improve or extend the life of the respective assets are charged to expense as incurred. Impairment of long-lived assets Long-lived assets consist of property and equipment. We evaluate the recoverability of our long-lived assets when circumstances indicate that an event of impairment may have occurred. We recognize an impairment loss only if the carrying amount of a long-lived asset is not recoverable based on its undiscounted future cash flows. Impairment is measured based on the difference between the carrying value of the related assets and the fair value of such assets. We recorded less than $0.1 million and $5.8 million in impairment losses on long-lived assets during the years ended December 31, 2023 and 2022, respectively. We did not record any impairment losses on long-lived assets during the year ended December 31, 2021. Leases We determine whether a contract is, or contains, a lease at inception. We classify each of our leases as operating or financing considering factors such as the length of the lease term, the present value of the lease payments, the nature of the asset being leased, and the potential for ownership of the asset to transfer during the lease term. Leases with terms greater than one-year are recognized on the consolidated balance sheets as right-of-use assets and lease liabilities and are measured at the present value of the fixed payments due over the expected lease term less the present value of any incentives, rebates or abatements we expect to receive from the lessor. Options to extend a lease are included in the expected lease term if exercise of the option is deemed reasonably certain. Costs determined to be variable and not based on an index or rate are not included in the measurement of the lease liability and are expensed as incurred. The interest rate implicit in lease contracts is typically not readily determinable. As such, we utilize the appropriate incremental borrowing rate, which is the rate incurred to borrow on a collateralized basis an amount equal to the lease payments over a similar term and in a similar economic environment. To estimate our incremental borrowing rate, a credit rating applicable to our company is estimated using a synthetic credit rating analysis since we do not currently have a rating agency-based credit rating. We record expense to recognize fixed lease payments on a straight-line basis over the expected lease term. We have elected the practical expedient not to separate lease and non-lease components for real estate leases. Segment information We have determined that our chief executive officer is the chief operating decision maker, or CODM. The CODM reviews financial information presented on a consolidated basis. Resource allocation decisions are made by the CODM based on consolidated results. There are no segment managers who are held accountable by the CODM for operations, operating results, and planning for levels or components below the consolidated unit level. As such, we have concluded that we operate as one segment. All long-lived assets are located in the United States. Research and development costs Research and development costs are expensed as incurred. Research and development expenses consist of costs incurred in performing research and development activities, including salaries and bonuses, stock-based compensation, employee benefits, facilities costs, laboratory supplies, depreciation and amortization, manufacturing expenses and external costs of vendors engaged to conduct preclinical development activities as well as the cost of licensing technology. Upfront payments and milestone payments made for the licensing of technology with no alternative future use are expensed as research and development expenses in the period in which they are incurred. Advance payments for goods or services to be received in the future for use in research and development activities are recorded as prepaid expenses. The prepaid amounts are expensed as the related goods are delivered or the services are performed. Research and manufacturing contract costs and accruals We have entered into various research and development and manufacturing contracts. These agreements are generally cancelable, and costs for research and development and manufacturing activities are expensed in the period in which they are incurred. Payments for such activities are based on the terms of the individual arrangements, which may differ from the pattern of costs incurred. We record accruals for costs incurred but not billed. When evaluating the adequacy of the accrued liabilities, we analyze progress of the research studies and manufacturing activities, including the phase or completion of events, invoices received and contracted costs. Significant judgments and estimates are made in determining the accrued balances at the end of any reporting period. Actual results could differ from our estimates. Our historical accrual estimates have not been materially different from the actual costs. Patent costs All patent-related costs incurred in connection with filing and prosecuting patent applications are expensed as incurred due to the uncertainty about the recovery of the expenditure. Amounts incurred are classified as general and administrative expenses. Stock-based compensation We measure stock options with service-based vesting or performance-based vesting granted to employees, non-employees and directors based on the fair value on the date of grant using the Black-Scholes option-pricing model. We measure restricted common stock awards and units using the difference between the purchase price per share of the award or unit, if any, and the fair value of our common stock at the date of grant. Compensation expense for awards with service-based vesting is generally recognized over the vesting period of the award using the straight-line method to record the expense. We use the graded-vesting method to record the expense of awards with both service-based and performance-based vesting conditions, commencing once achievement of the performance condition becomes probable. We account for forfeitures of share-based awards as they occur. We classify stock-based compensation expense in our consolidated statements of operations and comprehensive loss in the same manner in which the award recipient’s payroll costs are classified or in which the award recipient’s service payments are classified. Comprehensive loss Comprehensive loss includes net loss as well as other changes in stockholders’ equity that result from transactions and economic events other than those with stockholders. For the years ended December 31, 2023, 2022 and 2021, our only element of other comprehensive loss was unrealized gains (losses) on available-for-sale debt securities. Income taxes We account for income taxes using the asset and liability method, which requires the recognition of deferred tax assets and liabilities for the expected future tax consequences of events that have been recognized in the consolidated financial statements or in our tax returns. Deferred tax assets and liabilities are determined on the basis of the differences between the financial statements and tax basis of assets and liabilities using enacted tax rates in effect for the year in which the differences are expected to reverse. Changes in deferred tax assets and liabilities are recorded in the provision for income taxes. We assess the likelihood that our deferred tax assets will be recovered from future taxable income and, to the extent we believe, based upon the weight of available evidence, that it is more likely than not that all or a portion of the deferred tax assets will not be realized, a valuation allowance is established through a charge to income tax expense. Potential for recovery of deferred tax assets is evaluated by estimating the future taxable profits expected and considering prudent and feasible tax planning strategies. We account for uncertainty in income taxes recognized in the consolidated financial statements by applying a two-step process to determine the amount of tax benefit to be recognized. First, the tax position must be evaluated to determine the likelihood that it will be sustained upon external examination by the taxing authorities. If the tax position is deemed more-likely-than-not to be sustained, the tax position is then assessed to determine the amount of benefit to recognize in the consolidated financial statements. The amount of the benefit that may be recognized is the largest amount that has a greater than 50% likelihood of being realized upon ultimate settlement. The provision for income taxes includes the effects of any resulting tax reserves, or unrecognized tax benefits, that are considered appropriate as well as the related net interest and penalties. Our policy is to recognize interest and/or penalties related to income tax matters in income tax expense. We have accrued no amounts for interest and penalties on our consolidated balance sheets at December 31, 2023 and 2022. Net loss per share Basic net income (loss) per common share is computed by dividing the net income (loss) by the weighted average number of shares of common stock outstanding for the period. Diluted net income (loss) per common share is computed by dividing net income (loss) by the weighted average number of shares of common stock outstanding for the period, including potential dilutive common shares assuming the dilutive effect of outstanding stock awards. For periods in which we report a net loss, diluted net loss per common share is the same as basic net loss per common share, since dilutive common shares are not assumed to have been issued if their effect is anti-dilutive. We reported a net loss for each of the years ended December 31, 2023, 2022 and 2021. Revenue Recognition We enter into collaboration agreements that are within the scope of ASC Topic 606, “Revenue from Contracts with Customers,” or ASC 606, under which we license rights to certain of our potential product candidates and perform research and development services. The terms of these contracts typically include payment of the following: non-refundable, upfront fees; reimbursement of research and development costs; development, regulatory, and commercial milestone payments; royalties on net sales of licensed products, and a premium or discount on the sale of our common stock. Under ASC 606, an entity recognizes revenue when its customer obtains control of promised goods or services, in an amount that reflects the consideration that the entity expects to receive in exchange for those goods or services. To determine the appropriate amount of revenue to be recognized for contracts determined to be within the scope of ASC 606, we perform the following five steps: (i) identification of the promised goods or services in the contract; (ii) determination of whether the promised goods or services are performance obligations, including whether they are distinct in the context of the contract; (iii) measurement of the transaction price, including the constraint on variable consideration; (iv) allocation of the transaction price to the performance obligations; and (v) recognition of revenue when, or as, we satisfy each performance obligation. We only apply the five-step model to contracts when it is probable that we will collect consideration we are entitled to in exchange for the goods or services we transfer to the customer. The promised goods or services in our arrangements typically consist of license rights to our intellectual property and research and development services. We provide options to additional items in the contract, which will be accounted for as separate contracts if and when the other party elects to exercise such options, unless the option provides a material right to such party. We evaluate the other party’s options for material rights, or options to acquire additional goods or services for free or at a discount. If the other party’s options are determined to represent a material right, the material right is recognized as a separate performance obligation at the outset of the contract. Performance obligations are promised goods or services in a contract to transfer a distinct good or service to the other party and are considered distinct when (i) the other party can benefit from the good or service on its own or together with other readily available resources and (ii) the promised good or service is separately identifiable from other promises in the contract. In assessing whether promised goods or services are distinct, we consider factors such as the stage of development of the underlying intellectual property, the capabilities of the other party to develop the intellectual property on its own or whether the required expertise is readily available and whether the goods or services are integral or dependent to other goods or services in the contract. We estimate the transaction price based on the amount expected to be received for transferring the promised goods or services in the contract. The consideration may include fixed consideration or variable consideration. At the inception of each contract that includes variable consideration, we evaluate the number of potential payments and the likelihood that the payments will be received. We utilize either the most likely amount method or expected amount method to estimate the amount expected to be received based on which method best predicts the amount expected to be received. The amount of variable consideration that is included in the transaction price may be constrained and is included in the transaction price only to the extent that it is probable that a significant reversal in the amount of the cumulative revenue recognized will not occur in a future period. Our contracts include development, regulatory, and commercial milestone payments that will be assessed under the most likely amount method and constrained if it is probable that a significant revenue reversal would occur. Milestone payments that are not within our control or the counterparty’s control, such as regulatory approvals, are not considered probable of being achieved until those approvals are received. At the end of each reporting period, we re-evaluate the probability of achievement of such development, regulatory, and commercial milestones and any related constraint, and if necessary, we adjust our estimate of the overall transaction price. Any such adjustments are recorded on a cumulative catch-up basis, which would affect collaboration revenues in the period of adjustment. To date, we have not recognized any consideration related to the achievement of development, regulatory, or commercial milestone revenue resulting from our collaboration contracts. For contracts that include sales-based royalties, including milestone payments based on the level of sales, and where the license is deemed to be the predominant item to which the royalties relate, we recognize revenue at the later of (i) when the related sales occur, or (ii) when the performance obligation to which some or all of the royalty has been allocated has been satisfied (or partially satisfied). To date, we have not recognized any consideration related to sales-based royalty revenue resulting from our collaboration contract. We allocate the transaction price based on the relative estimated stand-alone selling price of each of the performance obligations. We must develop assumptions that require judgment to determine the stand-alone selling price for each performance obligation identified in the contract. We utilize key assumptions to determine the stand-alone selling price for service obligations, which may include other comparable transactions, pricing considered in negotiating the transaction and the estimated costs. Additionally, in determining the standalone selling price for material rights, we utilize comparable transactions, clinical trial success probabilities, and estimates of option exercise likelihood. Variable consideration is allocated specifically to one or more performance obligations in a contract when the terms of the variable consideration relate to the satisfaction of the performance obligation and the resulting amounts allocated are consistent with the amounts we would expect to receive for the satisfaction of each performance obligation. The consideration allocated to each performance obligation is recognized as revenue when control is transferred for the related goods or services. For performance obligations that consist of licenses and other promises, we utilize judgment to assess the nature of the combined performance obligation to determine whether the combined performance obligation is satisfied over time or at a point in time and, if over time, the appropriate method of measuring progress. We evaluate the measure of progress each reporting period and, if necessary, adjust the measure of performance and related revenue recognition. For performance obligations satisfied as of a point in time, the allocated consideration is deferred until the goods and services are transferred, or, in the case of options to deliver additional goods and services, the option is exercised or expires. Upfront payments and fees are recorded as deferred revenue upon receipt or when due until we perform our obligations. Amounts are recorded as accounts receivable when our right to consideration is unconditional. Restructuring and other costs Restructuring and other costs are comprised of employee separation costs, asset impairments, and other associated costs primarily related to implementing the restructuring plan. Employee separation costs principally consist of one-time termination benefits and contractual termination benefits for severance, other termination benefit costs, and stock-based compensation expense for the acceleration of stock awards. We record restructuring charges based on whether the termination benefits are provided under an on-going benefit arrangement or under a one-time benefit arrangement. We account for on-going benefit arrangements, such as those documented by separation agreements, in accordance with ASC 712, Nonretirement Postemployment Benefits. Under ASC 712, liabilities for post-employment benefits are recorded at the time the obligations are probable of being incurred and can be reasonably estimated. We account for one-time employment benefit arrangements in accordance with ASC 420 Exit or Disposal Cost Obligations. One-time termination benefits are expensed at the date the entity notifies the employee, unless the employee must provide future service, in which case the benefits are expensed ratably over the future service period. Other associated costs are recognized in the period in which the liability is incurred. ​ ​ ​ Recently issued accounting pronouncements ​ In December 2023, the FASB issued its final standard to improve income tax disclosures. This standard, issued as ASU 2023-09, requires public business entities to annually disclose specific categories in the rate reconciliation and provide additional information for reconciling items that meet a quantitative threshold. This update is effective for annual periods beginning after December 15, 2025. We are currently evaluating when to adopt the guidance and the impact the guidance will have our consolidated financial statements. ​

Marketable Securities and Fair

Marketable Securities and Fair Value Measurements	12 Months Ended
	Dec. 31, 2023
Marketable Securities and Fair Value Measurements
Marketable Securities and Fair Value Measurements	3. Marketable securities and fair value measurements The following tables present our marketable securities by security type as of December 31, 2023 and 2022: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ As of December 31, 2023 ​ ​ ​ Gross Gross ​ ​ ​ Amortized ​ Unrealized ​ Unrealized ​ Fair (in thousands) ​ Cost ​ Gains ​ Losses ​ Value U.S. treasury securities ​ $ 197,644 ​ $ 274 ​ $ — ​ $ 197,918 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ As of December 31, 2022 ​ ​ ​ Gross Gross ​ ​ ​ Amortized ​ Unrealized ​ Unrealized ​ Fair (in thousands) ​ Cost ​ Gains ​ Losses ​ Value U.S. treasury securities ​ $ 186,003 ​ $ 13 ​ $ (96) ​ $ 185,920 ​ Our marketable securities, as of December 31, 2023 and 2022, consisted of investments that matured within one year. The following tables present our assets that are measured at fair value on a recurring basis and indicate the level within the fair value hierarchy of the valuation techniques that we utilized to determine such fair value: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Fair Value Measurements at December 31, 2023 Using: (in thousands) ​ Level 1 ​ Level 2 ​ Level 3 ​ Total Cash equivalents: ​ ​ ​ ​ Money market funds ​ $ 38,210 ​ $ — ​ $ — ​ $ 38,210 Marketable securities: ​ ​ ​ ​ U.S. treasury securities ​ — ​ 197,918 ​ — ​ 197,918 Totals ​ $ 38,210 ​ $ 197,918 ​ $ — ​ $ 236,128 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Fair Value Measurements at December 31, 2022 Using: (in thousands) Level 1 Level 2 Level 3 Total Cash equivalents: ​ ​ ​ ​ ​ Money market funds ​ $ 77,010 ​ $ — ​ $ — ​ $ 77,010 Marketable securities: ​ ​ ​ ​ U.S. treasury securities ​ — ​ 185,920 ​ — ​ 185,920 Totals ​ $ 77,010 ​ $ 185,920 ​ $ — ​ $ 262,930 ​ ​

Collaboration and License Agree

Collaboration and License Agreement	12 Months Ended
	Dec. 31, 2023
ModernaTX, Inc
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Collaboration and License Agreement	4. Collaboration and License Agreement Moderna Collaboration and License Agreement In March 2023, we entered into a Collaboration and License Agreement, or the Collaboration Agreement, with Moderna to collaborate on developing treatments for certain diseases by targeting delivery of nucleic acids to liver cells and certain cells outside of the liver. Under the Collaboration Agreement, the parties have agreed to collaborate on preclinical research programs relating to lipid nanoparticle, or LNP, delivery systems and nucleic acid payloads, with each party obtaining certain rights to intellectual property used in and arising out of such research programs. Each party will be solely responsible for its own clinical development and commercialization of products under the Collaboration Agreement. Moderna will reimburse us for the internal and external costs incurred by us in conducting the research programs, to the extent consistent with such research plans and budgets. Moderna has exclusive options, upon payment of option exercise fees, to obtain worldwide, exclusive, sublicensable licenses under specified company intellectual property to develop, manufacture and commercialize (a) products comprising LNP delivery systems and nucleic acid payloads that are directed to (i) up to two liver targets, (ii) up to two non-liver targets and (iii) a third liver or non-liver target and (b) Exclusive Targets, which are Independent Program Products (as defined below) that include messenger RNA, or mRNA, that are directed to gene and protein targets in any of certain agreed-upon immune cell types, referred to as the Cell Target Types. Subject to the exclusivity obligations described below, each party has granted to the other a worldwide, non-exclusive, sublicensable license under certain LNP-related intellectual property arising out of the non-liver ctLNP program, or the Joint Collaboration ctLNP Intellectual Property, to develop, manufacture and commercialize products comprising LNP delivery systems and nucleic acid payloads directed to gene and protein targets in any of the Cell Target Types, or Independent Program Products. Each party is obligated to use commercially reasonable efforts to complete the activities assigned to it under the research plans, and Moderna is further obligated to use commercially reasonable efforts to develop, seek regulatory approval for and commercialize at least one product directed to each target for which Moderna exercises its exclusive license option in at least one indication in the United States and in specified European countries. We have agreed not to, directly or indirectly, alone or with, for or through any third party, develop, manufacture, commercialize or exploit (a) products containing mRNA that are directed to any of the Cell Target Types, during an agreed-upon exclusivity period, which may be extended by payment of extension fees, (b) products directed to any liver target or non-liver target during the option periods for those targets, (c) products directed to any liver target or non-liver target for which Moderna has exercised its exclusive license option or (d) products containing mRNA that are directed to any Exclusive Target for which Moderna has exercised its exclusive license option. Under the terms of the Collaboration Agreement, in April 2023, Moderna made an upfront payment to us of $40.0 million, and paid us $7.5 million in prepaid research funding. In addition, we are eligible to receive up to $1.8 billion in milestone payments upon the achievement of specified development, regulatory, commercial, and sales milestone events, research term extension fees and exclusivity extension fees. Subject to reductions in specified circumstances, we will also be entitled to receive tiered royalties: (i) ranging from high-single-digits to low-double-digits on sales of licensed products that are directed to any liver target or non-liver target with respect to which Moderna has exercised its exclusive license option, and (ii) in the single digits on sales of Independent Program Products, including the exclusively licensed Independent Program Products directed to the Exclusive Targets. In consideration for the non-exclusive license granted by Moderna to us under the Joint Collaboration ctLNP Intellectual Property, we have agreed to pay Moderna tiered royalties in the single digits on sales of Independent Program Products that include mRNA, subject to reductions in specified circumstances. Royalties will be paid by each party, on a licensed product-by-licensed product and country-by-country basis, until the latest to occur of: (i) expiration of the last-to-expire of specified licensed patent rights; (ii) expiration of regulatory exclusivity; or (iii) ten years after the first commercial sale of the applicable licensed product. In addition, in connection with the execution of the Collaboration Agreement, we entered into a Share Purchase Agreement, or the Share Purchase Agreement, with Moderna, pursuant to which we issued and sold 5,859,375 shares of our common stock to Moderna, at a price of $6.14 per share, for an aggregate purchase price of $36.0 million, which closed concurrently with the execution of the Collaboration Agreement. Under the Share Purchase Agreement, Moderna has the right, subject to certain terms and conditions, to purchase up to 3.06% of the outstanding shares of our common stock (on a post-closing basis) in connection with a future equity financing of at least $25.0 million by us. Moderna Agreement Assessment ​ We assessed the promised goods and services under the Collaboration Agreement, in accordance with ASC 606. At inception, the Collaboration Agreement included one combined performance obligation, which includes the license to the ctLNP technology to target indications outside of the liver and the related research services to develop such technology, as the two items are not distinct in context of the contract. The Collaboration Agreement also provides Moderna with options to receive additional research services and options to receive exclusive licenses. The options to receive exclusive licenses allow Moderna to develop and commercialize product candidates that utilize our ctLNP and ceDNA technology for targets within the liver, as well as utilizing the ctLNP technology to be developed as part of the Collaboration Agreement and our ceDNA technology for targets outside the liver. These options are considered to be priced at a discount to its standalone selling price and therefore are considered to be material rights. ​ The initial transaction price included a $40.0 million upfront fee, premium paid over the fair value of the common stock of $13.3 million in connection with shares issued and sold to Moderna under the Share Purchase Agreement, and estimated revenue associated with the payment for research services, including $7.5 million in prepaid research services. We utilized the expected amount method to determine the amount of reimbursement for these activities. We utilized the most likely amount method to determine the amount of consideration to include in the transaction price related to any variable consideration related to exclusivity fees, and milestones, and the royalty payments are constrained based on the royalty constraint. No amounts are included in the transaction price related to these elements. ​ We initially allocated the transaction price to each unit of account as follows: ​ ​ ​ ​ ​ ​ ​ ​ Performance Obligations (in thousands) ​ Standalone Selling Price ​ Transaction Price Allocated ctLNP technology and research license ​ $ 52,500 ​ $ 42,576 First liver program commercialization option license ​ ​ 7,000 ​ ​ 5,677 Second liver program commercialization option license ​ 7,000 ​ 5,677 First non-liver program commercialization option license ​ 11,700 ​ 9,488 Second non-liver program commercialization option license ​ 11,700 ​ 9,488 Third liver or non-liver program commercialization option license ​ ​ 6,150 ​ ​ 4,987 Total ​ $ 96,050 ​ $ 77,893 ​ ​ The transaction price was allocated to each unit of account based on the relative estimated standalone selling prices, over which management has applied significant judgment, of each element. We developed the estimated standalone selling price for combined performance obligation and each of the options to receive licenses primarily based on the probability-weighted present value of expected future cash flows associated with each license related to each specific program and an estimate of the costs to provide services including a reasonable return. In developing such estimate, we also considered applicable market conditions and relevant entity-specific factors, including those factors contemplated in negotiating the agreement, the probability of success and the time needed to commercialize a product candidate pursuant to the associated license. ​ On a quarterly basis, we measure proportional performance of the combined performance obligation over time using an input method based on cost incurred relative to the total estimated costs by determining the proportion of effort incurred as a percentage of total effort we expect to expend. This ratio is then applied to the transaction price allocated to the combined performance obligation and each of the options to receive licenses. Any changes to these estimates will be recognized in the period in which they change as a cumulative catch up. All allocated consideration for the material rights is deferred until such time that Moderna exercises its options or the right to exercise the options expires. Upon exercise, we will determine the appropriate revenue recognition methodology and any other implications on the accounting treatment for the arrangement. ​ The following table provides a summary of the transaction price allocated to each unit of account, in addition to revenue activity during the period: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Performance Obligations ​ Transaction Price Allocated ​ Revenue Recognized During ​ Deferred Revenue (in thousands) ​ As of December 31, 2023 ​ Year Ended December 31, 2023 ​ As of December 31, 2023 ctLNP technology and research license ​ $ 41,290 ​ $ 5,904 ​ $ 27,310 First liver program commercialization option license ​ ​ 5,505 ​ ​ — ​ ​ 4,429 Second liver program commercialization option license ​ 5,505 ​ — ​ 4,429 First non-liver program commercialization option license ​ 9,202 ​ — ​ 7,402 Second non-liver program commercialization option license ​ 9,202 ​ — ​ 7,402 Third liver or non-liver program commercialization option license ​ ​ 4,837 ​ ​ — ​ ​ 3,889 Total ​ $ 75,541 ​ $ 5,904 ​ $ 54,861 ​

Prepaid expenses and other curr

Prepaid expenses and other current assets	12 Months Ended
	Dec. 31, 2023
Prepaid expenses and other current assets
Prepaid expenses and other current assets	5. Prepaid expenses and other current assets Prepaid expenses and other current assets consisted of the following: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ December 31, (in thousands) 2023 ​ 2022 Prepaid research and development expenses ​ $ 2,050 ​ $ 4,555 Prepaid insurance ​ ​ 680 ​ ​ 1,180 Prepaid facilities-related expenses ​ ​ 953 ​ ​ 922 Prepaid employee compensation and benefits ​ ​ 133 ​ ​ 349 Interest income receivable ​ ​ 188 ​ ​ 265 Other ​ ​ 290 ​ ​ 259 Total ​ $ 4,294 ​ $ 7,530 ​ As of December 31, 2023 and 2022, there was $0.1 million and $1.7 million, respectively, of prepaid research and development expenses included in other long-term assets on our consolidated balance sheet. ​

Property and equipment, net

Property and equipment, net	12 Months Ended
	Dec. 31, 2023
Property and equipment, net.
Property and equipment, net	6. Property and equipment, net Property and equipment, net consisted of the following: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ December 31, (in thousands) ​ 2023 ​ 2022 Laboratory equipment ​ $ 14,859 ​ $ 13,619 Computer equipment and software ​ 1,447 ​ 1,189 Furniture and fixtures ​ 1,293 ​ 1,146 Leasehold improvements ​ 20,865 ​ 20,786 Construction in progress ​ 7,030 ​ 13 ​ ​ ​ 45,494 ​ ​ 36,753 Less: Accumulated depreciation and amortization ​ ​ (19,695) ​ ​ (14,538) Total ​ $ 25,799 ​ $ 22,215 ​ Depreciation and amortization expense for the years ended December 31, 2023, 2022, and 2021 was $5.3 million, $5.1 million, and $4.5 million, respectively. In July 2021, we entered into a lease agreement, or the Seyon Lease, to build out a current Good Manufacturing Practice, or cGMP, -compliant manufacturing facility in Waltham, Massachusetts. On January 31, 2024, we notified the landlord of termination of the Seyon Lease due to the landlord’s breach of its obligations to us under the Seyon Lease and returned possession of the premises to the landlord, effective January 31, 2024, and The balance of construction in progress on December 31, 2023, was comprised primarily of the capitalization of construction costs to renovate the property for alternative use.

Leases

Leases	12 Months Ended
	Dec. 31, 2023
Leases [Abstract]
Leases	7. Leases We lease our office and laboratory space under a noncancelable operating lease that expires in 2029, or the Office and Lab Lease. We have an option to extend the Office and Lab Lease term for one additional term of five years at the greater of the then-current base rent or the then-current fair market value. Exercise of this option was not determined to be reasonably certain and thus was not considered in determining the operating lease liability on the consolidated balance sheet as of December 31, 2023. We posted a letter of credit in the amount of approximately $2.1 million as a security deposit. The letter of credit is subject to increase if we were to sublease any portion of the leased premises. The Office and Lab Lease does not include any restrictions or covenants that had to be accounted for under the lease guidance. The Seyon Lease commenced in December 2021, when we were granted access to the facility and monthly rent payments began in September 2022, and the total rent payment was expected to be approximately $104.3 million for the 12-year lease term. We had an option to extend the Seyon Lease term for two additional terms of five years each at the greater of the then-current base rent or the then-current fair market value. Exercise of this option was not determined to be reasonably certain and thus was not considered in determining the operating lease liability on the consolidated balance sheet as of December 31, 2023. In connection with the Seyon Lease, we provided a security deposit of $3.6 million in the form of a letter of credit. We paid an initial monthly base rent of approximately $0.4 million that increased annually, up to a monthly base rent of $0.6 million. We were obligated to pay operating costs, taxes and utilities applicable to the facility. We were responsible for costs of constructing interior improvements within the facility that exceed a construction allowance of $26.0 million provided by the landlord. On January 31, 2024, we notified the landlord of termination of the Seyon Lease due to the landlord’s breach of its obligations to us under the Seyon Lease and returned possession of the premises to the landlord, effective January 31, 2024. On February 20, 2024, the landlord served us with a complaint, filed in Massachusetts Superior Court, with respect to the Seyon Lease. The complaint seeks declaratory judgment that we unlawfully terminated the Seyon Lease and also asserts a claim for breach of contract damages. For additional information, refer to Note 17, Subsequent event. ​ The following table presents our costs included in operating expenses related to our noncancelable operating leases: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Year ended December 31, (in thousands) ​ 2023 ​ 2022 Operating lease cost ​ $ 12,585 ​ $ 13,796 Variable lease cost ​ ​ 2,951 ​ ​ 2,572 Total ​ $ 15,536 ​ $ 16,368 ​ Net cash paid for the amounts included in the measurement of the operating lease liability on the consolidated balance sheet and operating activities in our consolidated statement of cash flows was $13.4 million and $9.2 million for the years ended December 31, 2023 and 2022, respectively. The weighted-average remaining lease term and weighted-average incremental borrowing rate for all leases as of December 31, 2023 was approximately 11 years 10 years Future lease payments for our noncancelable operating leases as of December 31, 2023 and a reconciliation to the carrying amount of the operating lease liability presented in the consolidated balance sheet as of December 31, 2023 are as follows: ​ ​ ​ ​ Year Ending December 31, ​ (in thousands) 2024 ​ $ 15,207 2025 ​ 15,638 2026 ​ 16,094 2027 ​ 16,550 2028 ​ 17,058 Thereafter ​ 56,161 Total undiscounted payments due under operating leases ​ ​ 136,708 Total undiscounted unearned tenant improvements ​ ​ 39 Less imputed interest ​ ​ (38,853) Total ​ $ 97,894 ​ ​ ​ ​ Current operating lease liability ​ $ 8,120 Non-current operating lease liability ​ ​ 89,774 Total ​ $ 97,894 ​

Accrued Expenses

Accrued Expenses	12 Months Ended
	Dec. 31, 2023
Accrued Expenses
Accrued Expenses	8. Accrued expenses Accrued expenses and other current liabilities consisted of the following: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ December 31, (in thousands) 2023 ​ 2022 Accrued employee compensation and benefits ​ $ 13,208 ​ $ 7,970 Accrued external research and development expenses ​ 1,169 ​ 1,959 Accrued professional fees ​ 908 ​ 1,047 Property and equipment ​ ​ 838 ​ ​ — Other ​ 406 ​ 426 Total ​ $ 16,529 ​ $ 11,402 ​ Accrued employee compensation and benefits as of December 31, 2023 includes salary continuation and severance costs of $5.3 million related to our strategic reorganization announced in November 2023. For additional information, refer to Note 16, Reduction in Force.

Equity

Equity	12 Months Ended
	Dec. 31, 2023
Equity
Equity	9. Equity As of December 31, 2023, our amended and restated certificate of incorporation authorizes us to issue 150,000,000 shares of common stock, par value $0.0001 per share, and 5,000,000 shares of preferred stock, par value $0.0001 per share, all of which preferred stock is undesignated. In January 2021, we issued and sold 9,200,000 shares of our common stock, including 1,200,000 shares pursuant to the full exercise of the underwriters’ option to purchase additional shares, in a follow-on public offering, resulting in net proceeds of $211.3 million after deducting underwriting discounts and commissions and other offering expenses. In March 2023, in connection with the Share Purchase Agreement with Moderna, we issued and sold 5,859,375 shares of our common stock to Moderna at a price of $6.14 per share for an aggregate purchase price of $36.0 million. For additional information, refer to Note 4, Collaboration and License Agreements. Each share of common stock entitles the holder to one vote on all matters submitted to a vote of our stockholders. Holders of common stock are not entitled to receive dividends, unless declared by the board of directors.

Stock-Based Compensation

Stock-Based Compensation	12 Months Ended
	Dec. 31, 2023
Stock-Based Compensation
Stock-Based compensation	10. Stock-based compensation Stock incentive plans Our 2017 Stock Incentive Plan, or the 2017 Plan, provided for us to grant incentive or nonstatutory stock options, restricted stock, restricted stock units and other equity awards to employees, non-employees and directors. In May 2020 our board of directors adopted, and in June 2020 our stockholders approved, the 2020 Stock Incentive Plan, or the 2020 Plan, and, together with the 2017 Plan, the Plans, which became effective on June 11, 2020. The 2020 Plan provides for the grant of incentive stock options, nonstatutory stock options, stock appreciation rights, restricted stock awards, restricted stock units and other stock-based awards. The number of shares of common stock reserved for issuance under the 2020 Plan is the sum of (1) 2,547,698 shares; plus (2) the number of shares (up to a maximum of 7,173,014 shares) as was equal to the sum of (x) the number of shares of common stock reserved for issuance under the 2017 Plan that remained available for grant under the 2017 Plan on June 11, 2020 and (y) the number of shares of common stock subject to outstanding awards granted under the 2017 Plan that expire, terminate or are otherwise surrendered, cancelled, forfeited or repurchased by us at their original issuance price pursuant to a contractual repurchase right; plus (3) an annual increase, to be added on the first day of each fiscal year, beginning with the fiscal year ending December 31, 2021 and continuing until, and including, the fiscal year ending December 31, 2030, equal to the lesser of (i) 4% of the number of shares of common stock outstanding on such date, and (ii) an amount determined by the board of directors. In January 2022, 2023 and 2024, the number of shares of common stock authorized for issuance under the 2020 Plan was increased from 12,154,517 shares to 14,433,745 shares, from 14,433,745 shares to 16,813,962 shares, and from 16,813,962 shares to 19,462,688 shares, respectively. Upon the effectiveness of the 2020 Plan, we ceased granting additional awards under the 2017 Plan. The Plans are administered by the board of directors or, at the discretion of the board of directors, by a committee of the board of directors. The exercise prices, vesting and other restrictions on any award under the Plans are determined at the discretion of the board of directors, or its committee if so delegated. Stock options granted under the Plans with service-based vesting conditions generally vest over four years and expire after ten years. The exercise price for stock options granted is not less than the fair value of common stock as of the date of grant. Prior to our IPO, fair value of common stock was determined by the board of directors. Subsequent to our IPO, fair value of common stock is based on quoted market prices. As of December 31, 2023, 255,176 shares remained available for future issuance under the 2020 Plan. Shares subject to outstanding awards granted under the Plans that expire, terminate or are otherwise surrendered, cancelled, forfeited or repurchased by us at their original issuance price pursuant to a contractual repurchase right will be available for future awards under the 2020 Plan. Stock option valuation The fair value of stock option grants is estimated using the Black-Scholes option-pricing model. We completed our IPO in 2020 and, accordingly, historically lacked sufficient company-specific historical and implied volatility information. Therefore, we have estimated our expected stock volatility based on combination of company-specific historical volatility and that of a publicly traded set of peer companies. Beginning in the third quarter of 2023, we determine our expected stock volatility by using the historical volatility of our publicly traded common stock. For options with service-based vesting conditions, the expected term of our stock options was determined utilizing the “simplified” method for awards that qualified as “plain-vanilla” options. The risk-free interest rate was determined by reference to the U.S. Treasury yield curve in effect at the time of grant of the award for time periods approximately equal to the expected term of the award. Expected dividend yield was based on the fact that we have never paid cash dividends and do not expect to pay any cash dividends in the foreseeable future. ​ The following table presents, on a weighted average basis, the assumptions used in the Black-Scholes option-pricing model to determine the fair value of stock options granted: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Year Ended December 31, ​ 2023 2022 2021 ​ Risk-free interest rate 3.88 % 2.08 % 0.81 % Expected volatility 102.3 % 91.2 % 76.0 % Expected dividend yield — — ​ — ​ Expected term (in years) 6.0 6.0 ​ 6.0 ​ ​ The following table summarizes our stock option activity since December 31, 2022: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Weighted Weighted ​ ​ ​ ​ ​ ​ Average ​ Average ​ Aggregate ​ ​ Number of ​ Exercise ​ Contractual ​ Intrinsic ​ Shares Price Term Value ​ ​ ​ ​ ​ ​ ​ (in years) ​ (in thousands) Outstanding as of December 31, 2022 8,827,322 ​ $ 13.15 7.97 ​ $ 55 Granted 3,312,016 ​ 3.20 ​ Exercised — ​ — ​ Forfeited (1,212,540) ​ 9.29 ​ Outstanding as of December 31, 2023 10,926,798 ​ $ 10.56 6.21 ​ $ 404 Vested and expected to vest as of December 31, 2023 9,967,903 ​ $ 10.66 6.00 ​ $ 387 Options exercisable as of December 31, 2023 5,378,652 ​ $ 14.07 5.03 ​ $ 17 ​ ​ ​ The aggregate intrinsic value of stock options is calculated as the difference between the exercise price of the stock options and the fair value of our common stock for those stock options that had strike prices lower than the fair value of our common stock. The weighted average grant-date fair value of stock options granted during the years ended December 31, 2023, 2022 and 2021 was $2.54, $4.76, and $18.81 per share, respectively. There were no stock option exercises in the year ended December 31, 2023. The total intrinsic value of stock options exercised during the years ended December 31, 2022 and 2021 was $0.5 million and $19.4 million. Restricted common stock The following table summarizes our restricted common stock units activity since December 31, 2022: ​ ​ ​ ​ ​ ​ ​ ​ ​ Weighted Average ​ Shares Grant Date Fair Value Unvested restricted common stock units as of December 31, 2022 959,521 ​ $ 6.65 Issued 895,400 ​ 4.47 Vested (732,448) ​ 6.57 Forfeited (102,022) ​ ​ 5.59 Unvested restricted common stock units as of December 31, 2023 1,020,451 ​ $ 4.90 ​ The total fair value of restricted common stock units vested during the years ended December 31, 2023, 2022, and 2021 was approximately $3.3 million, $3.5 million, and $0.2 million, respectively. There were no restricted common stock awards vested during the year ended December 31, 2023. The total fair value of restricted common stock awards vested during the years ended December 31, 2022 and 2021 was approximately $0.1 million and $17.7 million, respectively. There are currently no outstanding unvested restricted common stock awards. Employee stock purchase plan In May 2020 our board of directors adopted, and in June 2020 our stockholders approved, the 2020 Employee Stock Purchase Plan, or the 2020 ESPP, which became effective June 11, 2020. The 2020 ESPP is administered by our board of directors or by a committee appointed by the board of directors. The 2020 ESPP provides participating employees with the opportunity to purchase shares of common stock through payroll deductions made over the term of the offering. The per-share purchase price at the end of each offering period is equal to the lesser of 85% of the closing price of our common stock at the beginning or end of the offering period. The number of shares of common stock reserved for issuance under the 2020 ESPP automatically increases on the first day of each fiscal year, beginning with the fiscal year that commenced on January 1, 2021 and continuing for each fiscal year until, and including the fiscal year commencing on, January 1, 2030, in an amount equal to the lowest of (1) 1,302,157 shares of common stock, (2) 1% of the number of shares of common stock outstanding on such date, and (3) an amount determined by the board of directors. In January 2022, 2023, and 2024, the number of shares of common stock authorized for issuance under the 2020 ESPP was increased from 950,931 shares to 1,520,738 shares, from 1,520,738 shares to 2,115,792 shares, and from 2,115,792 shares to 2,777,974 shares, respectively. ​ ​ The following table presents, on a weighted average basis, the assumptions used in the Black-Scholes option-pricing model to determine the fair value of shares of common stock granted under the 2020 ESPP: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Year Ended December 31, ​ ​ 2023 2022 2021 Risk-free interest rate 5.18 % 1.43 % 0.09 % Expected volatility 95.1 % 93.5 % 85.6 % Expected dividend yield — — — Expected term (in years) 0.5 0.5 0.5 ​ Stock-Based Compensation We record compensation cost for all share-based payment arrangements, including employee, non-employee and director stock options and restricted common stock. We recorded stock-based compensation expense in the following expense categories of our consolidated statements of operations and comprehensive loss: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Year Ended December 31, (in thousands) 2023 2022 ​ 2021 Research and development expenses $ 11,496 ​ $ 12,405 ​ $ 9,316 General and administrative expenses 12,845 ​ 12,047 ​ 8,541 Total $ 24,341 ​ $ 24,452 ​ $ 17,857 ​ The following table summarizes our unrecognized stock-based compensation expense by type of awards and the weighted-average period over which that expense is expected to be recognized: ​ ​ ​ ​ ​ ​ ​ Unrecognized Expense Weight-average Recognition Period ​ ​ (in thousands) ​ (in years) Type of award: ​ ​ ​ ​ Time-based stock options ​ 23,636 ​ 2.00 Restricted stock units ​ 3,292 ​ 2.49 ​

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2023
Income Tax Disclosure [Abstract]
Income taxes	11. Income taxes For the years ended December 31, 2023, 2022 and 2021, we recorded no income tax benefits for the net operating losses incurred or for the research and development tax credits generated in each year, due to the uncertainty of realizing a benefit from those items. All of our operating losses since inception have been generated in the United States. A reconciliation of the U.S. federal statutory income tax rate to our effective income tax rate is as follows: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Year Ended December 31, ​ ​ 2023 2022 2021 ​ Federal statutory income tax rate (21.0) % (21.0) % (21.0) % State income taxes, net of federal benefit (6.4) (6.1) ​ (6.7) ​ Federal and state research and development tax credits (4.3) (2.3) ​ (2.4) ​ Stock-based compensation expense 1.9 0.9 ​ (1.7) ​ Change in deferred tax asset valuation allowance 29.8 28.5 ​ 31.8 ​ Effective income tax rate — % — % — % ​ ​ ​ ​ Net deferred tax assets as of December 31, 2023 and 2022 consisted of the following: ​ ​ ​ ​ ​ ​ ​ ​ ​ December 31, (in thousands) 2023 2022 Deferred tax assets: ​ ​ Net operating loss carryforwards ​ $ 96,939 ​ $ 89,546 Research and development tax credit carryforwards ​ 20,836 ​ 15,368 Operating lease liability ​ ​ 26,745 ​ ​ 22,322 Capitalized research and development costs ​ 36,345 ​ 20,005 Deferred revenue ​ ​ 3,341 ​ ​ — Other ​ 13,332 ​ 10,359 Total deferred tax assets ​ 197,538 ​ 157,600 Deferred tax liabilities: ​ ​ Operating lease right-of-use assets ​ ​ (19,084) ​ ​ (16,176) Property and equipment ​ (2,912) ​ (3,566) Total deferred tax liabilities ​ (21,996) ​ (19,742) Valuation allowance ​ ​ (175,542) ​ ​ (137,858) Net deferred tax assets ​ $ — ​ $ — ​ ​ ​ As of December 31, 2023, we had federal net operating loss carryforwards of $353.0 million, which may be available to offset future taxable income, of which $8.2 million of the total net operating loss carryforwards expire at various dates beginning in 2036, while the remaining $344.8 million do not expire but may be limited in their usage to an annual deduction equal to 80% of annual taxable income. As of December 31, 2023, we had state net operating loss carryforwards of $360.8 million, which may be available to offset future taxable income and expire at various dates beginning in 2036. In addition, as of December 31, 2023, we also had federal and state research and development tax credit carryforwards of $14.5 million and $8.0 million, respectively, which may be available to reduce future tax liabilities and expire at various dates beginning in 2036 and 2033, respectively. Utilization of the U.S. federal and state net operating loss carryforwards and research and development tax credit carryforwards may be subject to a substantial annual limitation under Section 382 and Section 383 of the Internal Revenue Code of 1986, as amended, or the Code, and corresponding provisions of state law, due to ownership changes that have occurred previously or that could occur in the future. These ownership changes may limit the amount of carryforwards that can be utilized annually to offset future taxable income and tax liabilities. In general, an ownership change, as defined by Section 382, results from transactions increasing the ownership of certain stockholders or public groups in the stock of a corporation by more than 50% over a three-year period. We have performed an analysis of ownership changes through December 31, 2021. Based on this analysis, we believe that $0.7 million of our tax attributes will expire unutilized due to Section 382 limitations. If we experience any ownership changes subsequent to the effective date of our most recent analysis, utilization of the net operating loss carryforwards or research and development tax credit carryforwards may be subject to incremental annual limitations. We have evaluated the positive and negative evidence bearing upon our ability to realize the deferred tax assets, which consist primarily of net operating loss carryforwards and research and development tax credit carryforwards. Management has considered our history of cumulative net losses incurred since inception, estimated future taxable income and prudent and feasible tax planning strategies and has concluded that it is more likely than not that we will not realize the benefits of federal and state net deferred tax assets. Accordingly, a full valuation allowance has been established against the net deferred tax assets as of December 31, 2023 and 2022. We reevaluate the positive and negative evidence at each reporting period. The changes in the valuation allowance for deferred tax assets during the year ended December 31, 2023 and 2022 related primarily to the increases in net operating loss carryforwards and research and development tax credit carryforwards. The changes in the valuation allowance for 2023 and 2022 were as follows: ​ ​ ​ ​ ​ ​ ​ ​ ​ Year Ended December 31, (in thousands) 2023 2022 Valuation allowance as of beginning of year ​ $ 137,858 ​ $ 98,902 Increases recorded to income tax provision ​ 37,684 ​ 38,956 Valuation allowance as of end of year ​ $ 175,542 ​ $ 137,858 ​ ​ ​ We assess the uncertainty in our income tax positions to determine whether a tax position of ours is more likely than not to be sustained upon examination, including resolution of any related appeals of litigation processes, based on the technical merits of the position. For tax positions meeting the more-likely-than-not threshold, the tax amount recognized in the financial statements is reduced by the largest benefit that has a greater than 50% likelihood of being realized upon the ultimate settlement with the relevant taxing authority. No reserve for uncertain tax positions or related interest and penalties have been recorded at December 31, 2023 and 2022. We file income tax returns as prescribed by the tax laws of the jurisdictions in which we operate. In the normal course of business, we are subject to examination by federal and state jurisdictions, where applicable. There are currently no pending tax examinations. We are open to future tax examination under statute from 2020 to the present. To the extent we have tax attribute carryforwards, the tax years in which the attribute was generated may still be adjusted upon examination by the Internal Revenue Service, or state tax authorities to the extent utilized in a future period. We have not yet conducted a study of our research and development credit carry forwards. This study may result in an adjustment to our research and development credit carry forwards; however, a full valuation allowance has been provided against our research and development credit carry forwards, and if an adjustment is required, this adjustment would be offset by an adjustment to the valuation allowance. Thus, there would be no impact to the consolidated balance sheet or statement of operations if an adjustment were required. Section 174 of the Tax Cuts and Jobs Act of 2017, or the Tax Act, requires taxpayers to capitalize and amortize research and development costs for the tax years beginning after December 31, 2021. For the year ended December 31, 2023, the capitalization and amortization of research and development costs resulted in a $59.8 million reduction of federal and state net operating loss carryforwards. In accordance with Section 174 of the Tax Act, we will amortize research and development costs for tax purposes over 5 years for costs of research and development performed in the U.S. and over 15 years for costs of research and development performed outside the U.S.

License Agreements

License Agreements	12 Months Ended
	Dec. 31, 2023
License Agreements [Abstract]
License agreements	12. License agreements NIH We have an agreement with the U.S. Department of Health and Human Services, as represented by The National Heart, Lung, and Blood Institute, an Institute of the National Institutes of Health, or NIH, entered into in 2017, pursuant to which NIH granted us a non-exclusive license, with the right to grant sublicenses, under certain NIH intellectual property related to our ceDNA construct. In July 2019, the agreement was amended to include Association Institut de Myologie, Universite Pierre et Marie Curie, Centre National de la Recherche Scientifique, and Inserm Transfert SA, collectively referred to as the French Institutions, as a licensor. We are obligated to make future milestone payments of up to $0.4 million per licensed product upon the achievement of specified milestones as well as royalties on a licensed product-by-licensed product and country-by-country basis of a low single digit percentage of annual net sales of licensed products. We are obligated to pay a high single-digit royalty percentage of all sublicensing income. The royalties on net sales may be reduced by up to 25% in certain circumstances as defined in the agreement. Our royalty obligation expires on a licensed product-by-licensed product and country-by-country basis upon the expiration of the last-to-expire licensed intellectual property rights in such country. Additionally, we are required to reimburse the French Institutions for a portion of certain past and ongoing patent related expenses related to the licensed technology. The agreement requires us to use reasonable commercial efforts to meet certain performance milestones and execute a commercial development plan within specified timeframes. Unless terminated earlier, the agreement remains in effect until the last to expire of the licensed patent rights on a licensed product-by-licensed product and country-by-country basis. NIH and the French Institutions may terminate the agreement if we fail to perform our material obligations, including but not limited to our failure to meet the applicable performance milestones despite using commercially reasonable efforts, and have not remediated such deficiency within a specified time period. NIH and the French Institutions can terminate the agreement in the event we become insolvent, file a petition in bankruptcy, have such a petition filed against it, or determine to file a petition in bankruptcy. In addition, NIH and the French Institutions may terminate the agreement in the event of a material breach by us and failure to cure such breach within a certain period of time. We are currently in compliance with the terms of the agreement. We can voluntarily terminate the agreement with prior notice to NIH and the French Institutions. We recorded research and development expense of $0.1 million for the year ended December 31, 2023 and less than $0.1 million for each of the years ended December 31, 2022 and 2021 under this agreement. UMass We have an agreement with the University of Massachusetts as represented by and solely on behalf of its Medical School, or UMass, entered into in 2017, pursuant to which UMass granted us an exclusive license, with the right to grant sublicenses, under the UMass intellectual property related to our ceDNA construct. We are obligated to make future milestone payments of up to $0.8 million per licensed product upon the achievement of specified milestones as well as royalties on a licensed product-by-licensed product and country-by-country basis of a low single digit percentage of annual net sales of licensed products, subject to annual minimum royalties as defined in the agreement. Additionally, we have agreed to pay a low-to-mid single-digit royalty percentage of all sublicensing income, which will vary depending on when the sublicense agreement to a third party was executed. The milestone payments are non-refundable and non-creditable against any other payments due to UMass under the agreement. Our royalty obligation expires on a licensed product-by-licensed product and country-by-country basis upon the expiration of the last-to-expire licensed intellectual property rights in such country. The agreement requires us to use diligent efforts to meet certain performance milestones within specified timeframes. As part of the arrangement, we agreed to issue 125,677 shares of our common stock to UMass. We recorded $0.1 million as research and development expense and additional-paid-in capital at the time of the agreement in 2017, representing the fair value of the common stock at that time. The shares were issued in August 2019. Unless terminated earlier, the agreement will continue until the last-to-expire valid claim of the licensed patents. UMass may terminate the agreement if we fail to perform our material obligations, including but not limited to our failure to meet the applicable performance milestones despite using commercially reasonable efforts, and have not remediated such deficiency within a specified time period or negotiated a revised performance timeline. UMass can terminate the agreement if we fail to make any payments within a specified period after receiving written notice of such failure, or in the event of a material breach by us and failure to cure such breach within a certain period of time. We are currently in compliance with the terms of the agreement. We can voluntarily terminate the agreement with prior notice to UMass. As of December 31, 2023, we have paid the first milestone for the issuance of the first patent in the licensed patent application family, and we have recorded no royalty or other milestone liabilities under the UMass Agreement. We recorded research and development expense of $0.6 million for the year ended December 31, 2023 and less than $0.1 million for each of the years ended December 31, 2022 and 2021 under this agreement. Other license agreements We have other license agreements under which we may become subject to future additional fees and milestone payments. ​

Commitments and Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2023
Commitments and Contingencies.
Commitments and Contingencies	13. Commitments and contingencies 401(k) Plan We have a defined-contribution plan under Section 401(k) of the Code, or the 401(k) Plan. The 401(k) Plan covers all employees who meet defined minimum age and service requirements and allows participants to defer a portion of their annual compensation on a pre-tax and/or after-tax basis. In September 2020, we adopted a match program, beginning on January 1, 2021, for employee contributions to the 401(k) Plan up to a maximum of four percent of the employee’s salary, subject to the maximums established under the Code. Indemnification agreements In the ordinary course of business, we may provide indemnification of varying scope and terms to vendors, lessors, contract research organizations, business partners and other parties with respect to certain matters including, but not limited to, losses arising out of breach of such agreements or from intellectual property infringement claims made by third parties. In addition, we have entered into indemnification agreements with members of our board of directors and our officers that will require us, among other things, to indemnify them against certain liabilities that may arise by reason of their status or service as directors or officers. The maximum potential amount of future payments we could be required to make under these indemnification agreements is, in many cases, unlimited. We have not incurred any material costs as a result of such indemnifications and are not currently aware of any indemnification claims. Legal proceedings We, from time to time, may be subject to legal proceedings and claims in the ordinary course of business. We are not currently aware of any such proceedings or claims that we believe will have, individually or in the aggregate, a material adverse effect on our business, financial condition or results of operations.

Net Loss per Share

Net Loss per Share	12 Months Ended
	Dec. 31, 2023
Net Loss per Share
Net Loss per Share	14. Net loss per share Net loss per share We have generated a net loss in all periods presented, therefore the basic and diluted net loss per share attributable to common stockholders are the same as the inclusion of the potentially dilutive securities would be anti-dilutive. We excluded the following potential common shares, presented based on amounts outstanding at each period end, from the computation of diluted net loss per share attributable to common stockholders for the periods indicated: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ December 31, ​ 2023 2022 2021 Unvested restricted common stock ​ — ​ — ​ 11,083 Unvested restricted stock units ​ 1,020,451 ​ 959,521 ​ 13,120 Stock options to purchase common stock 10,926,798 8,827,322 ​ 6,205,088 Total 11,947,249 9,786,843 ​ 6,229,291 ​ ​ ​

Related Parties

Related Parties	12 Months Ended
	Dec. 31, 2023
Related Parties
Related Parties	15. Related Partie ​ In March 2023, we entered into the Collaboration Agreement with Moderna. In connection with the Share Purchase Agreement, we issued and sold 5,859,375 shares of our common stock to Moderna, which resulted in Moderna becoming the beneficial owner of 8.9% of our outstanding common stock and a related party. ​

Reduction In Force

Reduction In Force	12 Months Ended
	Dec. 31, 2023
Restructuring and Related Activities [Abstract]
Reduction in Force	16. Reduction in Force In November 2023, following a review of strategic priorities and a determination by our management and board of directors to implement a strategic reorganization, to invest in our ctLNP delivery platform to develop wholly-owned programs for extrahepatic cell types and to develop our iqDNA platform for our lead program in hemophilia A and other programs, we announced a reduction in our workforce of approximately 40%, or RIF, and implemented reductions in operational expenditures including Good Manufacturing Practice readiness and manufacturing expenses. We expect to complete the RIF by the end of the second quarter of 2024. In connection with the RIF, affected employees will be eligible to receive one-time severance benefits, including cash severance, temporary healthcare coverage, to the extent they are eligible for and elect such coverage, and transition support services, subject to each such employee entering into an effective separation agreement, which will include a general release of claims against us. We are also offering a retention bonus to certain affected employees if such employees remain in continuous employment with us through their respective separation dates and execute a general release of claims against us. Below is a summary of accrued restructuring costs recorded and included in accrued expenses and other current liabilities during the year ended December 31, 2023: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ (in thousands) ​ ​ Severance and Benefits Costs ​ Stock-Based Compensation ​ ​ Total Restructuring expenses ​ $ 6,513 ​ $ 512 ​ $ 7,025 Cash payments ​ (1,222) ​ ​ — ​ ​ (1,222) Non-cash expenses ​ — ​ ​ (512) ​ ​ (512) Balance at December 31, 2023 ​ $ 5,291 ​ $ — ​ $ 5,291 ​ In the year ended December 31, 2023, we recorded $7.0 million of restructuring expenses million of restructuring expense in the year ended December 31, 2024 in connection with the RIF. We did not recognize any restructuring expense in the years ending December 31, 2022 and 2021. ​

Subsequent Event

Subsequent Event	12 Months Ended
	Dec. 31, 2023
Subsequent Events [Abstract]
Subsequent event	17. Subsequent event In July 2021, we entered into the Seyon Lease and in January 2024, we notified the landlord of the termination of the Seyon Lease due to the landlord’s breach of its obligations to us under the Seyon Lease and returned possession of the premises to the landlord, effective January 31, 2024. On February 20, 2024, the landlord served us with a complaint, filed in Massachusetts Superior Court, with respect to the Seyon Lease. The complaint seeks declaratory judgment that we unlawfully terminated the Seyon Lease and also asserts a claim for breach of contract damages. Our responsive pleading is due April 1, 2024. We will vigorously defend the action and our rights with respect to this matter. As of December 31, 2023, we had $46.0 million in operating lease right-to-use assets, $62.2 million in operating lease liability, $6.2 million in construction in process, $4.0 million in tenant receivable, and $3.6 million in restricted cash related to the Seyon Lease on our consolidated balance sheet. As of March 6, 2024, the issuance date of this Annual Report on Form 10-K, we were unable to determine the impact of the Seyon Lease termination and the related pending litigation on our consolidated financial statements. ​

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2023
Accounting Policies [Abstract]
Use of estimates	Use of estimates The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities at the date of the financial statements and the reported amounts of expenses during the reporting periods. Significant estimates and assumptions reflected in these consolidated financial statements include, but are not limited to, the accrual of research and development expenses and stock-based compensation expense. We base our estimates on historical experience, known trends and other market-specific or other relevant factors that we believe to be reasonable under the circumstances. On an ongoing basis, management evaluates its estimates, as there are changes in circumstances, facts and experience. Changes in estimates are recorded in the period in which they become known. Actual results may differ from those estimates or assumptions.
Concentrations of credit risk and of significant suppliers	Concentrations of credit risk and of significant suppliers Financial instruments that potentially expose us to concentrations of credit risk consist primarily of cash, cash equivalents, and marketable securities. We believe that we are not exposed to significant credit risk due to the financial strength of the depository institutions in which our cash, cash equivalents, and marketable securities are held. We maintain our cash equivalents in money market funds that invest in U.S. treasury securities. We have adopted an investment policy that limits the amounts that we may invest in the securities of a single issuer with the exclusion of the U.S. government. We have not experienced any credit losses. We are dependent on a small number of third-party suppliers for our drug substance and drug product. In particular, we rely, and expect to continue to rely, on third-party suppliers for certain materials and components required for the production of any product candidates we may develop for our programs. These programs could be adversely affected by a significant interruption in the supply process.
Cash equivalents	Cash equivalents We consider all highly liquid investments with a remaining maturity when purchased of three months or less to be cash equivalents.
Restricted cash	Restricted cash Amounts included in restricted cash represent amounts pledged as collateral for letters of credit required for security deposits on our leased facilities. These amounts are classified as restricted cash on our consolidated balance sheets.
Marketable securities	Marketable securities Our marketable securities, which consisted of debt securities as of December 31, 2023, are classified as available-for-sale and are reported at fair value. Unrealized gains and losses on available-for-sale debt securities are reported as a component of accumulated other comprehensive income (loss) in stockholders’ equity. We assess our available-for-sale debt securities under the available-for-sale debt security impairment model in Topic 326 as of each reporting date in order to determine if a portion of any decline in fair value below carrying value recognized on our available-for-sale debt securities is the result of a credit loss. We record credit losses in the consolidated statements of operations and comprehensive loss as credit loss expense, which is limited to the difference between the fair value and the amortized cost of the security. To date, we have not recorded any credit losses on our available-for-sale debt securities.
Fair value measurements	Fair value measurements Certain of our assets and liabilities are carried at fair value under GAAP. Fair value is defined as the exchange price that would be received for an asset or paid to transfer a liability (an exit price) in the principal or most advantageous market for the asset or liability in an orderly transaction between market participants on the measurement date. Valuation techniques used to measure fair value must maximize the use of observable inputs and minimize the use of unobservable inputs. Financial assets and financial liabilities carried at fair value are to be classified and disclosed in one of the following three levels of the fair value hierarchy, of which the first two are considered observable and the last is considered unobservable: ● Level 1—Quoted prices in active markets for identical assets or liabilities. ● Level 2—Observable inputs (other than Level 1 quoted prices), such as quoted prices in active markets for similar assets or liabilities, quoted prices in markets that are not active for identical or similar assets or liabilities, or other inputs that are observable or can be corroborated by observable market data. ● Level 3—Unobservable inputs that are supported by little or no market activity and that are significant to determining the fair value of the assets or liabilities, including pricing models, discounted cash flow methodologies and similar techniques. Our cash equivalents and marketable securities are carried at fair value, determined according to the fair value hierarchy described above (see Note 3). The carrying values of our accounts payable and accrued expenses approximate their fair values due to the short-term nature of these liabilities.
Property and equipment	Property and equipment Property and equipment are stated at cost less accumulated depreciation and amortization. Depreciation and amortization expense is recognized using the straight-line method over the estimated useful life of each asset as follows: ​ ​ ​ ​ ​ Estimated Useful Life Laboratory equipment ​ 5 years Computer equipment and software 3 years Furniture and fixtures 5 years Leasehold improvements Shorter of remaining life of lease or useful life ​ Costs for capital assets not yet placed into service are capitalized as construction-in-progress and depreciated once placed into service. Upon retirement or sale, the cost of assets disposed of and the related accumulated depreciation and amortization are removed from the accounts and any resulting gain or loss is included in loss from operations. Expenditures for repairs and maintenance which do not improve or extend the life of the respective assets are charged to expense as incurred.
Impairment of long-lived assets	Impairment of long-lived assets Long-lived assets consist of property and equipment. We evaluate the recoverability of our long-lived assets when circumstances indicate that an event of impairment may have occurred. We recognize an impairment loss only if the carrying amount of a long-lived asset is not recoverable based on its undiscounted future cash flows. Impairment is measured based on the difference between the carrying value of the related assets and the fair value of such assets. We recorded less than $0.1 million and $5.8 million in impairment losses on long-lived assets during the years ended December 31, 2023 and 2022, respectively. We did not record any impairment losses on long-lived assets during the year ended December 31, 2021.
Leases	Leases We determine whether a contract is, or contains, a lease at inception. We classify each of our leases as operating or financing considering factors such as the length of the lease term, the present value of the lease payments, the nature of the asset being leased, and the potential for ownership of the asset to transfer during the lease term. Leases with terms greater than one-year are recognized on the consolidated balance sheets as right-of-use assets and lease liabilities and are measured at the present value of the fixed payments due over the expected lease term less the present value of any incentives, rebates or abatements we expect to receive from the lessor. Options to extend a lease are included in the expected lease term if exercise of the option is deemed reasonably certain. Costs determined to be variable and not based on an index or rate are not included in the measurement of the lease liability and are expensed as incurred. The interest rate implicit in lease contracts is typically not readily determinable. As such, we utilize the appropriate incremental borrowing rate, which is the rate incurred to borrow on a collateralized basis an amount equal to the lease payments over a similar term and in a similar economic environment. To estimate our incremental borrowing rate, a credit rating applicable to our company is estimated using a synthetic credit rating analysis since we do not currently have a rating agency-based credit rating. We record expense to recognize fixed lease payments on a straight-line basis over the expected lease term. We have elected the practical expedient not to separate lease and non-lease components for real estate leases.
Segment information	Segment information We have determined that our chief executive officer is the chief operating decision maker, or CODM. The CODM reviews financial information presented on a consolidated basis. Resource allocation decisions are made by the CODM based on consolidated results. There are no segment managers who are held accountable by the CODM for operations, operating results, and planning for levels or components below the consolidated unit level. As such, we have concluded that we operate as one segment. All long-lived assets are located in the United States.
Research and development costs	Research and development costs Research and development costs are expensed as incurred. Research and development expenses consist of costs incurred in performing research and development activities, including salaries and bonuses, stock-based compensation, employee benefits, facilities costs, laboratory supplies, depreciation and amortization, manufacturing expenses and external costs of vendors engaged to conduct preclinical development activities as well as the cost of licensing technology. Upfront payments and milestone payments made for the licensing of technology with no alternative future use are expensed as research and development expenses in the period in which they are incurred. Advance payments for goods or services to be received in the future for use in research and development activities are recorded as prepaid expenses. The prepaid amounts are expensed as the related goods are delivered or the services are performed.
Research and manufacturing contract costs and accruals	Research and manufacturing contract costs and accruals We have entered into various research and development and manufacturing contracts. These agreements are generally cancelable, and costs for research and development and manufacturing activities are expensed in the period in which they are incurred. Payments for such activities are based on the terms of the individual arrangements, which may differ from the pattern of costs incurred. We record accruals for costs incurred but not billed. When evaluating the adequacy of the accrued liabilities, we analyze progress of the research studies and manufacturing activities, including the phase or completion of events, invoices received and contracted costs. Significant judgments and estimates are made in determining the accrued balances at the end of any reporting period. Actual results could differ from our estimates. Our historical accrual estimates have not been materially different from the actual costs.
Patent costs	Patent costs All patent-related costs incurred in connection with filing and prosecuting patent applications are expensed as incurred due to the uncertainty about the recovery of the expenditure. Amounts incurred are classified as general and administrative expenses.
Stock-based compensation	Stock-based compensation We measure stock options with service-based vesting or performance-based vesting granted to employees, non-employees and directors based on the fair value on the date of grant using the Black-Scholes option-pricing model. We measure restricted common stock awards and units using the difference between the purchase price per share of the award or unit, if any, and the fair value of our common stock at the date of grant. Compensation expense for awards with service-based vesting is generally recognized over the vesting period of the award using the straight-line method to record the expense. We use the graded-vesting method to record the expense of awards with both service-based and performance-based vesting conditions, commencing once achievement of the performance condition becomes probable. We account for forfeitures of share-based awards as they occur. We classify stock-based compensation expense in our consolidated statements of operations and comprehensive loss in the same manner in which the award recipient’s payroll costs are classified or in which the award recipient’s service payments are classified.
Comprehensive loss	Comprehensive loss Comprehensive loss includes net loss as well as other changes in stockholders’ equity that result from transactions and economic events other than those with stockholders. For the years ended December 31, 2023, 2022 and 2021, our only element of other comprehensive loss was unrealized gains (losses) on available-for-sale debt securities.
Income taxes	Income taxes We account for income taxes using the asset and liability method, which requires the recognition of deferred tax assets and liabilities for the expected future tax consequences of events that have been recognized in the consolidated financial statements or in our tax returns. Deferred tax assets and liabilities are determined on the basis of the differences between the financial statements and tax basis of assets and liabilities using enacted tax rates in effect for the year in which the differences are expected to reverse. Changes in deferred tax assets and liabilities are recorded in the provision for income taxes. We assess the likelihood that our deferred tax assets will be recovered from future taxable income and, to the extent we believe, based upon the weight of available evidence, that it is more likely than not that all or a portion of the deferred tax assets will not be realized, a valuation allowance is established through a charge to income tax expense. Potential for recovery of deferred tax assets is evaluated by estimating the future taxable profits expected and considering prudent and feasible tax planning strategies. We account for uncertainty in income taxes recognized in the consolidated financial statements by applying a two-step process to determine the amount of tax benefit to be recognized. First, the tax position must be evaluated to determine the likelihood that it will be sustained upon external examination by the taxing authorities. If the tax position is deemed more-likely-than-not to be sustained, the tax position is then assessed to determine the amount of benefit to recognize in the consolidated financial statements. The amount of the benefit that may be recognized is the largest amount that has a greater than 50% likelihood of being realized upon ultimate settlement. The provision for income taxes includes the effects of any resulting tax reserves, or unrecognized tax benefits, that are considered appropriate as well as the related net interest and penalties. Our policy is to recognize interest and/or penalties related to income tax matters in income tax expense. We have accrued no amounts for interest and penalties on our consolidated balance sheets at December 31, 2023 and 2022.
Net loss per share	Net loss per share Basic net income (loss) per common share is computed by dividing the net income (loss) by the weighted average number of shares of common stock outstanding for the period. Diluted net income (loss) per common share is computed by dividing net income (loss) by the weighted average number of shares of common stock outstanding for the period, including potential dilutive common shares assuming the dilutive effect of outstanding stock awards. For periods in which we report a net loss, diluted net loss per common share is the same as basic net loss per common share, since dilutive common shares are not assumed to have been issued if their effect is anti-dilutive. We reported a net loss for each of the years ended December 31, 2023, 2022 and 2021.
Revenue Recognition	Revenue Recognition We enter into collaboration agreements that are within the scope of ASC Topic 606, “Revenue from Contracts with Customers,” or ASC 606, under which we license rights to certain of our potential product candidates and perform research and development services. The terms of these contracts typically include payment of the following: non-refundable, upfront fees; reimbursement of research and development costs; development, regulatory, and commercial milestone payments; royalties on net sales of licensed products, and a premium or discount on the sale of our common stock. Under ASC 606, an entity recognizes revenue when its customer obtains control of promised goods or services, in an amount that reflects the consideration that the entity expects to receive in exchange for those goods or services. To determine the appropriate amount of revenue to be recognized for contracts determined to be within the scope of ASC 606, we perform the following five steps: (i) identification of the promised goods or services in the contract; (ii) determination of whether the promised goods or services are performance obligations, including whether they are distinct in the context of the contract; (iii) measurement of the transaction price, including the constraint on variable consideration; (iv) allocation of the transaction price to the performance obligations; and (v) recognition of revenue when, or as, we satisfy each performance obligation. We only apply the five-step model to contracts when it is probable that we will collect consideration we are entitled to in exchange for the goods or services we transfer to the customer. The promised goods or services in our arrangements typically consist of license rights to our intellectual property and research and development services. We provide options to additional items in the contract, which will be accounted for as separate contracts if and when the other party elects to exercise such options, unless the option provides a material right to such party. We evaluate the other party’s options for material rights, or options to acquire additional goods or services for free or at a discount. If the other party’s options are determined to represent a material right, the material right is recognized as a separate performance obligation at the outset of the contract. Performance obligations are promised goods or services in a contract to transfer a distinct good or service to the other party and are considered distinct when (i) the other party can benefit from the good or service on its own or together with other readily available resources and (ii) the promised good or service is separately identifiable from other promises in the contract. In assessing whether promised goods or services are distinct, we consider factors such as the stage of development of the underlying intellectual property, the capabilities of the other party to develop the intellectual property on its own or whether the required expertise is readily available and whether the goods or services are integral or dependent to other goods or services in the contract. We estimate the transaction price based on the amount expected to be received for transferring the promised goods or services in the contract. The consideration may include fixed consideration or variable consideration. At the inception of each contract that includes variable consideration, we evaluate the number of potential payments and the likelihood that the payments will be received. We utilize either the most likely amount method or expected amount method to estimate the amount expected to be received based on which method best predicts the amount expected to be received. The amount of variable consideration that is included in the transaction price may be constrained and is included in the transaction price only to the extent that it is probable that a significant reversal in the amount of the cumulative revenue recognized will not occur in a future period. Our contracts include development, regulatory, and commercial milestone payments that will be assessed under the most likely amount method and constrained if it is probable that a significant revenue reversal would occur. Milestone payments that are not within our control or the counterparty’s control, such as regulatory approvals, are not considered probable of being achieved until those approvals are received. At the end of each reporting period, we re-evaluate the probability of achievement of such development, regulatory, and commercial milestones and any related constraint, and if necessary, we adjust our estimate of the overall transaction price. Any such adjustments are recorded on a cumulative catch-up basis, which would affect collaboration revenues in the period of adjustment. To date, we have not recognized any consideration related to the achievement of development, regulatory, or commercial milestone revenue resulting from our collaboration contracts. For contracts that include sales-based royalties, including milestone payments based on the level of sales, and where the license is deemed to be the predominant item to which the royalties relate, we recognize revenue at the later of (i) when the related sales occur, or (ii) when the performance obligation to which some or all of the royalty has been allocated has been satisfied (or partially satisfied). To date, we have not recognized any consideration related to sales-based royalty revenue resulting from our collaboration contract. We allocate the transaction price based on the relative estimated stand-alone selling price of each of the performance obligations. We must develop assumptions that require judgment to determine the stand-alone selling price for each performance obligation identified in the contract. We utilize key assumptions to determine the stand-alone selling price for service obligations, which may include other comparable transactions, pricing considered in negotiating the transaction and the estimated costs. Additionally, in determining the standalone selling price for material rights, we utilize comparable transactions, clinical trial success probabilities, and estimates of option exercise likelihood. Variable consideration is allocated specifically to one or more performance obligations in a contract when the terms of the variable consideration relate to the satisfaction of the performance obligation and the resulting amounts allocated are consistent with the amounts we would expect to receive for the satisfaction of each performance obligation. The consideration allocated to each performance obligation is recognized as revenue when control is transferred for the related goods or services. For performance obligations that consist of licenses and other promises, we utilize judgment to assess the nature of the combined performance obligation to determine whether the combined performance obligation is satisfied over time or at a point in time and, if over time, the appropriate method of measuring progress. We evaluate the measure of progress each reporting period and, if necessary, adjust the measure of performance and related revenue recognition. For performance obligations satisfied as of a point in time, the allocated consideration is deferred until the goods and services are transferred, or, in the case of options to deliver additional goods and services, the option is exercised or expires. Upfront payments and fees are recorded as deferred revenue upon receipt or when due until we perform our obligations. Amounts are recorded as accounts receivable when our right to consideration is unconditional.
Restructuring and other costs	Restructuring and other costs Restructuring and other costs are comprised of employee separation costs, asset impairments, and other associated costs primarily related to implementing the restructuring plan. Employee separation costs principally consist of one-time termination benefits and contractual termination benefits for severance, other termination benefit costs, and stock-based compensation expense for the acceleration of stock awards. We record restructuring charges based on whether the termination benefits are provided under an on-going benefit arrangement or under a one-time benefit arrangement. We account for on-going benefit arrangements, such as those documented by separation agreements, in accordance with ASC 712, Nonretirement Postemployment Benefits. Under ASC 712, liabilities for post-employment benefits are recorded at the time the obligations are probable of being incurred and can be reasonably estimated. We account for one-time employment benefit arrangements in accordance with ASC 420 Exit or Disposal Cost Obligations. One-time termination benefits are expensed at the date the entity notifies the employee, unless the employee must provide future service, in which case the benefits are expensed ratably over the future service period. Other associated costs are recognized in the period in which the liability is incurred.
Recently issued accounting pronouncements	Recently issued accounting pronouncements ​ In December 2023, the FASB issued its final standard to improve income tax disclosures. This standard, issued as ASU 2023-09, requires public business entities to annually disclose specific categories in the rate reconciliation and provide additional information for reconciling items that meet a quantitative threshold. This update is effective for annual periods beginning after December 15, 2025. We are currently evaluating when to adopt the guidance and the impact the guidance will have our consolidated financial statements.

Summary of Significant Accoun_3

Summary of Significant Accounting Policies (Tables)	12 Months Ended
	Dec. 31, 2023
Accounting Policies [Abstract]
Summary of Property and equipment	Property and equipment are stated at cost less accumulated depreciation and amortization. Depreciation and amortization expense is recognized using the straight-line method over the estimated useful life of each asset as follows: ​ ​ ​ ​ ​ Estimated Useful Life Laboratory equipment ​ 5 years Computer equipment and software 3 years Furniture and fixtures 5 years Leasehold improvements Shorter of remaining life of lease or useful life

Marketable Securities and Fai_2

Marketable Securities and Fair Value Measurements (Tables)	12 Months Ended
	Dec. 31, 2023
Marketable Securities and Fair Value Measurements
Summary of marketable securities	The following tables present our marketable securities by security type as of December 31, 2023 and 2022: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ As of December 31, 2023 ​ ​ ​ Gross Gross ​ ​ ​ Amortized ​ Unrealized ​ Unrealized ​ Fair (in thousands) ​ Cost ​ Gains ​ Losses ​ Value U.S. treasury securities ​ $ 197,644 ​ $ 274 ​ $ — ​ $ 197,918 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ As of December 31, 2022 ​ ​ ​ Gross Gross ​ ​ ​ Amortized ​ Unrealized ​ Unrealized ​ Fair (in thousands) ​ Cost ​ Gains ​ Losses ​ Value U.S. treasury securities ​ $ 186,003 ​ $ 13 ​ $ (96) ​ $ 185,920
Summary of assets measured at fair value on a recurring basis	The following tables present our assets that are measured at fair value on a recurring basis and indicate the level within the fair value hierarchy of the valuation techniques that we utilized to determine such fair value: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Fair Value Measurements at December 31, 2023 Using: (in thousands) ​ Level 1 ​ Level 2 ​ Level 3 ​ Total Cash equivalents: ​ ​ ​ ​ Money market funds ​ $ 38,210 ​ $ — ​ $ — ​ $ 38,210 Marketable securities: ​ ​ ​ ​ U.S. treasury securities ​ — ​ 197,918 ​ — ​ 197,918 Totals ​ $ 38,210 ​ $ 197,918 ​ $ — ​ $ 236,128 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Fair Value Measurements at December 31, 2022 Using: (in thousands) Level 1 Level 2 Level 3 Total Cash equivalents: ​ ​ ​ ​ ​ Money market funds ​ $ 77,010 ​ $ — ​ $ — ​ $ 77,010 Marketable securities: ​ ​ ​ ​ U.S. treasury securities ​ — ​ 185,920 ​ — ​ 185,920 Totals ​ $ 77,010 ​ $ 185,920 ​ $ — ​ $ 262,930 ​ ​

Collaboration And License Agr_2

Collaboration And License Agreement (Tables)	12 Months Ended
	Dec. 31, 2023
Collaboration Arrangement [Abstract]
Summary of allocation of transaction price to each unit of account	​ ​ ​ ​ ​ ​ ​ ​ Performance Obligations (in thousands) ​ Standalone Selling Price ​ Transaction Price Allocated ctLNP technology and research license ​ $ 52,500 ​ $ 42,576 First liver program commercialization option license ​ ​ 7,000 ​ ​ 5,677 Second liver program commercialization option license ​ 7,000 ​ 5,677 First non-liver program commercialization option license ​ 11,700 ​ 9,488 Second non-liver program commercialization option license ​ 11,700 ​ 9,488 Third liver or non-liver program commercialization option license ​ ​ 6,150 ​ ​ 4,987 Total ​ $ 96,050 ​ $ 77,893
Summary of transaction price allocated with revenue activity	​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Performance Obligations ​ Transaction Price Allocated ​ Revenue Recognized During ​ Deferred Revenue (in thousands) ​ As of December 31, 2023 ​ Year Ended December 31, 2023 ​ As of December 31, 2023 ctLNP technology and research license ​ $ 41,290 ​ $ 5,904 ​ $ 27,310 First liver program commercialization option license ​ ​ 5,505 ​ ​ — ​ ​ 4,429 Second liver program commercialization option license ​ 5,505 ​ — ​ 4,429 First non-liver program commercialization option license ​ 9,202 ​ — ​ 7,402 Second non-liver program commercialization option license ​ 9,202 ​ — ​ 7,402 Third liver or non-liver program commercialization option license ​ ​ 4,837 ​ ​ — ​ ​ 3,889 Total ​ $ 75,541 ​ $ 5,904 ​ $ 54,861 ​

Prepaid expenses and other cu_2

Prepaid expenses and other current assets (Tables)	12 Months Ended
	Dec. 31, 2023
Prepaid expenses and other current assets
Schedule of prepaid expenses and other current assets	​ ​ ​ ​ ​ ​ ​ ​ ​ ​ December 31, (in thousands) 2023 ​ 2022 Prepaid research and development expenses ​ $ 2,050 ​ $ 4,555 Prepaid insurance ​ ​ 680 ​ ​ 1,180 Prepaid facilities-related expenses ​ ​ 953 ​ ​ 922 Prepaid employee compensation and benefits ​ ​ 133 ​ ​ 349 Interest income receivable ​ ​ 188 ​ ​ 265 Other ​ ​ 290 ​ ​ 259 Total ​ $ 4,294 ​ $ 7,530

Property and equipment, net (Ta

Property and equipment, net (Tables)	12 Months Ended
	Dec. 31, 2023
Property and equipment, net.
Summary of Property and equipment, net	Property and equipment, net consisted of the following: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ December 31, (in thousands) ​ 2023 ​ 2022 Laboratory equipment ​ $ 14,859 ​ $ 13,619 Computer equipment and software ​ 1,447 ​ 1,189 Furniture and fixtures ​ 1,293 ​ 1,146 Leasehold improvements ​ 20,865 ​ 20,786 Construction in progress ​ 7,030 ​ 13 ​ ​ ​ 45,494 ​ ​ 36,753 Less: Accumulated depreciation and amortization ​ ​ (19,695) ​ ​ (14,538) Total ​ $ 25,799 ​ $ 22,215

Leases (Tables)

Leases (Tables)	12 Months Ended
	Dec. 31, 2023
Leases [Abstract]
Costs included in operating expenses related to the operating lease	​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Year ended December 31, (in thousands) ​ 2023 ​ 2022 Operating lease cost ​ $ 12,585 ​ $ 13,796 Variable lease cost ​ ​ 2,951 ​ ​ 2,572 Total ​ $ 15,536 ​ $ 16,368
Future lease payments for noncancelable operating lease	Future lease payments for our noncancelable operating leases as of December 31, 2023 and a reconciliation to the carrying amount of the operating lease liability presented in the consolidated balance sheet as of December 31, 2023 are as follows: ​ ​ ​ ​ Year Ending December 31, ​ (in thousands) 2024 ​ $ 15,207 2025 ​ 15,638 2026 ​ 16,094 2027 ​ 16,550 2028 ​ 17,058 Thereafter ​ 56,161 Total undiscounted payments due under operating leases ​ ​ 136,708 Total undiscounted unearned tenant improvements ​ ​ 39 Less imputed interest ​ ​ (38,853) Total ​ $ 97,894 ​ ​ ​ ​ Current operating lease liability ​ $ 8,120 Non-current operating lease liability ​ ​ 89,774 Total ​ $ 97,894 ​

Accrued Expenses (Tables)

Accrued Expenses (Tables)	12 Months Ended
	Dec. 31, 2023
Accrued Expenses
Summary of accrued expenses	​ ​ ​ ​ ​ ​ ​ ​ ​ ​ December 31, (in thousands) 2023 ​ 2022 Accrued employee compensation and benefits ​ $ 13,208 ​ $ 7,970 Accrued external research and development expenses ​ 1,169 ​ 1,959 Accrued professional fees ​ 908 ​ 1,047 Property and equipment ​ ​ 838 ​ ​ — Other ​ 406 ​ 426 Total ​ $ 16,529 ​ $ 11,402

Stock-Based Compensation (Table

Stock-Based Compensation (Tables)	12 Months Ended
	Dec. 31, 2023
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Summary of Inputs Used in Black-Scholes Option Pricing Model	The following table presents, on a weighted average basis, the assumptions used in the Black-Scholes option-pricing model to determine the fair value of stock options granted: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Year Ended December 31, ​ 2023 2022 2021 ​ Risk-free interest rate 3.88 % 2.08 % 0.81 % Expected volatility 102.3 % 91.2 % 76.0 % Expected dividend yield — — ​ — ​ Expected term (in years) 6.0 6.0 ​ 6.0 ​
Summarizes the Company's stock option activity	The following table summarizes our stock option activity since December 31, 2022: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Weighted Weighted ​ ​ ​ ​ ​ ​ Average ​ Average ​ Aggregate ​ ​ Number of ​ Exercise ​ Contractual ​ Intrinsic ​ Shares Price Term Value ​ ​ ​ ​ ​ ​ ​ (in years) ​ (in thousands) Outstanding as of December 31, 2022 8,827,322 ​ $ 13.15 7.97 ​ $ 55 Granted 3,312,016 ​ 3.20 ​ Exercised — ​ — ​ Forfeited (1,212,540) ​ 9.29 ​ Outstanding as of December 31, 2023 10,926,798 ​ $ 10.56 6.21 ​ $ 404 Vested and expected to vest as of December 31, 2023 9,967,903 ​ $ 10.66 6.00 ​ $ 387 Options exercisable as of December 31, 2023 5,378,652 ​ $ 14.07 5.03 ​ $ 17 ​ ​ ​
Schedule of restricted common stock units	​ ​ ​ ​ ​ ​ ​ ​ ​ Weighted Average ​ Shares Grant Date Fair Value Unvested restricted common stock units as of December 31, 2022 959,521 ​ $ 6.65 Issued 895,400 ​ 4.47 Vested (732,448) ​ 6.57 Forfeited (102,022) ​ ​ 5.59 Unvested restricted common stock units as of December 31, 2023 1,020,451 ​ $ 4.90
Summary of stock-based compensation expense was classified in the statements of operations and comprehensive loss	​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Year Ended December 31, (in thousands) 2023 2022 ​ 2021 Research and development expenses $ 11,496 ​ $ 12,405 ​ $ 9,316 General and administrative expenses 12,845 ​ 12,047 ​ 8,541 Total $ 24,341 ​ $ 24,452 ​ $ 17,857
Schedule of unrecognized stock based compensation expense	​ ​ ​ ​ ​ ​ ​ Unrecognized Expense Weight-average Recognition Period ​ ​ (in thousands) ​ (in years) Type of award: ​ ​ ​ ​ Time-based stock options ​ 23,636 ​ 2.00 Restricted stock units ​ 3,292 ​ 2.49 ​
Employee Stock Purchase Plan 2020
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Summary of Inputs Used in Black-Scholes Option Pricing Model	​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Year Ended December 31, ​ ​ 2023 2022 2021 Risk-free interest rate 5.18 % 1.43 % 0.09 % Expected volatility 95.1 % 93.5 % 85.6 % Expected dividend yield — — — Expected term (in years) 0.5 0.5 0.5

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2023
Income Tax Disclosure [Abstract]
Summary of reconciliation of the U.S. federal statutory income tax rate	A reconciliation of the U.S. federal statutory income tax rate to our effective income tax rate is as follows: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Year Ended December 31, ​ ​ 2023 2022 2021 ​ Federal statutory income tax rate (21.0) % (21.0) % (21.0) % State income taxes, net of federal benefit (6.4) (6.1) ​ (6.7) ​ Federal and state research and development tax credits (4.3) (2.3) ​ (2.4) ​ Stock-based compensation expense 1.9 0.9 ​ (1.7) ​ Change in deferred tax asset valuation allowance 29.8 28.5 ​ 31.8 ​ Effective income tax rate — % — % — % ​ ​ ​
Summary of Net deferred tax assets	​ Net deferred tax assets as of December 31, 2023 and 2022 consisted of the following: ​ ​ ​ ​ ​ ​ ​ ​ ​ December 31, (in thousands) 2023 2022 Deferred tax assets: ​ ​ Net operating loss carryforwards ​ $ 96,939 ​ $ 89,546 Research and development tax credit carryforwards ​ 20,836 ​ 15,368 Operating lease liability ​ ​ 26,745 ​ ​ 22,322 Capitalized research and development costs ​ 36,345 ​ 20,005 Deferred revenue ​ ​ 3,341 ​ ​ — Other ​ 13,332 ​ 10,359 Total deferred tax assets ​ 197,538 ​ 157,600 Deferred tax liabilities: ​ ​ Operating lease right-of-use assets ​ ​ (19,084) ​ ​ (16,176) Property and equipment ​ (2,912) ​ (3,566) Total deferred tax liabilities ​ (21,996) ​ (19,742) Valuation allowance ​ ​ (175,542) ​ ​ (137,858) Net deferred tax assets ​ $ — ​ $ — ​ ​ ​
Summary of changes in the valuation allowance for deferred tax assets	​ ​ ​ ​ ​ ​ ​ ​ ​ Year Ended December 31, (in thousands) 2023 2022 Valuation allowance as of beginning of year ​ $ 137,858 ​ $ 98,902 Increases recorded to income tax provision ​ 37,684 ​ 38,956 Valuation allowance as of end of year ​ $ 175,542 ​ $ 137,858 ​ ​

Net Loss per Share (Tables)

Net Loss per Share (Tables)	12 Months Ended
	Dec. 31, 2023
Net Loss per Share
Summary of potential dilutive securities, presented based on amounts outstanding	​ ​ ​ ​ ​ ​ ​ ​ ​ ​ December 31, ​ 2023 2022 2021 Unvested restricted common stock ​ — ​ — ​ 11,083 Unvested restricted stock units ​ 1,020,451 ​ 959,521 ​ 13,120 Stock options to purchase common stock 10,926,798 8,827,322 ​ 6,205,088 Total 11,947,249 9,786,843 ​ 6,229,291

Reduction In Force (Tables)

Reduction In Force (Tables)	12 Months Ended
	Dec. 31, 2023
Restructuring and Related Activities [Abstract]
Summary of accrued restructuring costs recorded and included in accrued expenses and other current liabilities	​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ (in thousands) ​ ​ Severance and Benefits Costs ​ Stock-Based Compensation ​ ​ Total Restructuring expenses ​ $ 6,513 ​ $ 512 ​ $ 7,025 Cash payments ​ (1,222) ​ ​ — ​ ​ (1,222) Non-cash expenses ​ — ​ ​ (512) ​ ​ (512) Balance at December 31, 2023 ​ $ 5,291 ​ $ — ​ $ 5,291

Nature of the Business and Ba_2

Nature of the Business and Basis of Presentation - Additional information (Detail) - USD ($) $ in Thousands			1 Months Ended		12 Months Ended
	Jan. 31, 2021	Jun. 16, 2020	Aug. 31, 2021	Jan. 31, 2021	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Nature Of The Business And Basis Of Presentation [Line Items]
Date of incorporation					Oct. 21, 2016
Net loss and net loss attributable to common stockholders					$ 126,612	$ 136,639	$ 119,151
Accumulated deficit					$ 571,377	444,765
Shares issued during the period			1,795,524
Net proceeds from shares issued	$ 211,300		$ 12,300	$ 211,300
Impairment loss						$ 5,800	$ 0
Impairment, Long-Lived Asset, Held-for-Use, Statement of Income or Comprehensive Income [Extensible Enumeration]					Nonoperating Income (Expense)	Nonoperating Income (Expense)
Common Stock
Nature Of The Business And Basis Of Presentation [Line Items]
Shares issued during the period	9,200,000			9,200,000			9,200,000
Common Stock | IPO
Nature Of The Business And Basis Of Presentation [Line Items]
Shares issued during the period		12,105,263
Net proceeds from shares issued		$ 210,700
Maximum
Nature Of The Business And Basis Of Presentation [Line Items]
Impairment loss					$ 100

Summary of Significant Accoun_4

Summary of Significant Accounting Policies - Summary Of Property And Equipment (Detail)	Dec. 31, 2023
Laboratory equipment
Property, Plant and Equipment [Line Items]
Property and equipment, Useful life	5 years
Computer equipment and software
Property, Plant and Equipment [Line Items]
Property and equipment, Useful life	3 years
Furniture and fixtures
Property, Plant and Equipment [Line Items]
Property and equipment, Useful life	5 years
Leasehold Improvements
Property, Plant and Equipment [Line Items]
Leasehold improvements, Useful life	us-gaap:UsefulLifeShorterOfTermOfLeaseOrAssetUtilityMember

Summary of Significant Accoun_5

Summary of Significant Accounting Policies - Additional Information (Detail) $ in Millions	12 Months Ended
	Dec. 31, 2023 USD ($) segment	Dec. 31, 2022 USD ($)
Accounting Policies [Abstract]
Number of operating segment | segment	1
Income Tax rate settlement	50%
Interest and penalties | $	$ 0	$ 0

Marketable Securities and Fai_3

Marketable Securities and Fair Value Measurements - Summary of marketable securities (Detail) - Fair Value, Recurring - U.S. treasury securities - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Marketable Securities [Line Items]
Amortized Cost	$ 197,644	$ 186,003
Gross Unrealized Gains	274	13
Gross Unrealized Losses		(96)
Fair Value	$ 197,918	$ 185,920

Marketable Securities and Fai_4

Marketable Securities and Fair Value Measurements - Summary of assets measured at fair value on a recurring basis (Detail) - Fair Value, Recurring - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Marketable securities:
Total	$ 236,128	$ 262,930
Level 1
Marketable securities:
Total	38,210	77,010
Level 2
Marketable securities:
Total	197,918	185,920
Money Market Funds
Cash equivalents:
Cash equivalents	38,210	77,010
Money Market Funds | Level 1
Cash equivalents:
Cash equivalents	38,210	77,010
U.S. treasury securities
Marketable securities:
Marketable securities	197,918	185,920
U.S. treasury securities | Level 2
Marketable securities:
Marketable securities	$ 197,918	$ 185,920

Collaboration and License Agr_3

Collaboration and License Agreement (Details) $ / shares in Units, $ in Thousands	1 Months Ended		12 Months Ended
	Mar. 31, 2023 USD ($) item product $ / shares shares	Aug. 31, 2021 shares	Dec. 31, 2023 USD ($)	Dec. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)	Apr. 30, 2023 USD ($)
Collaboration and License Agreement
Issuance/Sale of common stock, shares | shares		1,795,524
Revenue recognized			$ 5,904	$ 0	$ 0
Share Purchase Agreement | Beneficial Owner | ModernaTX, Inc
Collaboration and License Agreement
Issuance/Sale of common stock, shares | shares	5,859,375
Collaboration and License Agreement | ModernaTX, Inc
Collaboration and License Agreement
Upfront payments	$ 40,000					$ 40,000
Prepaid research services funding	7,500					$ 7,500
Milestone payments receivable	$ 1,800,000
Number of performance obligations | item	1
Premium paid on common stock	$ 13,300
Variable consideration	$ 0
Collaboration and License Agreement | ModernaTX, Inc | Minimum
Collaboration and License Agreement
Number of products to be commercialized | product	1
Royalty payment term	10 years
Collaboration and License Agreement | Share Purchase Agreement | Beneficial Owner | ModernaTX, Inc
Collaboration and License Agreement
Issuance/Sale of common stock, shares | shares	5,859,375
Shares issue price (in dollars per share) | $ / shares	$ 6.14
Proceeds from issuance/sale of common stock	$ 36,000
Percent of shares to be issued	3.06%
Equity financing	$ 25,000

Collaboration and License Agr_4

Collaboration and License Agreement - Schedule of Allocated Transaction Price and Revenue Activity (Details) - ModernaTX, Inc - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Jun. 30, 2023
Collaboration and License Agreement
Collaboration and License Agreement
Standalone Selling Price		$ 96,050
Transaction Price Allocated	$ 75,541	77,893
Revenue Recognized	5,904
Revenue Recognized	54,861
ctLNP technology and research license
Collaboration and License Agreement
Standalone Selling Price		52,500
Transaction Price Allocated	41,290	42,576
Revenue Recognized	5,904
Revenue Recognized	27,310
First liver program commercialization option license
Collaboration and License Agreement
Standalone Selling Price		7,000
Transaction Price Allocated	5,505	5,677
Revenue Recognized	0
Revenue Recognized	4,429
Second liver program commercialization option license
Collaboration and License Agreement
Standalone Selling Price		7,000
Transaction Price Allocated	5,505	5,677
Revenue Recognized	0
Revenue Recognized	4,429
First non-liver program commercialization option license
Collaboration and License Agreement
Standalone Selling Price		11,700
Transaction Price Allocated	9,202	9,488
Revenue Recognized	0
Revenue Recognized	7,402
Second non-liver program commercialization option license
Collaboration and License Agreement
Standalone Selling Price		11,700
Transaction Price Allocated	9,202	9,488
Revenue Recognized	0
Revenue Recognized	7,402
Third liver or non-liver program commercialization option license
Collaboration and License Agreement
Standalone Selling Price		6,150
Transaction Price Allocated	4,837	$ 4,987
Revenue Recognized	0
Revenue Recognized	$ 3,889

Prepaid expenses and other cu_3

Prepaid expenses and other current assets (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Prepaid expenses and other current assets
Prepaid research and development expenses	$ 2,050	$ 4,555
Prepaid insurance	680	1,180
Prepaid facilities-related expenses	953	922
Prepaid employee compensation and benefits	133	349
Interest income receivable	188	265
Other	290	259
Total	$ 4,294	$ 7,530

Prepaid expenses and other cu_4

Prepaid expenses and other current assets - Additional Information (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Prepaid Expenses and Other Current Assets [Line Items]
Prepaid research and development expenses	$ 2,050	$ 4,555
Other Noncurrent Assets
Prepaid Expenses and Other Current Assets [Line Items]
Prepaid research and development expenses	$ 100	$ 1,700

Property and equipment, net - S

Property and equipment, net - Summary of Property and Equipment, Net (Detail) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 45,494	$ 36,753
Less: Accumulated depreciation and amortization	(19,695)	(14,538)
Property and equipment, net	25,799	22,215
Laboratory equipment
Property, Plant and Equipment [Line Items]
Property and equipment, gross	14,859	13,619
Computer equipment and software
Property, Plant and Equipment [Line Items]
Property and equipment, gross	1,447	1,189
Furniture and fixtures
Property, Plant and Equipment [Line Items]
Property and equipment, gross	1,293	1,146
Leasehold Improvements
Property, Plant and Equipment [Line Items]
Property and equipment, gross	20,865	20,786
Construction in progress
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 7,030	$ 13

Property and equipment, net - A

Property and equipment, net - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Property and equipment, net
Depreciation and amortization expense	$ 5,263	$ 5,130	$ 4,532

Leases - Narrative (Details)

Leases - Narrative (Details) - USD ($) $ in Thousands	1 Months Ended	12 Months Ended
	Jul. 31, 2021	Dec. 31, 2023	Dec. 31, 2022
Lessee, Lease, Description [Line Items]
Operating lease term			12 years
Total rent payments		$ 136,708	$ 104,300
Initial monthly base rent on operating lease	$ 400
Option to extend lease	two
Operating lease agreement option to renewal term	5 years
Construction allowance in operating lease	$ 26,000
Lease existence of option to extend		true
Operating lease agreement option to renewal term		5 years
Letter of credit for security deposit		$ 2,100
Operating lease payments		$ 13,400	$ 9,200
Weighted-average incremental borrowing		7.50%	7.40%
Weighted-average remaining lease term		11 years	10 years
Maximum
Lessee, Lease, Description [Line Items]
Estimated monthly base rent on operating lease	600
Letter of Credit
Lessee, Lease, Description [Line Items]
Security deposit	$ 3,600

Leases - Costs included in oper

Leases - Costs included in operating expenses related to the operating lease (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Leases [Abstract]
Operating lease cost	$ 12,585	$ 13,796
Variable lease cost	2,951	2,572
Total	$ 15,536	$ 16,368

Leases - Future Lease Payments

Leases - Future Lease Payments (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Leases [Abstract]
2024	$ 15,207
2025	15,638
2026	16,094
2027	16,550
2028	17,058
Thereafter	56,161
Total undiscounted payments due under operating leases	136,708	$ 104,300
Total undiscounted unearned tenant improvements	(39)
Less imputed interest	(38,853)
Current operating lease liability	8,120	7,086
Non-current operating lease liability	89,774	$ 74,621
Total	$ 97,894

Accrued Expenses - Schedule of

Accrued Expenses - Schedule of Accrued Expenses (Detail) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Accrued Liabilities [Line Items]
Accrued employee compensation and benefits	$ 13,208	$ 7,970
Accrued external research and development expenses	1,169	1,959
Accrued professional fees	908	1,047
Property and equipment	838	0
Other	406	426
Total	16,529	$ 11,402
Restructuring Reserve	5,291
Reduction In Force (RIF)
Accrued Liabilities [Line Items]
Restructuring Reserve	$ 5,300

Equity - Additional Information

Equity - Additional Information (Detail) $ / shares in Units, $ in Millions			1 Months Ended			12 Months Ended
	Jan. 31, 2021 USD ($) shares	Jun. 16, 2020 shares	Mar. 31, 2023 USD ($) $ / shares shares	Aug. 31, 2021 USD ($) shares	Jan. 31, 2021 USD ($) shares	Dec. 31, 2023 security $ / shares shares	Dec. 31, 2021 shares	Dec. 31, 2022 $ / shares shares
Capital Unit [Line Items]
Common stock, shares authorized						150,000,000		150,000,000
Common stock, par value per share | $ / shares						$ 0.0001		$ 0.0001
Preferred stock, shares authorized						5,000,000		5,000,000
Preferred stock, par value per share | $ / shares						$ 0.0001		$ 0.0001
Shares issued during the period				1,795,524
Net proceeds from shares issued | $	$ 211.3			$ 12.3	$ 211.3
Number of vote per common share | security						1
Beneficial Owner | ModernaTX, Inc | Share Purchase Agreement
Capital Unit [Line Items]
Shares issued during the period			5,859,375
Maximum
Capital Unit [Line Items]
Aggregate gross offering price | $				$ 250
Common Stock
Capital Unit [Line Items]
Shares issued during the period	9,200,000				9,200,000		9,200,000
Over-Allotment Option | Common Stock
Capital Unit [Line Items]
Shares issued during the period	1,200,000	1,578,947			1,200,000
Share Purchase Agreement | Beneficial Owner | ModernaTX, Inc | Share Purchase Agreement
Capital Unit [Line Items]
Shares issued during the period			5,859,375
Shares issue price (in dollars per share) | $ / shares			$ 6.14
Proceeds from Issuance of Common Stock | $			$ 36

Stock-Based Compensation - Stoc

Stock-Based Compensation - Stock incentive plans (Detail) - USD ($) $ / shares in Units, $ in Millions		1 Months Ended	12 Months Ended
	Jun. 11, 2020	Aug. 31, 2021	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	Jan. 31, 2024
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
No of Intrinsic stock option exercised			$ 0	$ 0.5	$ 19.4
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period, Fair Value			$ 3.3	$ 3.5	$ 0.2
Per-share purchase price, percentage			85%
Shares issued during the period		1,795,524
Shares issued			895,400
Equity Option
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted average grant date fair value of Unvested restricted common stock, Vested | $ / shares			$ 2.54	$ 4.76	$ 18.81
Unvested restricted common stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period, Fair Value			$ 0
Fair value of restricted common stock				$ 0.1	$ 17.7
2020 Stock Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock reserved for future issuance	2,547,698		255,176
Stock based compensation vesting period service based			4 years
Stock based compensation period of expiry			10 years
2020 Stock Incentive Plan | Maximum | Additional From 2017 Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock reserved for future issuance	7,173,014
2020 Stock Incentive Plan | Maximum | Annual Increase 2021 To 2030
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage of common stock shares additionally reserved for issuance	4%
Employee Stock Purchase Plan 2020
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock reserved for future issuance			2,115,792	1,520,738	950,931
Shares issued during the period			235,214
Shares issued			1,698,616
Employee Stock Purchase Plan 2020 | Subsequent Event
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock reserved for future issuance						2,777,974
Employee Stock Purchase Plan 2020 | Maximum | Annual Increase 2021 To 2030
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage of common stock shares additionally reserved for issuance	1%
Number of share instruments newly issued under a share-based compensation plan	1,302,157
Common Stock | 2020 Stock Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share based compensation by share based payment arrangement number of shares authorized for issuance			16,813,962	14,433,745	12,154,517
Common Stock | 2020 Stock Incentive Plan | Subsequent Event
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share based compensation by share based payment arrangement number of shares authorized for issuance						19,462,688

Stock-Based Compensation - St_2

Stock-Based Compensation - Stock Option Valuation (Detail)	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free interest rate	3.88%	2.08%	0.81%
Expected volatility	102.30%	91.20%	76%
Expected term (in years)	6 years	6 years	6 years
Employee Stock Purchase Plan 2020
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free interest rate	5.18%	1.43%	0.09%
Expected volatility	95.10%	93.50%	85.60%
Expected dividend yield	0%	0%	0%
Expected term (in years)	6 months	6 months	6 months

Stock-Based Compensation - Summ

Stock-Based Compensation - Summarizes the Company's Stock Option Activity (Detail) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Stock-Based Compensation
Number of shares Outstanding , Beginning	8,827,322
Number of Shares, Issued, Shares	3,312,016
Number of shares | Forfeited	(1,212,540)
Number of shares Outstanding , Ending	10,926,798	8,827,322
Number of shares | Vested and expected to vest	9,967,903
Number of shares | Options exercisable	5,378,652
Weighted average exercise price Outstanding , Beginning	$ 13.15
Weighted average exercise price | Granted	3.20
Weighted average exercise price | Forfeited	9.29
Weighted average exercise price Outstanding , Ending	10.56	$ 13.15
Weighted average exercise price | Vested and expected to vest	10.66
Weighted average exercise price | Options exercisable	$ 14.07
Weighted average contractual term (in years) Outstanding	6 years 2 months 15 days	7 years 11 months 19 days
Weighted average contractual term (in years) | Vested and expected to vest	6 years
Weighted average contractual term (in years) | Options exercisable	5 years 10 days
Aggregate intrinsic value (in thousands) Outstanding	$ 404	$ 55
Aggregate intrinsic value (in thousands) | Vested and expected to vest	387
Aggregate intrinsic value (in thousands) | Options exercisable	$ 17

Stock-Based Compensation - Su_2

Stock-Based Compensation - Summarizes the Company's Restricted Stock Units (Detail) - $ / shares	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Shares
Unvested restricted common stock units, Beginning balance	959,521
Granted	895,400
Vested	(732,448)
Forfeited	(102,022)
Unvested restricted common stock units, Ending balance	1,020,451
Weighted Average Grant Date Fair Value
Weighted Average Grant Date Fair Value, Beginning balance	$ 4.90	$ 6.65
Weighted Average Grant Date Fair Value, Granted	4.47
Weighted Average Grant Date Fair Value, Vested	6.57
Weighted Average Grant Date Fair Value, Forfeited	5.59
Weighted Average Grant Date Fair Value, Ending balance	$ 4.90

Stock-Based Compensation - Expe

Stock-Based Compensation - Expense Classification (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Stock-based compensation expense	$ 24,341	$ 24,452	$ 17,857
Research and development expense
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Stock-based compensation expense	11,496	12,405	9,316
General and administrative expenses
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Stock-based compensation expense	$ 12,845	$ 12,047	$ 8,541

Stock-Based Compensation - Unre

Stock-Based Compensation - Unrecognized Expense (Detail) $ in Thousands	12 Months Ended
	Dec. 31, 2023 USD ($)
Time Based Stock Option
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized Expense	$ 23,636
Unrecognized stock compensation on account of unvested options remaining period over which the compensation is to be recognized	2 years
Unvested restricted common stock units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized Expense	$ 3,292
Unrecognized stock compensation on account of unvested options remaining period over which the compensation is to be recognized	2 years 5 months 26 days

Income Taxes - Additional Infor

Income Taxes - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Schedule Of Income Tax Disclosure [Line Items]
Income tax benefits	$ 0	$ 0	$ 0
Net operating loss carry forwards	$ 344,800
Percentage on annual taxable income	80%
Tax Credit Carryforward, Amount	$ 8,000
Percentage of change in ownership	50%
Deferred tax assets tax attribute	$ 700
Unrecognized Tax Benefits of income tax rate	50%
Tax Credit Carry forward, Expiration Year	2036
Operating Loss Carry forwards, Expiration Year	2036
Loss from operations	$ (138,563)	(141,182)	$ (119,101)
Reserve for uncertain tax positions	0	0
Interest and penalties	0	$ 0
Operating loss carryforwards	59,800
Federal income tax authority
Schedule Of Income Tax Disclosure [Line Items]
Net operating loss carry forwards	353,000
State and Local Jurisdiction
Schedule Of Income Tax Disclosure [Line Items]
Net operating loss carry forwards	360,800
Federal and State R&D Tax Credit Carryforward
Schedule Of Income Tax Disclosure [Line Items]
Operating loss carryforward with expiration date	8,200
Tax Credit Carryforward, Amount	$ 14,500

Income Taxes - Summary of Recon

Income Taxes - Summary of Reconciliation of the U.S. Federal Statutory Income Tax Rate (Detail)	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Federal statutory income tax rate	(21.00%)	(21.00%)	(21.00%)
State income taxes, net of federal benefit	(6.40%)	(6.10%)	(6.70%)
Federal and state research and development tax credits	(4.30%)	(2.30%)	(2.40%)
Stock-based compensation expense	1.90%	0.90%	(1.70%)
Change in deferred tax asset valuation allowance	29.80%	28.50%	31.80%
Effective income tax rate	0%	0%	0%

Income Taxes - Summary of Net D

Income Taxes - Summary of Net Deferred Tax Assets (Detail) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Deferred tax assets:
Net operating loss carryforwards	$ 96,939	$ 89,546
Research and development tax credit carryforwards	20,836	15,368
Operating lease liability	26,745	22,322
Capitalized research and development costs	36,345	20,005
Deferred revenue	3,341	0
Other	13,332	10,359
Total deferred tax assets	197,538	157,600
Deferred tax liabilities:
Operating lease right-of-use assets	(19,084)	(16,176)
Property and equipment	(2,912)	(3,566)
Total deferred tax liabilities	(21,996)	(19,742)
Valuation allowance	(175,542)	(137,858)	$ (98,902)
Total deferred tax assets	$ 0	$ 0

Income Taxes - Summary of Chang

Income Taxes - Summary of Changes in the Valuation Allowance for Deferred Tax Assets (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Valuation Allowance [Abstract]
Valuation allowance as of beginning of year	$ 137,858	$ 98,902
Increases recorded to income tax provision	37,684	38,956
Valuation allowance as of end of year	$ 175,542	$ 137,858

License Agreement - Additional

License Agreement - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Research and development	$ 93,617	$ 96,718	$ 85,247
NIH
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Research and development	100
NIH | License Agreement Terms
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Future milestone obligation	$ 400
Percentage of increase decrease in net sales	25%
NIH | Maximum
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Research and development		100	100
UMass | License Agreement Terms
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Future milestone obligation	$ 800
Shares Issued For Services	125,677
Research and development	$ 600
Adjustments to Additional Paid in Capital, Other	$ 100
UMass | Maximum | License Agreement Terms
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Research and development		$ 100	$ 100

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2023
Commitments and Contingencies.
Defined contribution plan, percentage	4%

Net Loss per Share - Summary of

Net Loss per Share - Summary of Potential Dilutive Securities, Presented Based on Amounts Outstanding (Detail) - shares	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive net loss per share	11,947,249	9,786,843	6,229,291
Unvested restricted common stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive net loss per share	0	0	11,083
Unvested restricted common stock units
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive net loss per share	1,020,451	959,521	13,120
Employee Stock Option [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive net loss per share	10,926,798	8,827,322	6,205,088

Related Parties (Details)

Related Parties (Details) - shares	1 Months Ended
	Mar. 31, 2023	Aug. 31, 2021
Related Party Transaction [Line Items]
Shares issued during the period		1,795,524
Beneficial Owner | ModernaTX, Inc | Share Purchase Agreement
Related Party Transaction [Line Items]
Shares issued during the period	5,859,375
Beneficial ownership percentage	8.90%

Reduction in Force (Details)

Reduction in Force (Details) - USD ($) $ in Thousands	1 Months Ended	12 Months Ended
	Nov. 30, 2023	Dec. 31, 2023
Restructuring Cost and Reserve [Line Items]
Reduction in workforce	40%
Restructuring and Related Cost, Incurred Cost		$ 7,000
Restructuring, Incurred Cost, Statement of Income or Comprehensive Income [Extensible Enumeration]		General and Administrative Expense, Research and Development Expense
Additional restructuring expense		$ 400
Restructuring expenses		7,025
Research and development expense
Restructuring Cost and Reserve [Line Items]
Restructuring expenses		5,100
General and administrative expenses
Restructuring Cost and Reserve [Line Items]
Restructuring expenses		$ 1,900

Reduction In Force - Accrued Re

Reduction In Force - Accrued Restructuring Costs (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2023 USD ($)
Restructuring Cost and Reserve [Line Items]
Restructuring expenses	$ 7,025
Cash payments	(1,222)
Non-cash expenses	(512)
Balance at December 31, 2023	5,291
Severance and Benefits Costs
Restructuring Cost and Reserve [Line Items]
Restructuring expenses	6,513
Cash payments	(1,222)
Balance at December 31, 2023	5,291
Stock-Based Compensation
Restructuring Cost and Reserve [Line Items]
Restructuring expenses	512
Non-cash expenses	(512)
Reduction In Force (RIF)
Restructuring Cost and Reserve [Line Items]
Balance at December 31, 2023	$ 5,300

Subsequent Event (Details)

Subsequent Event (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Subsequent Event [Line Items]
Subsequent event	17. Subsequent event In July 2021, we entered into the Seyon Lease and in January 2024, we notified the landlord of the termination of the Seyon Lease due to the landlord’s breach of its obligations to us under the Seyon Lease and returned possession of the premises to the landlord, effective January 31, 2024. On February 20, 2024, the landlord served us with a complaint, filed in Massachusetts Superior Court, with respect to the Seyon Lease. The complaint seeks declaratory judgment that we unlawfully terminated the Seyon Lease and also asserts a claim for breach of contract damages. Our responsive pleading is due April 1, 2024. We will vigorously defend the action and our rights with respect to this matter. As of December 31, 2023, we had $46.0 million in operating lease right-to-use assets, $62.2 million in operating lease liability, $6.2 million in construction in process, $4.0 million in tenant receivable, and $3.6 million in restricted cash related to the Seyon Lease on our consolidated balance sheet. As of March 6, 2024, the issuance date of this Annual Report on Form 10-K, we were unable to determine the impact of the Seyon Lease termination and the related pending litigation on our consolidated financial statements. ​
Operating lease right-of-use assets	$ 69,852	$ 59,208
Lease liability	97,894
Property and equipment, net	25,799	22,215
Tenant receivable	3,960	395
Restricted cash	5,791	$ 5,692
Seyon Lease
Subsequent Event [Line Items]
Operating lease right-of-use assets	46,000
Lease liability	62,200
Property and equipment, net	6,200
Tenant receivable	4,000
Restricted cash	$ 3,600

Pay vs Performance Disclosure

Pay vs Performance Disclosure - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Pay vs Performance Disclosure
Net Income (Loss)	$ (126,612)	$ (136,639)	$ (119,151)

Insider Trading Arrangements

Insider Trading Arrangements	12 Months Ended
	Dec. 31, 2023
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