Solid Biosciences (SLDB) 10-K 14 Mar 22 2021 FY Annual report Financial data

Document and Entity Information

Document and Entity Information - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Feb. 16, 2022	Jun. 30, 2021
Cover [Abstract]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2021
Document Fiscal Year Focus	2021
Document Fiscal Period Focus	FY
Trading Symbol	SLDB
Title of 12(b) Security	Common Stock $0.001 par value per share
Security Exchange Name	NASDAQ
Entity Registrant Name	Solid Biosciences Inc.
Entity Central Index Key	0001707502
Current Fiscal Year End Date	--12-31
Entity File Number	001-38360
Entity Tax Identification Number	90-0943402
Entity Incorporation, State or Country Code	DE
Entity Address, Address Line One	141 Portland Street, Fifth Floor
Entity Address, City or Town	Cambridge
Entity Address, State or Province	MA
Entity Address, Postal Zip Code	02139
City Area Code	617
Local Phone Number	337-4680
Document Annual Report	true
Document Transition Report	false
Entity Filer Category	Non-accelerated Filer
Entity Small Business	true
Entity Emerging Growth Company	true
Entity Ex Transition Period	true
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		110,472,962
Entity Public Float			$ 271.5
Entity Well-known Seasoned Issuer	No
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
ICFR Auditor Attestation Flag	false
Auditor Firm ID	238
Auditor Name	PricewaterhouseCoopers LLP
Auditor Location	Boston, Massachusetts

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020
Current assets:
Cash and cash equivalents	$ 119,136	$ 154,744
Available-for-sale securities	88,643
Prepaid expenses and other current assets	14,723	4,157
Accounts receivable - related party	110
Total current assets	222,612	158,901
Operating lease, right of use asset	1,142	3,579
Property and equipment, net	6,462	8,153
Other non-current assets	94	209
Restricted cash	2,070	327
Total assets	232,380	171,169
Current liabilities:
Accounts payable	4,463	3,274
Accrued expenses	9,528	8,640
Operating lease liabilities	1,263	1,999
Finance lease liabilities	232	208
Deferred revenue - related party	8,080	10,359
Other current liabilities	35
Total current liabilities	23,601	24,480
Operating lease liabilities, excluding current portion	275	2,419
Finance lease obligations, excluding current portion	293	525
Deferred revenue - related party, excluding current portion		10,359
Other non-current liabilities		1,300
Total liabilities	24,169	39,083
Commitments and Contingencies (Note 11)
Stockholders’ Equity:
Preferred stock, $0.001 par value; 10,000,000 shares authorized at December 31, 2021 and December 31, 2020; no shares issued and outstanding at December 31, 2021 and December 31, 2020
Common stock, $0.001 par value; 300,000,000 shares authorized at December 31, 2021 and December 31, 2020; 110,340,281 shares issued and outstanding at December 31, 2021 and 84,893,994 shares issued and outstanding at December 31, 2020; 2,158,329 pre-funded warrants outstanding at December 31, 2021 and December 31, 2020	112	87
Additional paid-in capital	684,901	536,568
Accumulated other comprehensive loss	(45)
Accumulated deficit	(476,757)	(404,569)
Total stockholders' equity	208,211	132,086
Total liabilities and stockholders' equity	$ 232,380	$ 171,169

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2021	Dec. 31, 2020
Statement Of Financial Position [Abstract]
Preferred stock, par value	$ 0.001	$ 0.001
Preferred stock, shares authorized	10,000,000	10,000,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value	$ 0.001	$ 0.001
Common stock, shares authorized	300,000,000	300,000,000
Common stock, shares issued	110,340,281	84,893,994
Common stock, shares outstanding	110,340,281	84,893,994
Pre-funded warrants outstanding	2,158,329	2,158,329

Consolidated Statements of Oper

Consolidated Statements of Operations - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Income Statement [Abstract]
Revenue	$ 13,620	$ 0	$ 0
Operating expenses:
Research and development	58,739	64,881	94,737
General and administrative	27,135	21,581	24,581
Restructuring expense		1,944
Total operating expenses	85,874	88,406	119,318
Loss from operations	(72,254)	(88,406)	(119,318)
Other income, net:
Interest income, net	64	115	1,580
Other income	2	1	515
Total other income, net	66	116	2,095
Net loss	$ (72,188)	$ (88,290)	$ (117,223)
Net loss per share, basic and diluted	$ (0.68)	$ (1.70)	$ (2.91)
Weighted average common stock outstanding, basic and diluted	106,770,372	51,937,147	40,289,290

Consolidated Statements of Comp

Consolidated Statements of Comprehensive Loss - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Statement Of Income And Comprehensive Income [Abstract]
Net loss	$ (72,188)	$ (88,290)	$ (117,223)
Other comprehensive (loss) income:
Unrealized (loss) gain on available-for-sale securities	(45)	(1)	6
Comprehensive loss	$ (72,233)	$ (88,291)	$ (117,217)

Consolidated Statements of Stoc

Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	Common Stock [Member]	Additional Paid In Capital [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Accumulated Deficit [Member]
Beginning balance at Dec. 31, 2018	$ 125,183	$ 35	$ 324,209	$ (5)	$ (199,056)
Beginning balance, units at Dec. 31, 2018		35,432,460
Equity-based compensation	14,207		14,207
Sale of common stock, net of issuance costs	47,223	$ 11	47,212
Sale of common stock, net of issuance costs, shares		10,607,525
Sale of pre-funded warrants	10,652	$ 2	10,650
Sale of pre-funded warrants, shares		2,295,699
Forfeiture of restricted stock awards units		(52,414)
Unrealized gain (loss) on available-for-sale securities	6			6
Net loss	(117,223)				(117,223)
Ending balance at Dec. 31, 2019	80,048	$ 48	396,278	1	(316,279)
Ending balance, units at Dec. 31, 2019		48,283,270
Equity-based compensation	11,629		11,629
Sale of common stock, net of issuance costs	109,418	$ 31	109,387
Sale of common stock, net of issuance costs, shares		30,633,952
Issuance of common stock,to a related party, in connectionwith the Stock Purchase Agreement	19,282	$ 8	19,274
Issuance of common stock, to a related party, in connection with the Stock Purchase Agreement, shares		7,825,797
Vesting of restricted stock units		407,700
Forfeiture of restricted stock awards	0
Forfeiture of restricted stock awards units		(98,396)
Unrealized gain (loss) on available-for-sale securities	(1)			(1)
Net loss	(88,290)				(88,290)
Ending balance at Dec. 31, 2020	132,086	$ 87	536,568		(404,569)
Ending balance, units at Dec. 31, 2020		87,052,323
Equity-based compensation	13,373		13,373
Sale of common stock, net of issuance costs	134,878	$ 25	134,853
Sale of common stock, net of issuance costs, shares		25,000,000
Exercise of common stock options	$ 41		41
Exercise of common stock options, shares	11,625	11,625
Issuance of ESPP shares	$ 66		66
Issuance of ESPP shares, shares		44,681
Vesting of restricted stock units		419,193
Forfeiture of restricted stock awards units		(29,212)
Unrealized gain (loss) on available-for-sale securities	(45)			(45)
Net loss	(72,188)				(72,188)
Ending balance at Dec. 31, 2021	$ 208,211	$ 112	$ 684,901	$ (45)	$ (476,757)
Ending balance, units at Dec. 31, 2021		112,498,610

Consolidated Statements of St_2

Consolidated Statements of Stockholders' Equity (Parenthetical) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Statement Of Financial Position [Abstract]
Issuance costs	$ 3,790	$ 2,102	$ 8,872

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Operating activities:
Net loss	$ (72,188)	$ (88,290)	$ (117,223)
Adjustments to reconcile net loss to net cash used in operating activities:
Net amortization of premium/(discount) on available-for-sale securities	1,117	(20)	(279)
Equity-based compensation expense	13,373	11,629	14,207
Depreciation and amortization expense	2,964	3,922	2,824
Loss on sale of property and equipment	92		2
Gain on lease termination	(81)
Changes in operating assets and liabilities:
Prepaid expenses and other current and non-current assets	(9,247)	30	4,486
Accounts receivable - related party	(110)
Accounts payable	1,209	(3,431)	3,779
Accrued expenses and other current and non-current liabilities	(2,255)	(1,157)	(510)
Deferred revenue - related party, current and non-current	(12,638)	20,718
Net cash used in operating activities	(77,764)	(56,599)	(92,714)
Investing activities:
Purchases of property and equipment	(1,281)	(899)	(4,387)
Proceeds from sales and maturities of available-for-sale securities	51,444	7,900	60,399
Purchases of available-for-sale securities	(141,249)	(401)	(31,496)
Net cash (used in) provided by investing activities	(91,086)	6,600	24,516
Financing activities:
Proceeds from the issuance of common stock to a related party in connection with the Stock Purchase Agreement		19,282
Proceeds from issuance of common stock, net of issuance costs	134,878	113,208	49,325
Payment of offering costs		(3,790)	(2,102)
Proceeds from issuance of pre-funded warrants			10,652
Proceeds for exercise of stock options	41
Employee stock purchases and withholdings	66
Net cash provided by financing activities	134,985	128,700	57,875
Net (decrease) increase in cash, cash equivalents and restricted cash	(33,865)	78,701	(10,323)
Cash, cash equivalents, and restricted cash at beginning of period	155,071	76,370	86,693
Cash, cash equivalents, and restricted cash at end of period	121,206	155,071	76,370
Supplemental disclosure of non-cash investing and financing activities:
Decrease in ROU asset and lease liability due to lease termination	(1,233)
Property and equipment included in accounts payable and accruals	$ 104	$ 20	$ 490

Nature of the Business and Basi

Nature of the Business and Basis of Presentation	12 Months Ended
	Dec. 31, 2021
Accounting Policies [Abstract]
Nature of the Business and Basis of Presentation	1. Nature of the Business and Basis of Presentation Nature of Business Solid Biosciences Inc. was organized in March 2013 under the name SOLID Ventures Management, LLC and operated as a Delaware limited liability company until immediately prior to the effectiveness of its registration statement on Form S-1 on January 25, 2018, at which time it completed a statutory corporate conversion into a Delaware corporation (the “Corporate Conversion”) and changed its name to Solid Biosciences Inc. (the “Company”). The Company’s mission is to cure Duchenne muscular dystrophy (“Duchenne”), a genetic muscle-wasting disease predominantly affecting boys. It is caused by mutations in the dystrophin gene, which result in the absence or near-absence of dystrophin protein. Dystrophin protein works to strengthen muscle fibers and protect them from daily wear and tear. Without functioning dystrophin and certain associated proteins, muscles suffer excessive damage from normal daily activities and are unable to regenerate, leading to the build-up of fibrotic, or scar, and fat tissue. The Company’s lead product candidate, SGT-001, is a gene transfer candidate under investigation for its ability to drive functional dystrophin protein expression in patients’ muscles and improve the course of the disease. SGT-001 has been granted Rare Pediatric Disease Designation and Fast Track Designation in the United States and Orphan Drug Designations in both the United States and European Union. The Company filed an Investigational New Drug application (“IND”) in September 2017 and initiated a Phase I/II clinical trial for SGT-001 in the United States during the fourth quarter of 2017, which is called IGNITE DMD. In November 2019, IGNITE DMD was placed on clinical hold by the U.S. Food and Drug Administration (“FDA”). In October 2020, following changes to the clinical protocol designed to enhance patient safety, improvements to its manufacturing process, and the provision of additional efficacy and safety data to the FDA, the Company announced that the FDA lifted the clinical hold placed on the Company’s IGNITE DMD Phase I/II clinical trial. In February 2021, the Company resumed treatment of patients in the IGNITE DMD clinical trial. In May 2021, the Company announced the advancement of a next-generation Duchenne microdystrophin gene transfer program, SGT-003, a preclinical candidate that combines a novel, rationally-designed capsid candidate with the Company’s proprietary neuronal Nitric Oxide Synthase (nNOS) containing microdystrophin construct. The Company is subject to risks and uncertainties common to early-stage companies in the biotechnology industry, including, but not limited to, development by competitors of new technological innovations, dependence on licenses, protection of proprietary technology, dependence on key personnel, compliance with government regulations and the need to obtain additional financing to fund operations. Product candidates currently under development will require significant additional research and development efforts, including extensive preclinical studies and clinical trials and regulatory approval, prior to commercialization. These efforts require significant amounts of additional capital, adequate personnel infrastructure and extensive compliance and reporting capabilities. The Company’s product candidates are in development. There can be no assurance that the Company’s research and development will be successfully completed, that adequate protection for the Company’s intellectual property will be obtained, that any products developed will obtain necessary government regulatory approval or that any approved products will be commercially viable. Even if the Company’s product development efforts are successful, it is uncertain when, if ever, the Company will generate significant revenue from product sales. The Company operates in an environment of rapid change in technology and substantial competition from, among others, other pharmaceutical and biotechnology companies. In addition, the Company is dependent upon the services of its employees, partners and consultants. Liquidity The accompanying consolidated financial statements have been prepared on a basis that assumes the Company will continue as a going concern and which contemplates the realization of assets and satisfaction of liabilities and commitments in the ordinary course of business. Through December 31, 2021, the Company has funded its operations primarily with the proceeds from the sale of redeemable preferred units and member units as well as the sale of common stock and prefunded warrants to purchase shares of its common stock in private placements and the sale of common stock in its initial public offering and follow-on public offering in March 2021 and under its at-the-market sales agreement. In accordance with Accounting Standards Codification (“ASC”) 205-40, Going Concern, the Company has evaluated whether there are conditions and events, considered in the aggregate, that raise substantial doubt about the Company’s ability to continue as a going concern within one year after the date the financial statements are issued. As of December 31, 2021, the Company had an accumulated deficit of $476,757. During the year ended December 31, 2021, 2020 and 2019, the Company incurred a net loss of $72,188, $88,290 and $117,223, respectively. The Company used $77,764 of cash in operations for the year ended December 31, 2021. The Company expects to continue to generate operating losses in the foreseeable future. Based upon its current operating plan, the Company expects that its cash, cash equivalents and available-for-sale securities of $207,779 as of December 31, 2021, will be sufficient to fund its operating expenses and capital expenditure requirements for at least twelve months from the date of issuance of these financial statements. The accompanying consolidated financial statements have been prepared in accordance with accounting principles generally accepted in the United States of America (“GAAP”). The accompanying condensed consolidated financial statements include the accounts of the Company and its wholly owned or controlled subsidiaries. All intercompany accounts and transactions have been eliminated.

Summary of Significant Accounti

Summary of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2021
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	2. Summary of Significant Accounting Policies Use of Estimates The preparation of the Company’s consolidated financial statements in conformity with GAAP requires management to make estimates, judgments and assumptions that affect the reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities at the date of the consolidated financial statements and the reported amounts of revenues and expenses during the reporting periods. Significant estimates and assumptions reflected in these consolidated financial statements include, but are not limited to, estimates related to revenue recognition, the recognition of research and development expenses and equity-based compensation. Estimates are periodically reviewed in light of changes in circumstances, facts and experience. Changes in estimates are recorded in the period in which they become known. Actual results could differ from the Company’s estimates. The full extent to which the COVID-19 pandemic will directly or indirectly impact the Company’s business, results of operations and financial condition, including clinical trials and employee-related amounts, will depend on future developments that are highly uncertain, including new information that may emerge concerning COVID-19 and the actions taken to contain Cash Equivalents The Company considers all short-term, highly liquid investments with original maturities of 90 days or less at acquisition date to be cash equivalents. Restricted Cash The Company held restricted cash of $2,070 and $327 in separate restricted bank accounts as security deposits for leases of the Company’s facilities as of December 31, 2021 and December 31, 2020, respectively. The Company has included restricted cash of $2,070 and $327 as a non-current asset as of December 31, 2021 and December 31, 2020, respectively. A reconciliation of the amounts of cash and cash equivalents and restricted cash from the cash flow statement to the balance sheet is as follows: December 31, 2021 December 31, 2020 December 31, 2019 December 31, 2018 Cash and cash equivalents $ 119,136 $ 154,744 $ 76,043 $ 86,366 Restricted cash, non-current 2,070 327 327 327 Cash and cash equivalents and restricted cash $ 121,206 $ 155,071 $ 76,370 $ 86,693 Available-for-Sale Securities Available-for-sale securities consist of investments with original maturities greater than 90 days at acquisition date. The Company has classified its investments with maturities beyond one year as short term, based on their highly liquid nature and because such available-for-sale securities represent the investment of cash that is available for current operations. The Company classifies all of its investments as available-for-sale securities. The Company’s investments are measured and reported at fair value using quoted prices in active markets for similar securities. Unrealized gains and losses on available-for-sale debt securities are reported as a separate component of stockholders’ equity. The cost of debt securities sold is determined on a specific identification basis, and realized gains and losses are included in other income (expense) within the consolidated statement of operations. If any adjustment to fair value reflects a decline in the value of the investment that the Company considers to be “other than temporary,” the Company reduces the investment to fair value through a charge to the consolidated statement of operations. No such adjustments were necessary during the periods presented. Concentration of Credit Risk and of Significant Suppliers Financial instruments that potentially expose the Company to concentrations of credit risk consist primarily of cash, cash equivalents and available-for-sale securities. Periodically, the Company maintains deposits in accredited financial institutions in excess of federally insured limits. The Company maintains each of its cash, cash equivalents and available-for-sale securities balances with high-quality and accredited financial institutions and accordingly, such funds are not exposed to significant credit risk. The Company does not believe that it is subject to unusual credit risk beyond the normal credit risk associated with commercial banking relationships. The Company is dependent on third-party manufacturers to supply products for research and development activities of its programs, including clinical and preclinical testing. These programs could be adversely affected by a significant interruption in the supply of such drug substance products. Fair Value Measurements Certain 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 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. The Company’s cash equivalents and available-for-sale securities are carried at fair value, determined according to the fair value hierarchy described above. See Note 4 Fair Value of Financial Assets and Liabilities , for additional information. The carrying values of the Company’s accounts payable and accrued expenses and other current liabilities approximate their fair value due to the short-term nature of these liabilities. Leases At inception of a contract, the Company determines if a contract meets the definition of a lease. A lease is a contract, or part of a contract, that conveys the right to control the use of identified property, plant, or equipment (an identified asset) for a period of time in exchange for consideration. The Company determines if the contract conveys the right to control the use of an identified asset for a period of time. The Company assesses throughout the period of use whether the Company has both of the following: (1) the right to obtain substantially all of the economic benefits from use of the identified asset and (2) the right to direct the use of the identified asset. This determination is reassessed if the terms of the contract are changed. Leases are classified as operating or finance leases based on the terms of the lease agreement and certain characteristics of the identified asset. Right-of-use assets and lease liabilities are recognized at the lease commencement date based on the present value of the minimum future lease payments. The Company’s policy is to not record leases with an original term of twelve months or less on the consolidated balance sheets. The Company recognizes lease expense for these short-term leases on a straight-line basis over the lease term. Certain lease agreements include rental payments that are adjusted periodically for inflation or other variables. In addition to rent, the leases may require the Company to pay additional amounts for taxes, insurance, maintenance and other expenses, which are generally referred to as non-lease components. Such adjustments to rental payments and variable non-lease components are treated as variable lease payments and recognized in the period in which the obligation for these payments was incurred. Variable lease components and variable non-lease components are not measured as part of the right of use asset and liability. Only when lease components and their associated non-lease components are fixed are they accounted for as a single lease component and recognized as part of a right of use asset and liability. Total contract consideration is allocated to the combined fixed lease and non-lease components. Property and Equipment Property and equipment are stated at cost less accumulated depreciation. Depreciation expense is recognized using the straight-line method over the useful life of the asset. Laboratory equipment is depreciated over five years. Computer equipment is depreciated over three years. Computer software is depreciated over two years. Furniture and office equipment are depreciated over five years. Leasehold improvements are amortized over the shorter of the lease term or the estimated useful life of the related asset. Expenditures for repairs and maintenance of assets are charged to expense as incurred. Upon retirement or sale, the cost and related accumulated depreciation of assets disposed of are removed from the accounts and any resulting gain or loss is included in loss from operations. Equipment under a finance lease is stated at fair value at the inception of the lease less accumulated depreciation and is depreciated over the remaining lease term or the estimated useful life of the equipment. Impairment of Long-Lived Assets Long-lived assets, comprised of property and equipment, to be held and used are tested for recoverability whenever events or changes in business circumstances indicate that the carrying amount of the assets may not be fully recoverable. Factors that the Company considers in deciding when to perform an impairment review include significant underperformance of the business in relation to expectations, significant negative industry or economic trends and significant changes or planned changes in the use of the assets. If an impairment review is performed to evaluate a long-lived asset for recoverability, the Company compares forecasts of undiscounted cash flows expected to result from the use and eventual disposition of the long-lived asset to its carrying value. An impairment loss would be recognized when estimated undiscounted future cash flows expected to result from the use of an asset are less than its carrying amount. The impairment loss would be based on the excess of the carrying value of the impaired asset over its fair value, determined based on discounted cash flows. To date, the Company has not recorded any impairment losses or disposals on long-lived assets. Revenue Recognition The Company recognizes revenue in accordance with ASC Topic 606, Revenue from Contracts with Customers (“ASC 606”). Under ASC 606, an entity recognizes revenue when its customer obtains control of promised goods or services, in an amount that reflects the consideration which the entity expects to receive in exchange for those goods or services. To determine the appropriate amount of revenue to be recognized for arrangements determined to be within the scope of ASC 606, the Company performs the following five steps: (i) identification of the contract(s) with the customer; (ii) identification of the promised goods or services in the contract and determination of whether the promised goods or services are performance obligations; (iii) measurement of the transaction price; (iv) allocation of the transaction price to the performance obligations; and (v) recognition of revenue when (or as) the Company satisfies each performance obligation. The Company only applies the five-step model to contracts when it is probable that the entity will collect consideration it is entitled to in exchange for the goods or services it transfers to the customer. At contract inception, once the contract is determined to be within the scope of ASC 606, the Company assesses the goods or services promised within each contract and determines those that are performance obligations and assesses whether each promised good or service is distinct. The Company then recognizes as revenue the amount of the transaction price that is allocated to the respective performance obligation when (or as) the performance obligation is satisfied. As part of the accounting for these arrangements, the Company must use significant judgment to determine: a) the number of performance obligations based on the determination under step (ii) above and whether those performance obligations are distinct from other performance obligations in the contract; b) the transaction price under step (iii) above; and c) the standalone selling price for each performance obligation identified in the contract for the allocation of transaction price in step (iv) above. The Company uses judgment to determine whether milestones or other variable consideration, except for royalties, should be included in the transaction price as described further below. The transaction price is allocated to each performance obligation on a relative stand-alone selling price basis, for which the Company recognizes revenue as or when the performance obligations under the contract are satisfied. In determining the stand-alone selling price of a license to the Company’s proprietary technology or a material right provided by a customer option, the Company considers market conditions as well as entity-specific factors, including those factors contemplated in negotiating the agreements as well as internally developed estimates that include assumptions related to the market opportunity, estimated development costs, probability of success and the time needed to commercialize a product candidate pursuant to the license. In validating its estimated stand-alone selling prices, the Company evaluates whether changes in the key assumptions used to determine its estimated stand-alone selling prices will have a significant effect on the allocation of arrangement consideration between performance obligations. The Company estimates the transaction price based on the amount of consideration the Company expects to be received for transferring the promised goods or services in the contract. The consideration may include both fixed consideration and variable consideration. At the inception of each arrangement that includes variable consideration, the Company evaluates the amount of the potential payments and the likelihood that the payments will be received. The Company utilizes either the most likely amount method or expected value method to estimate the transaction price based on which method better predicts the amount of consideration expected to be received. If it is probable that a significant reversal in the amount of cumulative revenue recognized would not occur, the variable consideration is included in the transaction price. Performance obligations are promised goods or services in a contract to transfer a distinct good or service to the customer. Promised goods or services are considered distinct when: (i) the customer 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, the Company considers factors such as the stage of development of the underlying intellectual property, the capabilities of the customer to develop the intellectual property on their own and whether the required expertise is readily available. For performance obligations which consist of licenses and other promises, the Company utilizes judgment to assess the nature of the combined performance obligation in order to determine whether the combined performance obligation is satisfied over time or at a point in time. Up-front payments and fees are recorded as deferred revenue upon receipt or when due until the Company performs its obligations under these arrangements. Amounts expected to be recognized as revenue within the 12 months following the balance sheet date are classified as current portion of deferred revenue in the accompanying consolidated balance sheets. Amounts not expected to be recognized as revenue within the 12 months following the balance sheet date are classified as deferred revenue, net of current portion. Amounts are recorded as accounts receivable when the Company’s right to consideration is unconditional. Amounts recognized as revenue, but not yet received or invoiced are generally recognized as contract assets. Exclusive Licenses If the license granted in the arrangement is determined to be distinct from the other promises or performance obligations identified in the arrangement, which generally include research and development services, the Company recognizes revenue from non-refundable, upfront fees allocated to the license when the license is transferred to the customer and the customer is able to use and benefit from the license In assessing whether a license is distinct from the other promises, the Company considers relevant facts and circumstances of each arrangement, including the research and development capabilities of the collaboration partner and the availability of the associated expertise in the general marketplace. In addition, the Company considers whether the collaboration partner can benefit from the license for its intended purpose without the receipt of the remaining promise, whether the value of the license is dependent on the unsatisfied promise, whether there are other vendors that could provide the remaining promise, and whether it is separately identifiable from the remaining promise. For licenses that are combined with other promises, the Company utilizes 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 for purposes of recognizing revenue. The Company evaluates the measure of progress each reporting period and, if necessary, adjusts the measure of performance and related revenue recognition. The measure of progress, and thereby periods over which revenue should be recognized, are subject to estimates by management and may change over the course of the arrangement. Research and Development Services The promises under the Company’s collaboration and license agreements generally include research and development services to be performed by the Company on behalf of the collaboration partner. For performance obligations that include research and development services, the Company generally recognizes revenue allocated to such performance obligations based on an appropriate measure of progress. The Company utilizes judgment to determine the appropriate method of measuring progress for purposes of recognizing revenue, which is generally an input measure, such as costs incurred. Milestone Payments At the inception of each arrangement that includes milestone payments based on certain events, the Company evaluates whether the milestones are considered probable of being achieved and estimates the amount to be included in the transaction price using the most likely amount method. If it is probable that a significant reversal in the amount of cumulative revenue recognized would not occur, the associated milestone value is included in the transaction price. Milestone payments that are not within the control of the Company, such as regulatory approvals, are not considered probable of being achieved until those approvals are received. The Company evaluates factors such as the scientific, clinical, regulatory, commercial, and other risks that must be overcome to achieve the particular milestone in making this assessment. There is considerable judgment involved in determining whether it is probable that a significant reversal in the amount of cumulative revenue recognized would not occur. At the end of each subsequent reporting period, the Company reevaluates the probability of achievement of all milestones subject to constraint and, if necessary, adjusts its estimate of the overall transaction price. Any such adjustments are recorded on a cumulative catch-up basis, which would affect revenues and earnings in the period of adjustment. If a milestone or other variable consideration relates specifically to the Company’s efforts to satisfy a single performance obligation or to a specific outcome from satisfying the performance obligation, the Company generally allocates the milestone amount entirely to that performance obligation once it is probable that a significant reversal in the amount of cumulative revenue recognized would not occur. Royalties For arrangements that include sales-based royalties, including milestone payments based on a level of sales, and the license is deemed to be the predominant item to which the royalties relate, the Company recognizes 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, the Company has not recognized any royalty revenue resulting from any of its licensing arrangements. Collaboration Revenue The Company analyzes its collaboration arrangements to assess whether they are within the scope of ASC 808, Collaborative Arrangements Costs Associated with License and Collaborative Arrangements All costs associated with license and collaborative arrangements are expensed as incurred and recorded in research and development expense in the consolidated statements of operations. Research and Development Costs Research and development costs are expensed as incurred. Research and development expenses include salaries, equity-based compensation and benefits of employees, third-party license fees and other operational costs related to the Company’s research and development activities, including allocated facility-related expenses and external costs of outside vendors engaged to conduct both preclinical studies and clinical trials. Non-refundable pre-payments for goods or services that will be used or rendered for future research and development activities are deferred and capitalized. Such amounts are recognized as expense as the goods or services are delivered or the related services are performed, or until it is no longer expected that the goods will be delivered or the services rendered. The Company may in-license the rights to develop and commercialize product candidates. For each in-license transaction the Company evaluates whether it has acquired processes or activities along with inputs that would be sufficient to constitute a “business” as defined under GAAP. A “business” as defined under GAAP consists of inputs and processes applied to those inputs that have the ability to create outputs. Although businesses usually have outputs, outputs are not required for an integrated set of activities to qualify as a business. When the Company determines that it has not acquired sufficient processes or activities to constitute a business, any up-front payments, as well as milestone payments, are immediately expensed as acquired research and development in the period in which they are incurred. Research Contract Costs and Accruals The Company has entered into various research and development contracts with research institutions and other companies. These agreements are generally cancelable, and related payments are recorded as research and development expenses as incurred. The Company records accruals for estimated ongoing research costs. When evaluating the adequacy of the accrued liabilities, the Company analyzes progress of the studies, 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 the Company’s estimates. The Company’s historical accrual estimates have not been materially different from the actual costs. Patent Costs All patent-related costs incurred for 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. Equity-Based Compensation The Company measures all stock options and other stock-based awards granted to employees, directors and non-employees based on the fair value on the date of the grant and recognizes compensation expense of those awards, over the requisite service period, which is generally the vesting period of the respective award. Forfeitures are accounted for as they occur. The Company applies the straight-line method of expense recognition to all awards with only service-based vesting conditions. The Company has not issued any awards with performance-based vesting conditions. The fair value of each stock option grant is estimated on the date of grant using the Black-Scholes option-pricing model. The Company historically has been a private company and lacks company-specific historical and implied volatility information. Therefore, it estimates its expected stock volatility based on the historical volatility of a publicly traded set of peer companies and expects to continue to do so until such time as it has adequate historical data regarding the volatility of its own traded stock price. For options with service-based vesting conditions, the expected term of the Company’s stock options has been determined utilizing the “simplified” method for awards that qualify as “plain-vanilla” options. Through December 31, 2018, the expected term of stock options granted to non-employees is equal to the contractual term of the option award and effective January 1, 2019, the “simplified” method is used. The risk-free interest rate is 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 is based on the fact that the Company has never paid cash dividends and does not expect to pay any cash dividends in the foreseeable future. The fair value for restricted stock units, “RSU”, was calculated using the closing price of the Company’s common stock on the date of grant. The Company classifies stock-based compensation expense in its consolidated statement of operations in the same Income Taxes Income taxes are accounted for under the asset and liability method. Under the asset and liability method, deferred tax assets and liabilities are recognized for the future tax consequences attributable to differences between the financial statement carrying amounts of existing assets and liabilities and their respective tax bases. Deferred tax assets and liabilities are measured using enacted tax rates expected to apply to taxable income in the years in which those temporary differences are expected to be recovered or settled. The effect on deferred tax assets and liabilities of a change in tax rates is recognized in income in the period that includes the enactment date. The Company records valuation allowances to reduce deferred income tax assets to the amount that is more likely than not to be realized. The Company determines whether it is more likely than not that a tax position will be sustained upon examination. If it is not more likely than not that a position will be sustained, no amount of benefit attributable to the position is recognized. The tax benefit to be recognized of any tax position that meets the more likely than not recognition threshold is calculated as the largest amount that is more than 50% likely of being realized upon resolution of the contingency. Prior to January 25, 2018, the Company had not been subject to U.S. federal income taxes as the Company was organized as a limited liability company. As such, the taxable income or loss was passed through to and included in the tax returns of the members. Since January 25, 2018, the Company’s income has since been subject to U.S. federal, state, local, and foreign income taxes and taxed at the prevailing corporate tax rates. Segment Data The Company manages its operations as a single segment for the purposes of assessing performance and making operating decisions. The Company’s singular focus is on developing treatments through gene therapy and other means for patients with Duchenne. All of the Company’s tangible assets are held in the United States. 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 members. The Company’s only element of other comprehensive income (loss) in all periods presented was unrealized gains (losses) from available-for-sale securities. Net Loss per Share The Company follows the two-class method when computing net loss per share, as the Company has issued shares that meet the definition of participating securities. The two-class method determines net loss per share for each class of common and participating securities according to dividends declared or accumulated and participation rights in undistributed earnings. The two-class method requires income available to common stockholders for the period to be allocated between common and participating securities based upon their respective rights to receive dividends as if all income for the period had been distributed. Basic net loss per share is computed by dividing the net loss by the weighted average number of shares of common stock and pre-funded warrants outstanding for the period. Diluted net loss is computed by adjusting net loss to reallocate undistributed earnings based on the potential impact of dilutive securities. Diluted net loss per share is computed by dividing the diluted net loss by the weighted average number of shares of common stock and pre-funded warrants outstanding for the period, including potential dilutive shares of common stock assuming the dilutive effect of common stock equivalents. The Company’s preferred stock could entitle the holders of such shares to participate in dividends and not require the holders of such shares to participate in losses of the Company. Accordingly, in periods in which the Company reports a net loss, such losses are not allocated to such participating securities. In periods in which the Company reports a net loss, diluted net loss per share is the same as basic net loss per share, since dilutive shares of common stock are not assumed to have been issued if their effect is anti-dilutive. As of the year ended December 31, 2021 and 2020, there were no preferred stock issued or outstanding with any contractual rights. Funding from Charitable Organizations The Company has received funding from charitable organizations to perform research and development services to identify therapies for people with Duchenne. The amounts received are recognized as services are performed and research expenses are incurred. These are included in other income in the consolidated statements of operations as the arrangement between the Company and the charitable organizations are not part of the Company’s on-going, major or central operations. Any amount received in advance of services performed is recorded in other current liabilities in the consolidated balance sheets if the services are expected to be performed within the next twelve months. The Company recognized other income of $0, $1 and $515 for the years ended December 31, 2021, 2020 and 2019, respectively, which is included in the consolidated statements of operations. Contingencies Loss contingency provisions are recorded if the potential loss from any claim, asserted or unasserted, or legal proceeding, is considered probable and the amount can be reasonably estimated, or a range of loss can be determined. These accruals represent the Company’s best estimate of probable loss. Disclosure also is provided when it is reasonably possible that a loss will be incurred or when it is reasonably possible that the amount of a loss will exceed the recorded provision. The Company reviews the status of each significant matter and assesses its potential financial exposure. Significant judgment is required in both the determination of probability and the determination as to whether an exposure is reasonably estimable. Because of uncertainties related to

Collaborations

Collaborations	12 Months Ended
	Dec. 31, 2021
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Collaborations	3. Collaborations Ultragenyx Collaboration Agreement Collaboration Agreement On October 22, 2020 (the “Effective Date”), the Company entered into the Collaboration Agreement with Ultragenyx to focus on the development and commercialization of new gene therapies for Duchenne Muscular Dystrophy. The Company granted Ultragenyx an exclusive worldwide license for any pharmaceutical product that expresses the Company’s proprietary microdystrophin construct from AAV8 and variants thereof in clade E for the treatment of Duchenne Muscular Dystrophy and other diseases resulting from the lack of functional dystrophin (the “Licensed Products”). The Company retains exclusive rights to all other uses of its microdystrophin proteins, including under its existing SGT-001 program. The Company is conducting certain research and development activities with respect to the development of the Licensed Products. Ultragenyx will reimburse the Company for personnel and out-of-pocket costs that the Company incurs in conducting such development activities. In addition, Ultragenyx granted to the Company an exclusive Development Option or Income Share Option (each as defined and described below) exercisable in the Company’s sole discretion one time per Licensed Product. After the date of first achievement of clinical proof of concept, Ultragenyx will provide to the Company a data package with respect to the relevant Licensed Product. The Company will use the data package to determine whether to exercise the corresponding Development Option or Income Share Option with respect to such Licensed Product. With respect to each Licensed Product for which the Company has not exercised the Development Option or Income Share Option the Company will be entitled to milestone payments of up to $25,000 in the aggregate for each such Licensed Product that achieves specified development milestones and $65,000 in the aggregate for each such Licensed Product that achieves specified regulatory milestones. With respect to each Licensed Product for which the Company has not exercised the Income Share Option, the Company will also be entitled to milestone payments of up to $165,000 in the aggregate for each Licensed Product that archives specified annual worldwide net sales milestones. For Licensed Products for which the Company has not exercised the Development Option or Income Share Option, Ultragenyx will pay the Company tiered royalties on a Licensed Product-by-Licensed Product and country-by-country basis ranging from a low double digit percentage to a mid-teens percentage based on Ultragenyx’s annual worldwide net sales of such Licensed Products. For each Licensed Product for which Ultragenyx decides to initiate a registrational trial in humans, the Company will have the option to fund 30% of the development costs in the United States and European Union for such Licensed Product and forgo the development and regulatory milestones (the “Development Option”) and receive tiered royalties on a Licensed Product-by-Licensed Product and country-by-country basis ranging from a mid-teens percentage to a low twenties percentage based on Ultragenyx’s annual worldwide net sales of each such Licensed Product. For each Licensed Product for which the Company exercises the Development Option, the Company may also elect to share 30% of the net income and net losses on net sales of such Licensed Product in the United States and European Union (the “Income Share Option”). For Licensed Products for which the Company has exercised the Income Share Option, the Company will not be entitled to milestone payments and Ultragenyx will pay the Company tiered royalties on a Licensed Product-by-Licensed Product and country-by-country basis ranging from a mid-teens percentage to a low twenties percentage based on Ultragenyx’s annual net sales of each such Licensed Product outside of the United States and European Union. The Company may only exercise an Income Share Option if neither the Company nor any of its affiliates is then developing or commercializing a product that is competitive with the Licensed Product that is subject to such option. If the Company or any of its affiliates subsequently develops or commercializes a product that is competitive with a Licensed Product for which the Company has exercised an Income Share Option, then the Company and Ultragenyx will no longer share the net income and net losses on net sales of such Licensed Product and such Licensed Product will be treated as if the Company had exercised the Development Option with respect to such Licensed Product. Following the Company’s exercise of the Development Option or Income Share Option with respect to a Licensed Product, the Company also has the right to cease participation in the sharing of development costs and sharing in net income and net losses on net sales, as applicable, for such Licensed Product by written notice to Ultragenyx. Upon such notice, the Company will no longer share in the development costs and net income and net losses on net sales of such Licensed Product, as applicable, and will be eligible to receive payments on milestones achieved after the opt-out for such Licensed Product and royalties at the rates applicable to Licensed Products for which the Company has not exercised the Development Option or Income Share Option, as described above. The Collaboration Agreement continues on a country-by-country and Licensed Product-by-Licensed Product basis until the expiration of all payment obligations under the agreement. With respect to any Licensed Product for which the Company has exercised an Income Share Option, the Collaboration Agreement continues until there are no longer sales of such Licensed Product in the United States or Europe. Either party has the right to terminate the agreement if the other party has materially breached in the performance of its obligations under the agreement and such breach has not been cured within the applicable cure period. Ultragenyx may also terminate the Collaboration Agreement in its sole discretion upon 90 days’ prior written notice to the Company. Stock Purchase Agreement In connection with the execution of the Collaboration Agreement, Ultragenyx and the Company also entered into a stock purchase agreement (the “Stock Purchase Agreement”) on October 22, 2020 (the “Closing Date”), pursuant to which the Company issued and sold 7,825,797 shares of its common stock (the “Shares”) to Ultragenyx at a price of $5.1113 per share for an aggregate purchase price of approximately $40,000. The Stock Purchase Agreement contains customary representations, warranties and covenants of each of the parties thereto. Following the sale of the Shares, Ultragenyx beneficially owned approximately 14.45% of the Company’s outstanding common stock. As of December 31, 2021, Ultragenyx beneficially owned approximately 7.1% Investor Agreement In connection with the consummation of the transactions contemplated by the Stock Purchase Agreement, the Company and Ultragenyx entered into an Investor Agreement (the “Investor Agreement”) on the Effective Date. Pursuant to the terms of the Investor Agreement, Ultragenyx agreed that the Shares will be subject to a lock-up restriction, such that Ultragenyx will not, and will also cause its affiliates not to, without the prior approval of the Company and with certain exceptions, sell, transfer or otherwise dispose of the Shares until the earliest to occur of (i) 18 months after the Effective Date, (ii) the termination of the Collaboration Agreement or (iii) other specified events. Pursuant to the terms of the Investor Agreement, Ultragenyx agreed that, so long as it holds at least 10% of the Company’s outstanding common stock, the Shares will be subject to a voting agreement, such that until the earliest to occur of certain specified events, and subject to specified conditions, Ultragenyx will, and will cause its permitted transferees to, vote in accordance with the recommendation of the Company’s Board of Directors with respect to specified matters. Accounting Treatment The Company concluded that the Collaboration Agreement and the Stock Purchase Agreement should be combined and treated as a single arrangement for accounting purposes as the agreements were entered into contemporaneously and in contemplation of one another. The Company assessed this arrangement in accordance with ASC 606 and concluded that the contract counterparty, Ultragenyx, is a customer. The Company identified the following promises in the Collaboration Agreement that were evaluated under the scope of ASC 606: (1) an exclusive worldwide license to the Licensed Products; (2) an obligation to perform research and development services; and (iii) an obligation to participate in a joint steering committee. The Company assessed the promised goods and services to determine if they are distinct. Based on this assessment, the Company determined that Ultragenyx cannot benefit from the promised goods and services separately from the others as they are highly interrelated and therefore not distinct. Due to the early stage of the Licensed Products, the research and development services could not be performed by another party. The Company’s skill-set, knowledge and expertise are required to conduct the research and development services and the research and development services are expected to involve significant further development of the Licensed Products. Accordingly, the promised goods and services represent one combined performance obligation and the entire transaction price will be allocated to that single combined performance obligation. The Company determined the transaction price under ASC 606 at the inception of the Collaboration Agreement to be $22,513, which represents the excess proceeds from the equity investment under the Stock Purchase Agreement, when measured at fair value after taking into consideration a discount for lack of marketability, plus the estimated reimbursement of research and development costs, which represents variable consideration. The Company included the estimated reimbursement of research and development costs in the transaction price at the inception of the arrangement because the Company is required to perform research and development services and the contract requires Ultragenyx to reimburse the Company for costs incurred. Also, since the related revenue would be recognized only as the costs are incurred, the Company determined it is not probable that a significant reversal of cumulative revenue would occur. The Company evaluated how much variable consideration related to development and regulatory milestones, and the Company’s potential exercise of its Development Option or Income Share Option per Licensed Product, to include in the transaction price using the most likely amount approach and concluded that no amount should be included in the transaction price due to the high degree of uncertainty and risk associated with these potential payments. The Company also determined that royalties and sales milestones relate solely to the license of intellectual property and are therefore excluded from the transaction price under the sales- or usage-based royalty exception of ASC 606. Revenue related to these royalties and sales milestones will only be recognized when the associated sales occur, and relevant thresholds are met. The Company determined that revenue under the Collaboration Agreement should be recognized over time as Ultragenyx simultaneously receives the benefit from the Company as the Company performs under the single performance obligation over time. The Company will recognize revenue for the single performance obligation using a cost-to-cost input method as the Company has concluded it best depicts the research and development and joint steering committee participation services performed. Under this method, the transaction price is recognized over the contract’s entire performance period, using costs incurred relative to total estimated costs to determine the extent of progress towards completion. Ultragenyx is a related party since Ultragenyx is one of the Company’s significant stockholders. $13,620 and $0 has been recognized as related party revenue as the Company has performed services under the Collaboration Agreement for the year ended December 31, 2021 and December 31, 2020, respectively. Further, the Company has made no payments to Ultragenyx during the years ended December 31, 2021 and 2020. There is $110 and $0 due from Ultragenyx as of December 31, 2021 and December 31, 2020, respectively. The following table presents changes in the balances of the Company’s related party collaboration receivables and contract liabilities during the year ended December 31, 2021: Balance as of December 31, 2020 Additions Deductions Balance as of December 31, 2021 Related party collaboration receivable $ — $ 982 $ (872 ) $ 110 Contract liabilities: Deferred revenue 20,718 982 (13,620 ) 8,080

Fair Value of Financial Assets

Fair Value of Financial Assets and Liabilities	12 Months Ended
	Dec. 31, 2021
Fair Value Disclosures [Abstract]
Fair Value of Financial Assets and Liabilities	4. Fair Value of Financial Assets and Liabilities The following tables present information about the Company’s assets and liabilities that are measured at fair value on a recurring basis and indicate the level of the fair value hierarchy utilized to determine such fair values: Fair Value Measurements as of December 31, 2021: Level 1 Level 2 Level 3 Total Assets: Cash equivalents $ - $ 75,224 $ - $ 75,224 Available-for-sale securities - 88,643 - 88,643 $ - $ 163,867 $ - $ 163,867 Fair Value Measurements as of December 31, 2020: Level 1 Level 2 Level 3 Total Assets: Cash equivalents $ - $ 84,462 $ - $ 84,462 $ - $ 84,462 $ - $ 84,462 As of December 31, 2021 the fair values of the Company’s available-for-sale securities were determined using level two inputs. During the year ended December 31, 2021, there were no transfers between Level 1, Level 2 and Level 3. As of December 31, 2020 there were no available-for-sale securities. The fair value of the Company’s cash, restricted cash, accounts payable, accrued expenses and other current liabilities approximate their carrying value due to their short-term maturities.

Available-for-Sale Securities

Available-for-Sale Securities	12 Months Ended
	Dec. 31, 2021
Investments Debt And Equity Securities [Abstract]
Available-for-Sale Securities	5. Available-for-Sale Securities As of December 31, 2021 the fair value of available-for-sale debt securities by type of security was as follows: December 31, 2021 Amortized Cost Gross Unrealized Gain Gross Unrealized Loss Fair Value Investments: Treasury bill $ 2,800 $ - $ - $ 2,800 Corporate bond securities 83,889 - (45 ) 83,844 Commercial paper 1,999 - - 1,999 $ 88,688 $ $ (45 ) $ 88,643 As of December 31, 2020, there were no available-for-sale securities. The estimated fair value and amortized cost of the Company’s available-for-sale securities by contractual maturity are summarized as follows: December 31, 2021 Amortized Cost Fair Value Due in one year or less $ 88,688 $ 88,643 Total available-for-sale securities $ 88,688 $ 88,643 The average maturity of the Company’s available-for-sale securities as of December 31, 2021 was approximately 0.7 years.

Prepaid Expenses and Other Curr

Prepaid Expenses and Other Current Assets	12 Months Ended
	Dec. 31, 2021
Deferred Costs Capitalized Prepaid And Other Assets Disclosure [Abstract]
Prepaid Expenses and Other Current Assets	6. Prepaid Expenses and Other Current Assets Prepaid expenses and other current assets consist of the following: December 31, 2021 2020 Prepaid research and development expenses $ 6,015 $ 2,674 Prepaid expenses and other assets 8,708 1,483 $ 14,723 $ 4,157

Property and Equipment

Property and Equipment	12 Months Ended
	Dec. 31, 2021
Property Plant And Equipment [Abstract]
Property and Equipment	7. Property and Equipment Property and equipment consists of the following: December 31, 2021 2020 Furniture and fixtures $ 212 $ 203 Laboratory equipment 10,719 9,631 Leasehold improvements 4,713 4,713 Computer equipment 436 436 Computer software 553 553 Construction in process 1,490 1,330 18,123 16,866 Less accumulated depreciation 11,661 8,713 $ 6,462 $ 8,153 Depreciation expense was $2,964, $3,922 and $2,824 for the years ended December 31, 2021, 2020 and 2019, respectively.

Accrued Expenses

Accrued Expenses	12 Months Ended
	Dec. 31, 2021
Payables And Accruals [Abstract]
Accrued Expenses	8. Accrued Expenses Accrued expenses and other current liabilities consist of the following: December 31, 2021 2020 Accrued research and development $ 1,507 $ 2,091 Accrued compensation 3,084 3,834 Accrued other 4,937 2,715 $ 9,528 $ 8,640

Equity-Based Compensation

Equity-Based Compensation	12 Months Ended
	Dec. 31, 2021
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Equity-Based Compensation	9. Equity-Based Compensation Equity Incentive Plans In connection with the closing of the Company’s initial public offering, the Board of Directors and stockholders approved the 2018 Omnibus Incentive Plan (the “2018 Plan”), which provides for the reservation of 5,001,000 shares of common stock for equity awards. On June 16, 2020, the Company’s stockholders approved the 2020 Equity Incentive Plan (“2020 Plan”) which consists of (i) 3,000,000 shares of common stock and (ii) additional shares of common stock (up to 4,879,025) as is equal to (i) the number of shares reserved under the 2018 Plan that remain available for grant under the 2018 Plan as of immediately prior to the date the 2020 Plan was approved by the Company’s stockholders and (ii) the number of shares subject to awards granted under the 2018 Plan which awards expire, terminate or are otherwise surrendered, cancelled, forfeited or repurchased by the Company at their original issuance price pursuant to a contractual repurchase right. As of the effective date of the 2020 Plan, no further awards will be made under the 2018 Plan. Any options or awards outstanding under the 2018 Plan remain outstanding and effective and are governed by their existing terms. On June 16, 2021, the Company’s stockholders approved an amendment to the 2020 Plan to reserve an additional 7,000,000 shares of common stock for issuance under the plan. At December 31, 2021, 8,557,152 shares remained available for future issuance under the 2020 Plan. Under the 2020 Plan, stock options may not be granted at less than fair value on the date of grant . 2021 Employee Stock Purchase Plan The 2021 Employee Stock Purchase Plan (the “2021 ESPP”) was adopted by the Board of Directors on April 14, 2021, approved by the stockholders on June 16, 2021, and became effective on June 16, 2021. The number of shares of the Company’s common stock reserved for issuance under the 2021 ESPP is 1,102,885 shares. At December 31, 2021, 1,058,204 shares remained available for future issuance under the 2021 ESPP. Stock Options The following table summarizes the Company’s stock option activity for the year ended December 31, 2021: Number of Options Weighted Average Exercise Price Remaining Contractual Life (in years) Outstanding at December 31, 2020 3,117,775 $ 14.58 7.78 Granted 3,708,345 5.92 Exercised (11,625 ) 3.55 Forfeitures (801,859 ) 14.85 Outstanding at December 31, 2021 6,012,636 9.23 8.49 Vested and expected to vest as of December 31, 2021 6,012,636 $ 9.23 8.49 Exercisable at December 31, 2021 1,939,881 $ 13.50 7.63 At December 31, 2021, the Company had an aggregate of $15,606 of unrecognized equity-based compensation cost related to stock options outstanding which is expected to be recognized over a weighted average period of 2.59 years. The intrinsic value of stock options outstanding as of December 31, 2021 and 2020 was $0 and $910, respectively. The intrinsic value of stock options exercisable as of December 31, 2021 was $0. The intrinsic value of stock options exercised during the year ended December 31, 2021 was $41. The fair value of each option award is estimated on the date of grant using the Black-Scholes option-pricing model using the assumptions noted in the following table for the years ended December 31: 2021 2020 2019 Expected volatility 115.5% - 123.9% 21.3% - 119.5% 85.1% - 104.7% Expected dividends 0.0% 0.0% 0.0% Expected term (in years) 5.1 - 6.25 4.50 - 10.00 5.10 - 6.25 Risk-free rate 0.4% - 1.4% 0.6% - 3.4% 1.5% - 2.6% The weighted average fair value of options to purchase shares of common stock granted during the year ended December 31, 2021 2020 $5.05 and $2.69, respectively. Restricted Stock Units In 2021 and 2020, the Board of Directors issued restricted stock units to employees. Restricted stock unit grants typically vest over one or two years. The following table summarizes the Company’s restricted stock unit activity for the year ended December 31, 2021 : Units Weighted- Average Grant Date Fair Value Unvested at December 31, 2020 918,395 $ 3.17 Granted 220,750 2.25 Vested (419,193 ) 3.21 Forfeitures (210,258 ) 3.13 Outstanding at December 31, 2021 509,694 $ 2.75 Unvested as of December 31, 2021 509,694 $ 2.75 At December 31, 2021 Restricted Common Stock In connection with the Company’s Corporate Conversion on January 25, 2018, all restricted Series B and D common units were converted to restricted shares of common stock. The following table summarizes the Company’s unvested restricted shares of common stock activity for the year ended December 31, 2021: Units Weighted- Average Grant Date Fair Value Unvested restricted Common Units at December 31, 2020 70,052 $ 9.83 Releases (40,840 ) 10.50 Forfeitures (29,212 ) 8.89 Unvested restricted Common Units at December 31, 2021 - $ - The aggregate intrinsic value of restricted common units that vested during the years ended December 31, 2021 At December 31, 2021 The Company recorded equity-based compensation expense in the following captions within its consolidated statements of operations for the years ended December 31, 2021 For the Year Ended December 31, 2021 2020 2019 Research and development expenses $ 6,289 $ 5,822 $ 8,006 General and administrative expenses 7,084 4,956 6,201 Restructuring expenses - 851 - $ 13,373 $ 11,629 $ 14,207

Leases

Leases	12 Months Ended
	Dec. 31, 2021
Leases [Abstract]
Leases	10. Leases In January 2018, the Company executed a lease agreement for lab space in Cambridge, Massachusetts. The lease consists of approximately 9,500 square feet with an initial term of five years with the option to extend the term for one additional two year term. The future minimum rent commitment for the initial five-year term is approximately $1,900. The future minimum rent commitment for the lease term as of December 31, 2021 is approximately $1,090. In addition to rent, the lease requires the Company to pay additional amounts for taxes, insurance, maintenance and other operating expenses. In January 2018, the Company executed a lease agreement for office space in Cambridge, Massachusetts. The space serves as the Company’s corporate headquarters and consists of approximately 16,000 square feet. The initial term of the lease runs through February 2022 and was extended through May 2022. The future minimum rent commitment for the lease term as of December 31, 2021 is approximately $544. In addition to rent, the lease requires the Company to pay additional amounts for taxes, insurance, maintenance and other operating expenses. In November 2018, the Company entered into a 48-month capital lease for certain lab equipment to be used at its facility in Cambridge, Massachusetts. The future minimum lease commitment for the lease term as of December 31, 2021 is approximately $576. In January 2019, the Company executed a lease agreement for additional office space in Cambridge, Massachusetts. The space serves as office space supporting the Company’s lab operations and consists of approximately 5,000 square feet. The term of the lease runs through October 2025. In June 2021, the Company determined that it no longer needed a floor of office space that it was renting and terminated its lease with another landlord. As a result of the termination, the Company wrote off $1,314 and $1,233 of the remaining lease liability and right-of-use asset, respectively, associated with the lease. I n June 2021, the Company entered into a lease with Hood Park LLC (“Landlord”), pursuant to which the Company will lease approximately 49,869 square feet of office, laboratory, research and development and manufacturing space located in Charlestown, Massachusetts (“Premises”). The Company intends to relocate its corporate headquarters to the Premises in May 2022. The term of the lease commences on the later of (i) the date the Landlord delivers the Premises to the Company or (ii) the earlier of (a) the date the Company’s work on the Premises is substantially completed, (b) the date the Company commences business operations in the Premises, or (c) the one hundred twentieth (120th) day following the Landlord’s satisfaction of item (i) above. The initial term of the lease will be for a ten-year The lease provides the Company with an option to extend the lease for an additional five-year The Company was required to post a customary letter of credit in the amount of $1,833, subject to decrease on a set schedule, as a security deposit pursuant to the lease. For the year ended December 31, 2021, the Company incurred approximately $5,833 of construction costs that are reimbursable by the landlord. As of December 31, 2021 Finance Leases Operating Leases 2022 $ 276 $ 1,355 2023 300 279 2024 — — 2025 — — Thereafter — — Total 576 1,634 Less: Imputed Interest 51 96 Total Lease Liabilities $ 525 $ 1,538 The Company recorded rent expense of $2,568, $2,562 and $2,500 for the years ended December 31, 2021, 2020, and 2019 Short-term lease and variable lease costs were not material for the year ended December 31, 2021 and 2020 The supplemental disclosure of cash flow information related to the Company’s leases and the weighted average remaining lease term and weighted average discount rate of the Company’s leases are as follows: For the Year Ended December 31, For the Year Ended December 31, For the Year Ended December 31, 2021 2020 2019 Other information Cash paid for amounts included in the measurement of lease liabilities $ 2,408 $ 2,408 $ 2,288 Operating lease liabilities arising from obtaining right-of-use-assets $ 310 $ 1,629 $ 1,629 Finance lease liabilities arising from obtaining right-of-use assets $ — $ — $ — Weighted-average remaining lease term (in years) Operating lease 1.0 2.8 3.5 Finance lease 2.3 2.3 3.5 Weighted-average discount rate Operating lease 11.9 % 12.6 % 12.5 % Finance lease 10.7 % 10.7 % 10.7 %

Commitments and Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2021
Commitments And Contingencies Disclosure [Abstract]
Commitments and Contingencies	11. Commitments and Contingencies Letter of Credit The Company had outstanding letters of credit in the amounts of $2,070 and $327 at December 31, 2021 Indemnification Agreements In the ordinary course of business, the Company may provide indemnification of varying scope and terms to vendors, lessors, 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, the Company has entered into indemnification agreements with its executive officers and members of its Board of Directors that require the Company, among other things, to indemnify them against certain liabilities that may arise by reason of their status or service as executive officers or directors. The maximum potential amount of future payments the Company could be required to make under these indemnification agreements is, in many cases, unlimited. To date, the Company has not incurred any material costs as a result of such indemnification arrangements. The Company does not believe that the outcome of any claims under indemnification arrangements will have a material effect on its financial position, results of operations or cash flows, and it has not accrued any liabilities related to such obligations in its consolidated financial statements as of December 31, 2021 and 2020. Legal Proceedings The Company may periodically become subject to legal proceedings and claims arising in connection with ongoing business activities, including claims or disputes related to patents that have been issued or that are pending in the field of research on which the Company is focused. The Company is not aware of any material legal proceedings or claims as of December 31, 2021.

License Agreements

License Agreements	12 Months Ended
	Dec. 31, 2021
Organization Consolidation And Presentation Of Financial Statements [Abstract]
License Agreements	12. License Agreements University of Washington License Agreement In 2015, the Company entered into a license agreement with the University of Washington, acting through UW CoMotion, under which the Company obtained an exclusive, royalty-bearing, sublicensable, worldwide license under a patent application owned by the University of Washington relating to novel micro-dystrophins and all patents claiming priority to such patent to develop, manufacture, and commercialize products for use in the treatment of Duchenne and related disease indications caused by a lack of functional dystrophin. The Company has the right to grant sublicenses to third parties contingent upon written approval by the University of Washington prior to executing such sublicense, which approval may not be unreasonably withheld. In consideration for the rights granted by the agreement, the Company paid a one-time, non-refundable license fee, which was recorded as a research and development expense in 2015. The Company is required to reimburse the University of Washington for costs incurred in applying for, prosecuting and maintaining patents and pay up to an aggregate of approximately $1,000 upon the achievement of certain milestones. In October 2017, the first milestone was achieved under this agreement. The milestone payment was recorded as a research and development expense in the fourth quarter of 2017. In October 2020, the license agreement was amended such that the Company was required to pay the University of Washington $375 in connection with the execution of the Collaboration Agreement. This payment was recorded as a research and development expense in the fourth quarter of 2020. The license agreement was also amended such that the Company is required to pay an aggregate of approximately $3,400 upon the achievement of certain milestones. There were no milestones achieved during the years ended December 31, 2021 The license agreement remains in effect until the expiration of the last-to-expire patent licensed under the agreement. The Company may terminate the agreement at any time upon providing sixty days’ written notice to the University of Washington. The University of Washington may terminate the agreement upon the Company’s uncured, material breach of the agreement or if the Company enters into an insolvency-related event. The Company recorded research and development expense in the amount of $60, $446, and $38 for the years ended December 31, 2021 The University of Missouri License Agreement In 2015, the Company entered into a license agreement with the Curators of the University of Missouri (the “University of Missouri”), a public corporation of Missouri, under which the Company obtained an exclusive, royalty-bearing, sublicensable, worldwide license under certain patent and patent applications owned by the University of Missouri relating to a novel synthetic microdystrophin gene to make, sell and distribute products for use in the treatment of Duchene and related disease indications resulting from a lack of functional dystrophin. In consideration for the rights granted by the agreement, the Company paid a one-time, non-refundable license fee, which was recorded as a research and development expense in 2015. The Company is required to reimburse the University of Missouri for costs incurred in applying for, prosecuting and maintaining the licensed patents and pay up to an aggregate of approximately $1,000 upon the achievement of certain milestones for each product developed based on the licensed patents. In October 2017, the first milestone was achieved under this agreement. The milestone payment was recorded as a research and development expense in the fourth quarter of 2017. Under the agreement, in the event the Company grants a sublicenses to another party, the Company is required to pay the University of Missouri a percentage of the consideration received. The license agreement was amended such that the Company was required to pay the University of Missouri $750 in 2021 and $1,300 in 2022 as a result of the execution of the Collaboration Agreement with Ultragenyx in October 2020. These amounts were recorded as a research and development expense in the fourth quarter of 2020. The Company paid $750 in February 2021 and $1,300 in February 2022. The license agreement was also amended such that the Company is required to pay an aggregate of approximately $1,900 upon the achievement of certain milestones. There were no milestones achieved during the years ended December 31, 2021 Under the agreement, the Company granted the University of Missouri a non-exclusive, royalty-free, irrevocable, paid-up license, with the right to grant sublicenses to non-profit, academic, educational or governmental institutions, to practice and use improvements made by the Company using the licensed patent rights, solely for non-commercial research purposes. The license agreement remains in effect until the expiration of the last-to-expire patent or the abandonment of the last to be abandoned patent application licensed under the agreement. The University of Missouri may terminate the agreement, or render the license granted thereunder non-exclusive, in individual countries if the Company’s sublicensees fail to achieve certain milestones. The Company may terminate the license agreement at any time upon providing six months’ written notice to the University of Missouri and paying a termination fee. Each of the University of Missouri and the Company may also terminate the agreement for an uncured default or breach of the agreement by the other party. The Company’s ability to cure such breach only applies to the first two notices of such breach provided by the University of Missouri, and thereafter, the University of Missouri may terminate the agreement for the Company’s default or breach of the agreement upon thirty days’ written notice without an opportunity to cure such default or breach. The Company recorded research and development expense in the amount of $195, $2,111, and $23 for the years ended December 31, 2021 The University of Michigan License Agreement In 2016, the Company entered into a license agreement with the Regents of the University of Michigan, (the “University of Michigan”), a constitutional corporation of Michigan, under which the Company obtained an exclusive, royalty-bearing, sublicensable, worldwide license to make, sell and distribute products under certain patents owned by the University of Michigan related to microdystrophin and utrophin spectrin-like nucleic acid sequences for any use that, but for this agreement, would comprise an infringement of a valid claim included in the licensed patent rights. In consideration for the rights granted by the agreement, the Company paid a one-time license fee and a separate fee to cover past patent prosecution costs, which the Company recorded as a research and development expense in 2016. The Company is required to reimburse the University of Michigan for costs incurred in applying for, prosecuting and maintaining patents, and pay up to an aggregate of approximately $1,000 upon the achievement of certain milestones. There were no milestones achieved during the years ended December 31, 2021 Under the agreement, the University of Michigan reserves for itself and its affiliates the right to use the licensed rights for non-commercial research, public service, internal and educational purposes and the right to grant the same limited non-commercial rights to other non-profit research institutions. The license agreement remains in effect until the expiration of the last-to-expire patent licensed under the agreement. The University of Michigan may terminate the agreement upon the Company’s uncured material breach of the agreement, including failure to make required payments under the agreement or to achieve certain milestones, or if the Company becomes insolvent or bankrupt. The Company may terminate the license agreement at any time upon providing sixty days’ written notice to the University of Michigan. The Company recorded and research and development expense in the amount of $37, $35 and $39 for the years ended December 31, 2021 Harvard College License Agreements In 2016, the Company entered into a license agreement with the President and Fellows of Harvard College, (“Harvard College”), under which the Company obtained a non-exclusive, royalty-bearing, sublicensable, worldwide license to use certain intellectual property owned by Harvard College to develop, manufacture, and commercialize products for use in the treatment of Duchenne. In consideration for the rights granted by the agreement, the Company paid a one-time, non-refundable license fee, which was recorded as a research and development expense in 2016. The Company is required to pay an annual license maintenance fee until certain milestones are achieved, after which time the annual maintenance fee will increase annually. Such annual maintenance fee will further increase if the Company grants certain rights to a sublicensee or strategic partner with whom the Company collaborates on the development and commercialization of licensed products. The annual maintenance fee is creditable against royalty payments. The Company also must pay a milestone payment within thirty days after achieving certain milestones. There were no milestones achieved during the years ended December 31, 2021 , 2020 , and 2019 . The Company must pay a royalty of a low single digit percentage on future sales by the Company or its sublicensees of products developed using the licensed technology. The license agreement remains in effect for an initial term of fifteen years, with automatic three-year The Company recorded research and development expense in the amount of $10, $0 and $10 for the years ended December 31, 2021 In August 2017, the Company entered into another license agreement with Harvard College, under which the Company obtained a non-exclusive, royalty-bearing, sublicensable, worldwide license to use certain intellectual property owned by Harvard College to develop, manufacture, and commercialize products for use in the treatment of Duchenne. In consideration for the rights granted by the agreement, the Company paid a one-time, non-refundable license fee, which was recorded as a research and development expense in 2017. The Company is required to pay an annual license maintenance fee until certain milestones are achieved, after which time the annual maintenance fee will increase annually. Such annual maintenance fee will further increase if the Company grants certain rights to a sublicensee or strategic partner with whom the Company collaborates on the development and commercialization of licensed products. The annual maintenance fee is creditable against royalty payments. The Company also must pay a milestone payment within thirty days after achieving certain milestones. There were no milestones achieved during the years ended December 31, 2021 The license agreement remains in effect for an initial term of fifteen years, with automatic three-year The Company recorded research and development expense in the amount of $5, $0 and $5 for the years ended December 31, 2021 Other License Agreements In 2016, the Company entered into a license agreement with Life Technologies Corporation (“Life Technologies”). In consideration for obtaining a non-exclusive, royalty-free, worldwide license to use certain technologies and associated know-how to develop product candidates, the Company paid a one-time, non-refundable license fee. This fee was recorded as a research and development expense in 2016. The license agreement will remain effective in perpetuity unless earlier terminated. Life Technologies has the right to terminate the agreement upon the Company’s material, uncured breach of the agreement or in the event that it determines that continued performance of the agreement may violate any laws. The Company is obligated to diligently pursue regulatory approval necessary for the development, manufacture and sale of the licensed products. The Company has the right to terminate the agreement at any time upon providing thirty days’ written notice to Life Technologies.

Net Loss per Share

Net Loss per Share	12 Months Ended
	Dec. 31, 2021
Earnings Per Share [Abstract]
Net Loss per Share	13. Net Loss per Share Basic and diluted net loss per share were calculated as follows: The numerator for basic and diluted net loss per share is as follows: For the Year Ended December 31, 2021 2020 2019 Net loss $ (72,188 ) $ (88,290 ) $ (117,223 ) The denominator is as follows: For the Year Ended December 31, 2021 2020 2019 Weighted average common stock outstanding, basic and diluted 104,612,043 49,675,603 39,326,983 Weighted average pre-funded warrants to purchase common stock 2,158,329 2,261,544 962,307 Total 106,770,372 51,937,147 40,289,290 Net loss per share, basic and diluted is as follows: For the Year Ended December 31, 2021 2020 2019 Net loss per share, basic and diluted $ (0.68 ) $ (1.70 ) $ (2.91 ) The following potential common stock equivalents, presented based on amounts outstanding at each period end, were excluded from the calculation of diluted net loss per share for the periods indicated because including them would have had an anti-dilutive effect: For the Year Ended December 31, 2021 2020 2019 Options to purchase shares of common stock 6,012,636 3,117,775 2,610,975 Unvested restricted stock units 509,694 918,395 245,100 Unvested shares of common stock — 70,052 352,952 6,522,330 4,106,222 3,209,027

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2021
Income Tax Disclosure [Abstract]
Income Taxes	14. Income Taxes The Company recorded no tax benefit for the years ended December 31, 2021 and 2020 for the net operating losses incurred due to its uncertainty of realizing a benefit from those items. A reconciliation of income taxes computed using the U.S. federal statutory rate to that reflected in operations as of December 31, 2021 and 2020 is as follows: December 31, 2021 December 31, 2020 Income tax computed at federal statutory tax rate 21.0 % 21.0 % State taxes, net of federal benefit 6.4 % 6.0 % Permanent differences 0.5 % (0.4 )% Tax credits 12.1 % 9.6 % Change in deferred tax rate 0.3 % 0.0 % Stock compensation cancelations (1.9 )% 0.0 % Other 0.1 % (0.4 )% Valuation allowance (38.5 )% (35.8 )% 0.0 % 0.0 % The Company established deferred tax assets and liabilities on identified book to tax temporary differences as of the date of conversion to a C-corporation. Deferred income taxes reflect the net tax effects of these temporary differences between the carrying amounts of assets and liabilities for financial reporting purposes and the amounts used for income tax purposes. Significant components of the Company’s net deferred tax assets as of December 31, 2021 and 2020 are as follows: December 31, 2021 December 31, 2020 Deferred tax assets: Tax loss carryforwards $ 60,405 $ 53,321 Tax credit carryforwards 36,905 28,154 Deferred expenses 420 1,200 Accrued expenses 815 994 Stock compensation 7,821 5,909 Intangible assets 19,919 12,034 Depreciation 224 - Other 2,373 144 Total deferred tax assets 128,882 101,756 Valuation allowance (128,570 ) (100,740 ) Deferred tax liabilities: Right of use asset (312 ) (972 ) Depreciation — (44 ) Total deferred tax liabilities (312 ) (1,016 ) Net deferred taxes $ - $ - As of December 31, 2021, the Company has federal net operating loss carryforwards of $219,928 which may be available to offset future taxable income and do not expire, but are limited in their usage to an annual deduction equal to 80% of annual taxable income. In addition, as of December 31, 2021, the Company has state net operating loss carryforwards of approximately $218,052 which may be available to offset future taxable income and begin to expire in 2038. The Company also had federal and state tax credits of $34,606 and $2,910, respectively, which may be used to offset future tax liability and each of which begin to expire in 2033. The Company’s ability to utilize these federal and state carryforwards may be limited in the future if the Company experiences an ownership change pursuant to Internal Revenue Code Section 382. Ownership changes, as defined in the Internal Revenue Code, including those resulting from the issuance of common stock in connection with the Company’s public offerings, may limit the amount of net operating loss and tax credit carryforwards that can be utilized to offset future taxable income or tax liability. The Company has not conducted a study to assess whether a change of control has occurred or whether there have been multiple changes of control since inception due to the significant complexity and cost associated with such a study. If the Company has experienced a change of control, as defined by Section 382, at any time since inception, utilization of the net operating loss carryforwards tax credit carryforwards would be subject to an annual limitation under Section 382. Any limitation may result in expiration of a portion of the net operating loss carryforwards or research and development tax credit carryforwards before utilization. Further, until a study is completed and any limitation is known, no amounts are being presented as an uncertain tax position. A valuation allowance is recorded against deferred tax assets if it is more likely than not that some or all of the deferred tax assets will not be realized. The Company has evaluated the positive and negative evidence bearing upon the realizability of the deferred tax assets. The Company concluded, in accordance with the applicable accounting standards, that it is more likely than not that the Company will not realize the benefit of its deferred tax assets. Accordingly, the Company has recorded a full valuation allowance against its deferred tax assets. The Company files tax returns as prescribed by the tax laws of the jurisdictions in which it operates. In the normal course of business, the Company is subject to examination by federal and state jurisdictions, where applicable. There are currently no pending income tax examinations. The Company’s C-Corporation tax years beginning with the year ended December 31, 2018 are open under statute. Any tax credit or net operating loss carryforward can be adjusted in future periods after the respective year of generation’s statute of limitation has closed. As of December 31, 2021 and 2020, the Company did not have unrecognized tax benefits. The Company recognizes interest and penalties related to income taxes as a component of income tax expense. As of December 31, 2021 and 2020, no interest and penalties have been recorded.

Defined Contribution Plan

Defined Contribution Plan	12 Months Ended
	Dec. 31, 2021
Compensation And Retirement Disclosure [Abstract]
Defined Contribution Plan	15. Defined Contribution Plan The Company has a defined contribution savings plan under Section 401(k) of the Internal Revenue Code. The plan covers substantially all employees who meet minimum age and service requirements and allows participants to defer a portion of their annual compensation on a pretax basis. Company contributions to the plan may be made at the discretion of the Company’s Board of Directors. The Company made $241 of contributions during the year ended December 31, 2021. The company had made no contributions to the plan during the years ended December 31, 2020 and December 31, 2019.

Restructuring

Restructuring	12 Months Ended
	Dec. 31, 2021
Restructuring And Related Activities [Abstract]
Restructuring	16. Restructuring In 2021, the Company did not incur any restructuring expense. The company paid $62 during the first quarter of 2021 related to restructuring accrued as of December 31, 2020. In January 2020, the Company’s Board of Directors approved a restructuring plan to reduce operating costs and better align the Company’s workforce with the needs of its business following the Company’s November 2019 announcement that the SGT-001 IGNITE DMD trial was placed on clinical hold by the FDA. Under the restructuring plan, the Company made changes to its management team and reduced headcount by approximately 30 percent. Affected employees were eligible to receive severance payments and outplacement services in connection with the restructuring plan. During year ended December 31, 2020, the Company recorded aggregate restructuring charges of $1,944 related to severance payments and other employee-related costs. The Company does not expect to incur any additional significant costs associated with this restructuring. During the year ended December 31, 2020, $1,882 of the estimated restructuring charges were paid. The following table shows the total amount expected to be incurred and the liability related to the 2020 restructuring for the years ended December 31, 2021 and December 31, 2020: One-Time Employee Termination Benefits Accrued restructuring costs as of December 31, 2019 $ - Restructuring charges incurred during the period 1,944 Amounts paid during the period (1,882 ) Accrued restructuring costs as of December 31, 2020 $ 62 Restructuring charges incurred during the period - Amounts paid during the period (62 ) Accrued restructuring costs as of December 31, 2021 $ -

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2021
Accounting Policies [Abstract]
Use of Estimates	Use of Estimates The preparation of the Company’s consolidated financial statements in conformity with GAAP requires management to make estimates, judgments and assumptions that affect the reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities at the date of the consolidated financial statements and the reported amounts of revenues and expenses during the reporting periods. Significant estimates and assumptions reflected in these consolidated financial statements include, but are not limited to, estimates related to revenue recognition, the recognition of research and development expenses and equity-based compensation. Estimates are periodically reviewed in light of changes in circumstances, facts and experience. Changes in estimates are recorded in the period in which they become known. Actual results could differ from the Company’s estimates. The full extent to which the COVID-19 pandemic will directly or indirectly impact the Company’s business, results of operations and financial condition, including clinical trials and employee-related amounts, will depend on future developments that are highly uncertain, including new information that may emerge concerning COVID-19 and the actions taken to contain
Cash Equivalents	Cash Equivalents The Company considers all short-term, highly liquid investments with original maturities of 90 days or less at acquisition date to be cash equivalents.
Restricted Cash	Restricted Cash The Company held restricted cash of $2,070 and $327 in separate restricted bank accounts as security deposits for leases of the Company’s facilities as of December 31, 2021 and December 31, 2020, respectively. The Company has included restricted cash of $2,070 and $327 as a non-current asset as of December 31, 2021 and December 31, 2020, respectively. A reconciliation of the amounts of cash and cash equivalents and restricted cash from the cash flow statement to the balance sheet is as follows: December 31, 2021 December 31, 2020 December 31, 2019 December 31, 2018 Cash and cash equivalents $ 119,136 $ 154,744 $ 76,043 $ 86,366 Restricted cash, non-current 2,070 327 327 327 Cash and cash equivalents and restricted cash $ 121,206 $ 155,071 $ 76,370 $ 86,693
Available-for-Sale Securities	Available-for-Sale Securities Available-for-sale securities consist of investments with original maturities greater than 90 days at acquisition date. The Company has classified its investments with maturities beyond one year as short term, based on their highly liquid nature and because such available-for-sale securities represent the investment of cash that is available for current operations. The Company classifies all of its investments as available-for-sale securities. The Company’s investments are measured and reported at fair value using quoted prices in active markets for similar securities. Unrealized gains and losses on available-for-sale debt securities are reported as a separate component of stockholders’ equity. The cost of debt securities sold is determined on a specific identification basis, and realized gains and losses are included in other income (expense) within the consolidated statement of operations. If any adjustment to fair value reflects a decline in the value of the investment that the Company considers to be “other than temporary,” the Company reduces the investment to fair value through a charge to the consolidated statement of operations. No such adjustments were necessary during the periods presented.
Concentration of Credit Risk and of Significant Suppliers	Concentration of Credit Risk and of Significant Suppliers Financial instruments that potentially expose the Company to concentrations of credit risk consist primarily of cash, cash equivalents and available-for-sale securities. Periodically, the Company maintains deposits in accredited financial institutions in excess of federally insured limits. The Company maintains each of its cash, cash equivalents and available-for-sale securities balances with high-quality and accredited financial institutions and accordingly, such funds are not exposed to significant credit risk. The Company does not believe that it is subject to unusual credit risk beyond the normal credit risk associated with commercial banking relationships. The Company is dependent on third-party manufacturers to supply products for research and development activities of its programs, including clinical and preclinical testing. These programs could be adversely affected by a significant interruption in the supply of such drug substance products.
Fair Value Measurements	Fair Value Measurements Certain 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 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. The Company’s cash equivalents and available-for-sale securities are carried at fair value, determined according to the fair value hierarchy described above. See Note 4 Fair Value of Financial Assets and Liabilities , for additional information. The carrying values of the Company’s accounts payable and accrued expenses and other current liabilities approximate their fair value due to the short-term nature of these liabilities.
Leases	Leases At inception of a contract, the Company determines if a contract meets the definition of a lease. A lease is a contract, or part of a contract, that conveys the right to control the use of identified property, plant, or equipment (an identified asset) for a period of time in exchange for consideration. The Company determines if the contract conveys the right to control the use of an identified asset for a period of time. The Company assesses throughout the period of use whether the Company has both of the following: (1) the right to obtain substantially all of the economic benefits from use of the identified asset and (2) the right to direct the use of the identified asset. This determination is reassessed if the terms of the contract are changed. Leases are classified as operating or finance leases based on the terms of the lease agreement and certain characteristics of the identified asset. Right-of-use assets and lease liabilities are recognized at the lease commencement date based on the present value of the minimum future lease payments. The Company’s policy is to not record leases with an original term of twelve months or less on the consolidated balance sheets. The Company recognizes lease expense for these short-term leases on a straight-line basis over the lease term. Certain lease agreements include rental payments that are adjusted periodically for inflation or other variables. In addition to rent, the leases may require the Company to pay additional amounts for taxes, insurance, maintenance and other expenses, which are generally referred to as non-lease components. Such adjustments to rental payments and variable non-lease components are treated as variable lease payments and recognized in the period in which the obligation for these payments was incurred. Variable lease components and variable non-lease components are not measured as part of the right of use asset and liability. Only when lease components and their associated non-lease components are fixed are they accounted for as a single lease component and recognized as part of a right of use asset and liability. Total contract consideration is allocated to the combined fixed lease and non-lease components.
Property and Equipment	Property and Equipment Property and equipment are stated at cost less accumulated depreciation. Depreciation expense is recognized using the straight-line method over the useful life of the asset. Laboratory equipment is depreciated over five years. Computer equipment is depreciated over three years. Computer software is depreciated over two years. Furniture and office equipment are depreciated over five years. Leasehold improvements are amortized over the shorter of the lease term or the estimated useful life of the related asset. Expenditures for repairs and maintenance of assets are charged to expense as incurred. Upon retirement or sale, the cost and related accumulated depreciation of assets disposed of are removed from the accounts and any resulting gain or loss is included in loss from operations. Equipment under a finance lease is stated at fair value at the inception of the lease less accumulated depreciation and is depreciated over the remaining lease term or the estimated useful life of the equipment.
Impairment of Long-Lived Assets	Impairment of Long-Lived Assets Long-lived assets, comprised of property and equipment, to be held and used are tested for recoverability whenever events or changes in business circumstances indicate that the carrying amount of the assets may not be fully recoverable. Factors that the Company considers in deciding when to perform an impairment review include significant underperformance of the business in relation to expectations, significant negative industry or economic trends and significant changes or planned changes in the use of the assets. If an impairment review is performed to evaluate a long-lived asset for recoverability, the Company compares forecasts of undiscounted cash flows expected to result from the use and eventual disposition of the long-lived asset to its carrying value. An impairment loss would be recognized when estimated undiscounted future cash flows expected to result from the use of an asset are less than its carrying amount. The impairment loss would be based on the excess of the carrying value of the impaired asset over its fair value, determined based on discounted cash flows. To date, the Company has not recorded any impairment losses or disposals on long-lived assets.
Revenue Recognition	Revenue Recognition The Company recognizes revenue in accordance with ASC Topic 606, Revenue from Contracts with Customers (“ASC 606”). Under ASC 606, an entity recognizes revenue when its customer obtains control of promised goods or services, in an amount that reflects the consideration which the entity expects to receive in exchange for those goods or services. To determine the appropriate amount of revenue to be recognized for arrangements determined to be within the scope of ASC 606, the Company performs the following five steps: (i) identification of the contract(s) with the customer; (ii) identification of the promised goods or services in the contract and determination of whether the promised goods or services are performance obligations; (iii) measurement of the transaction price; (iv) allocation of the transaction price to the performance obligations; and (v) recognition of revenue when (or as) the Company satisfies each performance obligation. The Company only applies the five-step model to contracts when it is probable that the entity will collect consideration it is entitled to in exchange for the goods or services it transfers to the customer. At contract inception, once the contract is determined to be within the scope of ASC 606, the Company assesses the goods or services promised within each contract and determines those that are performance obligations and assesses whether each promised good or service is distinct. The Company then recognizes as revenue the amount of the transaction price that is allocated to the respective performance obligation when (or as) the performance obligation is satisfied. As part of the accounting for these arrangements, the Company must use significant judgment to determine: a) the number of performance obligations based on the determination under step (ii) above and whether those performance obligations are distinct from other performance obligations in the contract; b) the transaction price under step (iii) above; and c) the standalone selling price for each performance obligation identified in the contract for the allocation of transaction price in step (iv) above. The Company uses judgment to determine whether milestones or other variable consideration, except for royalties, should be included in the transaction price as described further below. The transaction price is allocated to each performance obligation on a relative stand-alone selling price basis, for which the Company recognizes revenue as or when the performance obligations under the contract are satisfied. In determining the stand-alone selling price of a license to the Company’s proprietary technology or a material right provided by a customer option, the Company considers market conditions as well as entity-specific factors, including those factors contemplated in negotiating the agreements as well as internally developed estimates that include assumptions related to the market opportunity, estimated development costs, probability of success and the time needed to commercialize a product candidate pursuant to the license. In validating its estimated stand-alone selling prices, the Company evaluates whether changes in the key assumptions used to determine its estimated stand-alone selling prices will have a significant effect on the allocation of arrangement consideration between performance obligations. The Company estimates the transaction price based on the amount of consideration the Company expects to be received for transferring the promised goods or services in the contract. The consideration may include both fixed consideration and variable consideration. At the inception of each arrangement that includes variable consideration, the Company evaluates the amount of the potential payments and the likelihood that the payments will be received. The Company utilizes either the most likely amount method or expected value method to estimate the transaction price based on which method better predicts the amount of consideration expected to be received. If it is probable that a significant reversal in the amount of cumulative revenue recognized would not occur, the variable consideration is included in the transaction price. Performance obligations are promised goods or services in a contract to transfer a distinct good or service to the customer. Promised goods or services are considered distinct when: (i) the customer 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, the Company considers factors such as the stage of development of the underlying intellectual property, the capabilities of the customer to develop the intellectual property on their own and whether the required expertise is readily available. For performance obligations which consist of licenses and other promises, the Company utilizes judgment to assess the nature of the combined performance obligation in order to determine whether the combined performance obligation is satisfied over time or at a point in time. Up-front payments and fees are recorded as deferred revenue upon receipt or when due until the Company performs its obligations under these arrangements. Amounts expected to be recognized as revenue within the 12 months following the balance sheet date are classified as current portion of deferred revenue in the accompanying consolidated balance sheets. Amounts not expected to be recognized as revenue within the 12 months following the balance sheet date are classified as deferred revenue, net of current portion. Amounts are recorded as accounts receivable when the Company’s right to consideration is unconditional. Amounts recognized as revenue, but not yet received or invoiced are generally recognized as contract assets. Exclusive Licenses If the license granted in the arrangement is determined to be distinct from the other promises or performance obligations identified in the arrangement, which generally include research and development services, the Company recognizes revenue from non-refundable, upfront fees allocated to the license when the license is transferred to the customer and the customer is able to use and benefit from the license In assessing whether a license is distinct from the other promises, the Company considers relevant facts and circumstances of each arrangement, including the research and development capabilities of the collaboration partner and the availability of the associated expertise in the general marketplace. In addition, the Company considers whether the collaboration partner can benefit from the license for its intended purpose without the receipt of the remaining promise, whether the value of the license is dependent on the unsatisfied promise, whether there are other vendors that could provide the remaining promise, and whether it is separately identifiable from the remaining promise. For licenses that are combined with other promises, the Company utilizes 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 for purposes of recognizing revenue. The Company evaluates the measure of progress each reporting period and, if necessary, adjusts the measure of performance and related revenue recognition. The measure of progress, and thereby periods over which revenue should be recognized, are subject to estimates by management and may change over the course of the arrangement. Research and Development Services The promises under the Company’s collaboration and license agreements generally include research and development services to be performed by the Company on behalf of the collaboration partner. For performance obligations that include research and development services, the Company generally recognizes revenue allocated to such performance obligations based on an appropriate measure of progress. The Company utilizes judgment to determine the appropriate method of measuring progress for purposes of recognizing revenue, which is generally an input measure, such as costs incurred. Milestone Payments At the inception of each arrangement that includes milestone payments based on certain events, the Company evaluates whether the milestones are considered probable of being achieved and estimates the amount to be included in the transaction price using the most likely amount method. If it is probable that a significant reversal in the amount of cumulative revenue recognized would not occur, the associated milestone value is included in the transaction price. Milestone payments that are not within the control of the Company, such as regulatory approvals, are not considered probable of being achieved until those approvals are received. The Company evaluates factors such as the scientific, clinical, regulatory, commercial, and other risks that must be overcome to achieve the particular milestone in making this assessment. There is considerable judgment involved in determining whether it is probable that a significant reversal in the amount of cumulative revenue recognized would not occur. At the end of each subsequent reporting period, the Company reevaluates the probability of achievement of all milestones subject to constraint and, if necessary, adjusts its estimate of the overall transaction price. Any such adjustments are recorded on a cumulative catch-up basis, which would affect revenues and earnings in the period of adjustment. If a milestone or other variable consideration relates specifically to the Company’s efforts to satisfy a single performance obligation or to a specific outcome from satisfying the performance obligation, the Company generally allocates the milestone amount entirely to that performance obligation once it is probable that a significant reversal in the amount of cumulative revenue recognized would not occur. Royalties For arrangements that include sales-based royalties, including milestone payments based on a level of sales, and the license is deemed to be the predominant item to which the royalties relate, the Company recognizes 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, the Company has not recognized any royalty revenue resulting from any of its licensing arrangements. Collaboration Revenue The Company analyzes its collaboration arrangements to assess whether they are within the scope of ASC 808, Collaborative Arrangements Costs Associated with License and Collaborative Arrangements All costs associated with license and collaborative arrangements are expensed as incurred and recorded in research and development expense in the consolidated statements of operations.
Research and Development Costs	Research and Development Costs Research and development costs are expensed as incurred. Research and development expenses include salaries, equity-based compensation and benefits of employees, third-party license fees and other operational costs related to the Company’s research and development activities, including allocated facility-related expenses and external costs of outside vendors engaged to conduct both preclinical studies and clinical trials. Non-refundable pre-payments for goods or services that will be used or rendered for future research and development activities are deferred and capitalized. Such amounts are recognized as expense as the goods or services are delivered or the related services are performed, or until it is no longer expected that the goods will be delivered or the services rendered. The Company may in-license the rights to develop and commercialize product candidates. For each in-license transaction the Company evaluates whether it has acquired processes or activities along with inputs that would be sufficient to constitute a “business” as defined under GAAP. A “business” as defined under GAAP consists of inputs and processes applied to those inputs that have the ability to create outputs. Although businesses usually have outputs, outputs are not required for an integrated set of activities to qualify as a business. When the Company determines that it has not acquired sufficient processes or activities to constitute a business, any up-front payments, as well as milestone payments, are immediately expensed as acquired research and development in the period in which they are incurred.
Research Contract Costs and Accruals	Research Contract Costs and Accruals The Company has entered into various research and development contracts with research institutions and other companies. These agreements are generally cancelable, and related payments are recorded as research and development expenses as incurred. The Company records accruals for estimated ongoing research costs. When evaluating the adequacy of the accrued liabilities, the Company analyzes progress of the studies, 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 the Company’s estimates. The Company’s historical accrual estimates have not been materially different from the actual costs.
Patent Costs	Patent Costs All patent-related costs incurred for 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.
Equity-Based Compensation	Equity-Based Compensation The Company measures all stock options and other stock-based awards granted to employees, directors and non-employees based on the fair value on the date of the grant and recognizes compensation expense of those awards, over the requisite service period, which is generally the vesting period of the respective award. Forfeitures are accounted for as they occur. The Company applies the straight-line method of expense recognition to all awards with only service-based vesting conditions. The Company has not issued any awards with performance-based vesting conditions. The fair value of each stock option grant is estimated on the date of grant using the Black-Scholes option-pricing model. The Company historically has been a private company and lacks company-specific historical and implied volatility information. Therefore, it estimates its expected stock volatility based on the historical volatility of a publicly traded set of peer companies and expects to continue to do so until such time as it has adequate historical data regarding the volatility of its own traded stock price. For options with service-based vesting conditions, the expected term of the Company’s stock options has been determined utilizing the “simplified” method for awards that qualify as “plain-vanilla” options. Through December 31, 2018, the expected term of stock options granted to non-employees is equal to the contractual term of the option award and effective January 1, 2019, the “simplified” method is used. The risk-free interest rate is 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 is based on the fact that the Company has never paid cash dividends and does not expect to pay any cash dividends in the foreseeable future. The fair value for restricted stock units, “RSU”, was calculated using the closing price of the Company’s common stock on the date of grant. The Company classifies stock-based compensation expense in its consolidated statement of operations in the same
Income Taxes	Income Taxes Income taxes are accounted for under the asset and liability method. Under the asset and liability method, deferred tax assets and liabilities are recognized for the future tax consequences attributable to differences between the financial statement carrying amounts of existing assets and liabilities and their respective tax bases. Deferred tax assets and liabilities are measured using enacted tax rates expected to apply to taxable income in the years in which those temporary differences are expected to be recovered or settled. The effect on deferred tax assets and liabilities of a change in tax rates is recognized in income in the period that includes the enactment date. The Company records valuation allowances to reduce deferred income tax assets to the amount that is more likely than not to be realized. The Company determines whether it is more likely than not that a tax position will be sustained upon examination. If it is not more likely than not that a position will be sustained, no amount of benefit attributable to the position is recognized. The tax benefit to be recognized of any tax position that meets the more likely than not recognition threshold is calculated as the largest amount that is more than 50% likely of being realized upon resolution of the contingency. Prior to January 25, 2018, the Company had not been subject to U.S. federal income taxes as the Company was organized as a limited liability company. As such, the taxable income or loss was passed through to and included in the tax returns of the members. Since January 25, 2018, the Company’s income has since been subject to U.S. federal, state, local, and foreign income taxes and taxed at the prevailing corporate tax rates.
Segment Data	Segment Data The Company manages its operations as a single segment for the purposes of assessing performance and making operating decisions. The Company’s singular focus is on developing treatments through gene therapy and other means for patients with Duchenne. All of the Company’s tangible assets are held in the United States.
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 members. The Company’s only element of other comprehensive income (loss) in all periods presented was unrealized gains (losses) from available-for-sale securities.
Net Loss per Share	Net Loss per Share The Company follows the two-class method when computing net loss per share, as the Company has issued shares that meet the definition of participating securities. The two-class method determines net loss per share for each class of common and participating securities according to dividends declared or accumulated and participation rights in undistributed earnings. The two-class method requires income available to common stockholders for the period to be allocated between common and participating securities based upon their respective rights to receive dividends as if all income for the period had been distributed. Basic net loss per share is computed by dividing the net loss by the weighted average number of shares of common stock and pre-funded warrants outstanding for the period. Diluted net loss is computed by adjusting net loss to reallocate undistributed earnings based on the potential impact of dilutive securities. Diluted net loss per share is computed by dividing the diluted net loss by the weighted average number of shares of common stock and pre-funded warrants outstanding for the period, including potential dilutive shares of common stock assuming the dilutive effect of common stock equivalents. The Company’s preferred stock could entitle the holders of such shares to participate in dividends and not require the holders of such shares to participate in losses of the Company. Accordingly, in periods in which the Company reports a net loss, such losses are not allocated to such participating securities. In periods in which the Company reports a net loss, diluted net loss per share is the same as basic net loss per share, since dilutive shares of common stock are not assumed to have been issued if their effect is anti-dilutive. As of the year ended December 31, 2021 and 2020, there were no preferred stock issued or outstanding with any contractual rights.
Funding from Charitable Organizations	Funding from Charitable Organizations The Company has received funding from charitable organizations to perform research and development services to identify therapies for people with Duchenne. The amounts received are recognized as services are performed and research expenses are incurred. These are included in other income in the consolidated statements of operations as the arrangement between the Company and the charitable organizations are not part of the Company’s on-going, major or central operations. Any amount received in advance of services performed is recorded in other current liabilities in the consolidated balance sheets if the services are expected to be performed within the next twelve months. The Company recognized other income of $0, $1 and $515 for the years ended December 31, 2021, 2020 and 2019, respectively, which is included in the consolidated statements of operations.
Contingencies	Contingencies Loss contingency provisions are recorded if the potential loss from any claim, asserted or unasserted, or legal proceeding, is considered probable and the amount can be reasonably estimated, or a range of loss can be determined. These accruals represent the Company’s best estimate of probable loss. Disclosure also is provided when it is reasonably possible that a loss will be incurred or when it is reasonably possible that the amount of a loss will exceed the recorded provision. The Company reviews the status of each significant matter and assesses its potential financial exposure. Significant judgment is required in both the determination of probability and the determination as to whether an exposure is reasonably estimable. Because of uncertainties related to these matters, accruals are based only on the best information available at the time. As additional information becomes available, the Company reassesses the potential liability related to pending claims and may change its estimates. These changes in the estimates of the potential liabilities could have a material impact on the Company’s consolidated results of operations and financial position.
Recently Adopted Accounting Pronouncements	Recently Adopted Accounting Pronouncements In December 2019, the FASB issued Accounting Standards Update (“ASU”) No. 2019-12, Income Taxes (Topic 740): Simplifying the Accounting for Income Taxes , which simplifies the accounting for income taxes by removing certain exceptions to the general principles in ASC 740 and clarifies and amends existing guidance to improve consistent application. The standard became effective for the Company beginning January 1, 2021. The amendments that are related to changes in ownership of foreign equity method investments or foreign subsidiaries are to be applied on a modified retrospective basis through a cumulative-effect adjustment to retained earnings as of the beginning of the fiscal year of adoption. The amendments that are related to franchise taxes that are partially based on income are to be applied on either a retrospective basis for all periods presented or a modified retrospective basis through a cumulative-effect adjustment to retained earnings as of the beginning of the fiscal year of adoption. All other amendments under this ASU are to be applied on a prospective basis. The Company adopted the guidance effective January 1, 2021. The adoption of this new standard did not have a material impact on the Company’s financial statements. Accounting Pronouncements Not Yet Adopted In August 2020, the FASB issued ASU No. 2020-06, Debt, Debt with Conversion and Other Options (Subtopic 470-20) and Derivatives and Hedging—Contracts in Entity’s Own Equity (Subtopic 815-40): Accounting for Convertible Instruments and Contracts in an Entity’s Own Equity, which, among other things, provides guidance on how to account for contracts on an entity’s own equity. This ASU simplifies the accounting for certain financial instruments with characteristics of liabilities and equity. Specifically, the ASU eliminated the need for the Company to assess whether a contract on the entity’s own equity (1) permits settlement in unregistered shares, (2) whether counterparty rights rank higher than shareholder’s rights, and (3) whether collateral is required. In addition, the ASU requires incremental disclosure related to contracts on the entity’s own equity and clarifies the treatment of certain financial instruments accounted for under this ASU on earnings per share. The ASU also simplifies the accounting for convertible instruments by removing the beneficial conversion feature and cash conversion feature separation models. This ASU may be applied on a full retrospective or modified retrospective basis. This ASU is effective for smaller reporting companies for fiscal years beginning after December 15, 2023, with early adoption permitted. The Company does not expect the adoption to materially impact its financial position and results of operations.
Related Party	Related Party In October 2020, the Company entered into a collaboration and license agreement (the “Collaboration Agreement”) with Ultragenyx Pharmaceutical Inc. (“Ultragenyx”). In connection with the Collaboration Agreement, Ultragenyx also purchased 7,825,797 shares of the Company’s common stock, which resulted in Ultragenyx becoming a related party of the Company. In November 2020, the Company entered into a consulting agreement with Danforth Advisors, LLC (“Danforth”), an affiliate of Stephen DiPalma, our interim chief financial officer. Pursuant to the consulting agreement, Danforth provides the Company with the chief financial officer services of Mr. DiPalma, and other services, including financial planning, offering support and accounting services, in exchange for fees payable to Danforth based on hourly rates. The Company has paid Danforth approximately $700 as of December 31, 2021. In accordance with the consulting agreement, in November 2020, the Company issued to Danforth a warrant to purchase 30,000 shares of its common stock at an exercise price per share of $3.29. The consulting agreement may be terminated by either party without cause upon 60 days’ prior written notice to the other party and with cause upon 30 days’ prior written notice to the other party.

Summary of Significant Accoun_3

Summary of Significant Accounting Policies (Tables)	12 Months Ended
	Dec. 31, 2021
Accounting Policies [Abstract]
Reconciliation of Amounts From Cash Flow Statement to Balance Sheet	A reconciliation of the amounts of cash and cash equivalents and restricted cash from the cash flow statement to the balance sheet is as follows: December 31, 2021 December 31, 2020 December 31, 2019 December 31, 2018 Cash and cash equivalents $ 119,136 $ 154,744 $ 76,043 $ 86,366 Restricted cash, non-current 2,070 327 327 327 Cash and cash equivalents and restricted cash $ 121,206 $ 155,071 $ 76,370 $ 86,693

Collaborations (Tables)

Collaborations (Tables)	12 Months Ended
	Dec. 31, 2021
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Summary of Changes in Balances of Company's Related Party Collaboration Receivables and Contract Liabilities	The following table presents changes in the balances of the Company’s related party collaboration receivables and contract liabilities during the year ended December 31, 2021: Balance as of December 31, 2020 Additions Deductions Balance as of December 31, 2021 Related party collaboration receivable $ — $ 982 $ (872 ) $ 110 Contract liabilities: Deferred revenue 20,718 982 (13,620 ) 8,080

Fair Value of Financial Asset_2

Fair Value of Financial Assets and Liabilities (Tables)	12 Months Ended
	Dec. 31, 2021
Fair Value Disclosures [Abstract]
Summary of Assets and Liabilities Measured at Fair Value on Recurring Basis and Indicate Level of Fair Value Hierarchy Utilized	The following tables present information about the Company’s assets and liabilities that are measured at fair value on a recurring basis and indicate the level of the fair value hierarchy utilized to determine such fair values: Fair Value Measurements as of December 31, 2021: Level 1 Level 2 Level 3 Total Assets: Cash equivalents $ - $ 75,224 $ - $ 75,224 Available-for-sale securities - 88,643 - 88,643 $ - $ 163,867 $ - $ 163,867 Fair Value Measurements as of December 31, 2020: Level 1 Level 2 Level 3 Total Assets: Cash equivalents $ - $ 84,462 $ - $ 84,462 $ - $ 84,462 $ - $ 84,462

Available-for-Sale Securities (

Available-for-Sale Securities (Tables)	12 Months Ended
	Dec. 31, 2021
Investments Debt And Equity Securities [Abstract]
Schedule of Fair Value of Available-for-Sale Debt Securities by Type of Security	As of December 31, 2021 the fair value of available-for-sale debt securities by type of security was as follows: December 31, 2021 Amortized Cost Gross Unrealized Gain Gross Unrealized Loss Fair Value Investments: Treasury bill $ 2,800 $ - $ - $ 2,800 Corporate bond securities 83,889 - (45 ) 83,844 Commercial paper 1,999 - - 1,999 $ 88,688 $ $ (45 ) $ 88,643
Summary Estimated Fair Value and Amortized Cost of Available-for-Sale Securities by Contractual Maturity	The estimated fair value and amortized cost of the Company’s available-for-sale securities by contractual maturity are summarized as follows: December 31, 2021 Amortized Cost Fair Value Due in one year or less $ 88,688 $ 88,643 Total available-for-sale securities $ 88,688 $ 88,643

Prepaid Expenses and Other Cu_2

Prepaid Expenses and Other Current Assets (Tables)	12 Months Ended
	Dec. 31, 2021
Deferred Costs Capitalized Prepaid And Other Assets Disclosure [Abstract]
Schedule of Prepaid Expenses and Other Current Assets	Prepaid expenses and other current assets consist of the following: December 31, 2021 2020 Prepaid research and development expenses $ 6,015 $ 2,674 Prepaid expenses and other assets 8,708 1,483 $ 14,723 $ 4,157

Property and Equipment (Tables)

Property and Equipment (Tables)	12 Months Ended
	Dec. 31, 2021
Property Plant And Equipment [Abstract]
Summary of Property and Equipment	Property and equipment consists of the following: December 31, 2021 2020 Furniture and fixtures $ 212 $ 203 Laboratory equipment 10,719 9,631 Leasehold improvements 4,713 4,713 Computer equipment 436 436 Computer software 553 553 Construction in process 1,490 1,330 18,123 16,866 Less accumulated depreciation 11,661 8,713 $ 6,462 $ 8,153

Accrued Expenses (Tables)

Accrued Expenses (Tables)	12 Months Ended
	Dec. 31, 2021
Payables And Accruals [Abstract]
Summary of Accrued Expenses and Other Current Liabilities	Accrued expenses and other current liabilities consist of the following: December 31, 2021 2020 Accrued research and development $ 1,507 $ 2,091 Accrued compensation 3,084 3,834 Accrued other 4,937 2,715 $ 9,528 $ 8,640

Equity-Based Compensation (Tabl

Equity-Based Compensation (Tables)	12 Months Ended
	Dec. 31, 2021
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Summary of Stock Option Activity	The following table summarizes the Company’s stock option activity for the year ended December 31, 2021: Number of Options Weighted Average Exercise Price Remaining Contractual Life (in years) Outstanding at December 31, 2020 3,117,775 $ 14.58 7.78 Granted 3,708,345 5.92 Exercised (11,625 ) 3.55 Forfeitures (801,859 ) 14.85 Outstanding at December 31, 2021 6,012,636 9.23 8.49 Vested and expected to vest as of December 31, 2021 6,012,636 $ 9.23 8.49 Exercisable at December 31, 2021 1,939,881 $ 13.50 7.63
Assumptions for Measuring Fair Values of Stock Options	The fair value of each option award is estimated on the date of grant using the Black-Scholes option-pricing model using the assumptions noted in the following table for the years ended December 31: 2021 2020 2019 Expected volatility 115.5% - 123.9% 21.3% - 119.5% 85.1% - 104.7% Expected dividends 0.0% 0.0% 0.0% Expected term (in years) 5.1 - 6.25 4.50 - 10.00 5.10 - 6.25 Risk-free rate 0.4% - 1.4% 0.6% - 3.4% 1.5% - 2.6%
Summary of Restricted Stock Unit Activity	The following table summarizes the Company’s restricted stock unit activity for the year ended December 31, 2021 : Units Weighted- Average Grant Date Fair Value Unvested at December 31, 2020 918,395 $ 3.17 Granted 220,750 2.25 Vested (419,193 ) 3.21 Forfeitures (210,258 ) 3.13 Outstanding at December 31, 2021 509,694 $ 2.75 Unvested as of December 31, 2021 509,694 $ 2.75
Summary of Unvested Restricted Shares of Common Stock Activity	The following table summarizes the Company’s unvested restricted shares of common stock activity for the year ended December 31, 2021 Units Weighted- Average Grant Date Fair Value Unvested restricted Common Units at December 31, 2020 70,052 $ 9.83 Releases (40,840 ) 10.50 Forfeitures (29,212 ) 8.89 Unvested restricted Common Units at December 31, 2021 - $ -
Schedule of Equity-based Compensation Expense	The Company recorded equity-based compensation expense in the following captions within its consolidated statements of operations for the years ended December 31, 2021 For the Year Ended December 31, 2021 2020 2019 Research and development expenses $ 6,289 $ 5,822 $ 8,006 General and administrative expenses 7,084 4,956 6,201 Restructuring expenses - 851 - $ 13,373 $ 11,629 $ 14,207

Leases (Tables)

Leases (Tables)	12 Months Ended
	Dec. 31, 2021
Leases [Abstract]
Future Minimum Payments for Operating and Finance Leases	As of December 31, 2021 Finance Leases Operating Leases 2022 $ 276 $ 1,355 2023 300 279 2024 — — 2025 — — Thereafter — — Total 576 1,634 Less: Imputed Interest 51 96 Total Lease Liabilities $ 525 $ 1,538
Supplemental Disclosure of Cash Flow Information Related to Leases and Weighted Average Remaining Lease Term and Discount Rate	The supplemental disclosure of cash flow information related to the Company’s leases and the weighted average remaining lease term and weighted average discount rate of the Company’s leases are as follows: For the Year Ended December 31, For the Year Ended December 31, For the Year Ended December 31, 2021 2020 2019 Other information Cash paid for amounts included in the measurement of lease liabilities $ 2,408 $ 2,408 $ 2,288 Operating lease liabilities arising from obtaining right-of-use-assets $ 310 $ 1,629 $ 1,629 Finance lease liabilities arising from obtaining right-of-use assets $ — $ — $ — Weighted-average remaining lease term (in years) Operating lease 1.0 2.8 3.5 Finance lease 2.3 2.3 3.5 Weighted-average discount rate Operating lease 11.9 % 12.6 % 12.5 % Finance lease 10.7 % 10.7 % 10.7 %

Net Loss per Share (Tables)

Net Loss per Share (Tables)	12 Months Ended
	Dec. 31, 2021
Earnings Per Share [Abstract]
Schedule of Basic and Diluted Net Loss per Share	The numerator for basic and diluted net loss per share is as follows: For the Year Ended December 31, 2021 2020 2019 Net loss $ (72,188 ) $ (88,290 ) $ (117,223 ) The denominator is as follows: For the Year Ended December 31, 2021 2020 2019 Weighted average common stock outstanding, basic and diluted 104,612,043 49,675,603 39,326,983 Weighted average pre-funded warrants to purchase common stock 2,158,329 2,261,544 962,307 Total 106,770,372 51,937,147 40,289,290 Net loss per share, basic and diluted is as follows: For the Year Ended December 31, 2021 2020 2019 Net loss per share, basic and diluted $ (0.68 ) $ (1.70 ) $ (2.91 )
Schedule of Antidilutive Securities Excluded from Calculation of Diluted Net Loss per Share	The following potential common stock equivalents, presented based on amounts outstanding at each period end, were excluded from the calculation of diluted net loss per share for the periods indicated because including them would have had an anti-dilutive effect: For the Year Ended December 31, 2021 2020 2019 Options to purchase shares of common stock 6,012,636 3,117,775 2,610,975 Unvested restricted stock units 509,694 918,395 245,100 Unvested shares of common stock — 70,052 352,952 6,522,330 4,106,222 3,209,027

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2021
Income Tax Disclosure [Abstract]
Schedule of Reconciliation of Income Taxes Computed by U.S. Federal Statutory Rate	A reconciliation of income taxes computed using the U.S. federal statutory rate to that reflected in operations as of December 31, 2021 and 2020 is as follows: December 31, 2021 December 31, 2020 Income tax computed at federal statutory tax rate 21.0 % 21.0 % State taxes, net of federal benefit 6.4 % 6.0 % Permanent differences 0.5 % (0.4 )% Tax credits 12.1 % 9.6 % Change in deferred tax rate 0.3 % 0.0 % Stock compensation cancelations (1.9 )% 0.0 % Other 0.1 % (0.4 )% Valuation allowance (38.5 )% (35.8 )% 0.0 % 0.0 %
Schedule of Net Deferred Tax Assets	Significant components of the Company’s net deferred tax assets as of December 31, 2021 and 2020 are as follows: December 31, 2021 December 31, 2020 Deferred tax assets: Tax loss carryforwards $ 60,405 $ 53,321 Tax credit carryforwards 36,905 28,154 Deferred expenses 420 1,200 Accrued expenses 815 994 Stock compensation 7,821 5,909 Intangible assets 19,919 12,034 Depreciation 224 - Other 2,373 144 Total deferred tax assets 128,882 101,756 Valuation allowance (128,570 ) (100,740 ) Deferred tax liabilities: Right of use asset (312 ) (972 ) Depreciation — (44 ) Total deferred tax liabilities (312 ) (1,016 ) Net deferred taxes $ - $ -

Restructuring (Tables)

Restructuring (Tables)	12 Months Ended
	Dec. 31, 2021
Restructuring And Related Activities [Abstract]
Schedule of Total Amount Expected to be Incurred and Liability Related to Restructuring	The following table shows the total amount expected to be incurred and the liability related to the 2020 restructuring for the years ended December 31, 2021 and December 31, 2020: One-Time Employee Termination Benefits Accrued restructuring costs as of December 31, 2019 $ - Restructuring charges incurred during the period 1,944 Amounts paid during the period (1,882 ) Accrued restructuring costs as of December 31, 2020 $ 62 Restructuring charges incurred during the period - Amounts paid during the period (62 ) Accrued restructuring costs as of December 31, 2021 $ -

Nature of the Business and Ba_2

Nature of the Business and Basis of Presentation - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Net loss	$ 72,188	$ 88,290	$ 117,223
Accumulated deficit	476,757	404,569
Net cash used in operating activities	77,764	56,599	92,714
Cash and cash equivalents	119,136	$ 154,744	$ 76,043	$ 86,366
Cash, cash equivalents and available-for-sale securities	$ 207,779

Summary of Significant Accoun_4

Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($)	1 Months Ended	12 Months Ended
	Nov. 30, 2020	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019	Oct. 31, 2020	Dec. 31, 2018
Significant Accounting Policies [Line Items]
Restricted cash deposit		$ 2,070,000	$ 327,000
Restricted cash noncurrent		2,070,000	327,000	$ 327,000		$ 327,000
Income tax expense (benefit)		$ 0	$ 0
Preferred stock, shares issued		0	0
Preferred stock, shares outstanding		0	0
Other income		$ 2,000	$ 1,000	515,000
Common stock, shares issued		110,340,281	84,893,994
Payment of exchange fees	$ 700,000
Common stock shares issued warrant to purchase	30,000
Common stock warrants exercise price per share	$ 3.29
Agreement termination period	60 days
Agreement written notice period	30 days
Funding From Charitable Organizations [Member]
Significant Accounting Policies [Line Items]
Other income		$ 0	$ 1,000	$ 515,000
Collaborative Arrangement, Transaction with Party to Collaborative Arrangement [Member] | Ultragenyx [Member]
Significant Accounting Policies [Line Items]
Common stock, shares issued					7,825,797
ASC Topic 606 [Member]
Significant Accounting Policies [Line Items]
Change in accounting principle, accounting standards update, adopted		true
Laboratory Equipment [Member]
Significant Accounting Policies [Line Items]
Useful life of asset		5 years
Computer Equipment [Member]
Significant Accounting Policies [Line Items]
Useful life of asset		3 years
Computer Software [Member]
Significant Accounting Policies [Line Items]
Useful life of asset		2 years
Furniture and Office Equipment [Member]
Significant Accounting Policies [Line Items]
Useful life of asset		5 years
Leasehold Improvements [Member]
Significant Accounting Policies [Line Items]
Useful life of asset		shorter of the lease term or the estimated useful life of the related asset

Summary of Significant Accoun_5

Summary of Significant Accounting Policies - Reconciliation of Amounts From Cash Flow Statement to Balance Sheet (Detail) - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Reconciliation Of Beginning And End Of Period Amounts From Cash Flow Statement To Balance Sheet [Abstract]
Cash and cash equivalents	$ 119,136	$ 154,744	$ 76,043	$ 86,366
Restricted cash, non-current	2,070	327	327	327
Cash and cash equivalents and restricted cash	$ 121,206	$ 155,071	$ 76,370	$ 86,693

Collaborations - Additional Inf

Collaborations - Additional Information (Detail) - USD ($)	Oct. 22, 2020	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Proceeds from issuance of common stock		$ 134,878,000	$ 113,208,000	$ 49,325,000
Ultragenyx [Member] | Collaborative Arrangement, Transaction with Party to Collaborative Arrangement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Milestone payments upon achievement of regulatory milestone		$ 65,000,000
Percentage of option to fund of development cost		30.00%
Percentage of net income and net losses on net sales		30.00%
Agreement termination notice period		90 days
Issued and sold shares of common stock	7,825,797
Common stock issued price per share	$ 5.1113
Proceeds from issuance of common stock	$ 40,000,000
Percentage of outstanding common stock	14.45%	7.10%
Proceeds from equity investment under stock purchase agreement		$ 22,513,000
Transaction price		0
Related party revenue recognized		13,620,000	0
Related party transaction, payment		0	0
Due from related parties		110,000	$ 0
Ultragenyx [Member] | Collaborative Arrangement, Transaction with Party to Collaborative Arrangement [Member] | Maximum [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Milestone payments upon achievement of development milestone		25,000,000
Milestone Payments upon achievement of net sales milestones		$ 165,000,000
Ultragenyx [Member] | Collaborative Arrangement, Transaction with Party to Collaborative Arrangement [Member] | Minimum [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Percentage of outstanding common stock		10.00%

Collaborations - Summary of Cha

Collaborations - Summary of Changes in Balances of Company's Related Party Collaboration Receivables and Contract Liabilities (Detail) $ in Thousands	12 Months Ended
	Dec. 31, 2021USD ($)
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Related party collaboration receivable, Ending balance	$ 110
Ultragenyx [Member] | Collaborative Arrangement, Transaction with Party to Collaborative Arrangement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Related party collaboration receivable, Additions	982
Related party collaboration receivable, Deductions	(872)
Related party collaboration receivable, Ending balance	110
Deferred revenue, Beginning balance	20,718
Deferred revenue, Additions	982
Deferred revenue, Deductions	(13,620)
Deferred revenue, Ending balance	$ 8,080

Fair Value of Financial Asset_3

Fair Value of Financial Assets and Liabilities - Summary of Assets and Liabilities Measured at Fair Value on Recurring Basis and Indicate Level of Fair Value Hierarchy Utilized (Detail) - USD ($)	Dec. 31, 2021	Dec. 31, 2020
Assets:
Cash equivalents	$ 75,224,000	$ 84,462,000
Available-for-sale securities	88,643,000	0
Assets Fair Value Disclosure	163,867,000	84,462,000
Level 2 [Member]
Assets:
Cash equivalents	75,224,000	84,462,000
Available-for-sale securities	88,643,000
Assets Fair Value Disclosure	$ 163,867,000	$ 84,462,000

Fair Value of Financial Asset_4

Fair Value of Financial Assets and Liabilities - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020
Fair Value Disclosures [Abstract]
Available-for-sale securities	$ 88,643,000	$ 0
Fair value, assets, level 1 to level 2 transfers, amount	0
Fair value, assets, level 2 to level 1 transfers, amount	0
Fair value, assets, transfers into level 3, amount	0
Fair value, assets, transfers out of level 3, amount	$ 0

Available-for-Sale Securities -

Available-for-Sale Securities - Schedule of Fair Value of Available-for-Sale Debt Securities by Type of Security (Detail) - USD ($)	Dec. 31, 2021	Dec. 31, 2020
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 88,688,000
Gross Unrealized Loss	(45,000)
Fair Value	88,643,000	$ 0
Treasury Bill [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	2,800,000
Fair Value	2,800,000
Corporate Bond Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	83,889,000
Gross Unrealized Loss	(45,000)
Fair Value	83,844,000
Commercial Paper [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,999,000
Fair Value	$ 1,999,000

Available-for-Sale Securities_2

Available-for-Sale Securities - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020
Investments Debt And Equity Securities [Abstract]
Available-for-sale securities	$ 88,643,000	$ 0
Available-for-sale securities average maturity period	8 months 12 days

Available-for-Sale Securities_3

Available-for-Sale Securities - Summary Estimated Fair Value and Amortized Cost of Available-for-Sale Securities by Contractual Maturity (Detail) - USD ($)	Dec. 31, 2021	Dec. 31, 2020
Investments Debt And Equity Securities [Abstract]
Due in one year or less, amortized cost	$ 88,688,000
Amortized Cost	88,688,000
Due in one year or less, fair value	88,643,000
Total available-for-sale securities, fair value	$ 88,643,000	$ 0

Prepaid Expenses and Other Cu_3

Prepaid Expenses and Other Current Assets - Schedule of Prepaid Expenses and Other Current Assets (Detail) - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020
Deferred Costs Capitalized Prepaid And Other Assets Disclosure [Abstract]
Prepaid research and development expenses	$ 6,015	$ 2,674
Prepaid expenses and other assets	8,708	1,483
Prepaid expenses and other current assets	$ 14,723	$ 4,157

Property and Equipment - Summar

Property and Equipment - Summary of Property and Equipment (Detail) - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 18,123	$ 16,866
Less accumulated depreciation	11,661	8,713
Property and equipment, net	6,462	8,153
Furniture and Fixtures [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	212	203
Laboratory Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	10,719	9,631
Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	4,713	4,713
Computer Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	436	436
Computer Software [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	553	553
Construction in Progress [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 1,490	$ 1,330

Property and Equipment - Additi

Property and Equipment - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Property Plant And Equipment [Abstract]
Depreciation expense	$ 2,964	$ 3,922	$ 2,824

Accrued Expenses - Summary of A

Accrued Expenses - Summary of Accrued Expenses and Other Current Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020
Accrued Expenses And Other Current Liabilities [Line Items]
Accrued expenses and other current liabilities	$ 9,528	$ 8,640
Accrued Research and Development [Member]
Accrued Expenses And Other Current Liabilities [Line Items]
Accrued expenses and other current liabilities	1,507	2,091
Accrued Compensation [Member]
Accrued Expenses And Other Current Liabilities [Line Items]
Accrued expenses and other current liabilities	3,084	3,834
Accrued Other [Member]
Accrued Expenses And Other Current Liabilities [Line Items]
Accrued expenses and other current liabilities	$ 4,937	$ 2,715

Equity-Based Compensation - Add

Equity-Based Compensation - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	Jun. 16, 2021	Jun. 16, 2020	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock options outstanding, weighted average recognition period			8 years 5 months 26 days	7 years 9 months 10 days
Stock Option [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized equity-based compensation cost related to stock option			$ 15,606
Stock options outstanding, weighted average recognition period			2 years 7 months 2 days
Stock options outstanding, intrinsic value			$ 0	$ 910
Intrinsic value of stock options exercisable			0
Intrinsic value of stock options exercised			$ 41
Weighted average fair value of options to purchase common stock granted			$ 5.05	$ 2.69
'Restricted Stock Units (RSUs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized equity-based compensation			$ 607
Unrecognized equity-based compensation cost, weighted average period			1 year 8 months 12 days
Restricted Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized equity-based compensation			$ 0
Aggregate intrinsic value of restricted common units vested			$ 199	$ 522	$ 135
Maximum [Member] | 'Restricted Stock Units (RSUs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period			2 years
Minimum [Member] | 'Restricted Stock Units (RSUs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period			1 year
2018 Omnibus Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares authorized for common stock for equity awards			5,001,000
2020 Equity Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares authorized for common stock for equity awards		3,000,000
Number of additional shares authorized for common stock for equity awards	7,000,000
Number of shares available for future issuance			8,557,152
2020 Equity Incentive Plan [Member] | Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of additional shares authorized for common stock for equity awards		4,879,025
2021 Employee Stock Purchase Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares available for future issuance	1,102,885		1,058,204

Equity-Based Compensation - Sum

Equity-Based Compensation - Summary of Stock Option Activity (Detail) - $ / shares	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020
Number of Options
Outstanding at December 31, 2020	3,117,775
Granted	3,708,345
Exercised	(11,625)
Forfeitures	(801,859)
Outstanding at December 31, 2021	6,012,636	3,117,775
Vested and expected to vest as of December 31, 2021	6,012,636
Exercisable at December 31, 2021	1,939,881
Weighted Average Exercise Price
Outstanding at December 31, 2020	$ 14.58
Granted	5.92
Exercised	3.55
Forfeitures	14.85
Outstanding at December 31, 2021	9.23	$ 14.58
Vested and expected to vest as of December 31, 2021	9.23
Exercisable at December 31, 2021	$ 13.50
Remaining Contractual Life (in years)
Outstanding	8 years 5 months 26 days	7 years 9 months 10 days
Vested and expected to vest as of December 31, 2021	8 years 5 months 26 days
Exercisable at December 31, 2021	7 years 7 months 17 days

Equity-Based Compensation - Ass

Equity-Based Compensation - Assumptions for Measuring Fair Values of Stock Options (Detail)	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected volatility, Minimum	115.50%	21.30%	85.10%
Expected volatility, Maximum	123.90%	119.50%	104.70%
Expected dividends	0.00%	0.00%	0.00%
Risk-free rate, Minimum	0.40%	0.60%	1.50%
Risk-free rate, Maximum	1.40%	3.40%	2.60%
Minimum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term (in years)	5 years 1 month 6 days	4 years 6 months	5 years 1 month 6 days
Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term (in years)	6 years 3 months	10 years	6 years 3 months

Equity-Based Compensation - S_2

Equity-Based Compensation - Summary of Restricted Stock Unit Activity (Detail) - 'Restricted Stock Units (RSUs) [Member]	12 Months Ended
	Dec. 31, 2021$ / sharesshares
Units
Unvested at December 31, 2020 | shares	918,395
Granted | shares	220,750
Vested | shares	(419,193)
Forfeitures | shares	(210,258)
Outstanding at December 31, 2021 | shares	509,694
Unvested as of December 31, 2021 | shares	509,694
Weighted-Average Grant Date Fair Value
Unvested at December 31, 2020 | $ / shares	$ 3.17
Granted | $ / shares	2.25
Vested | $ / shares	3.21
Forfeitures | $ / shares	3.13
Outstanding at December 31, 2021 | $ / shares	2.75
Unvested as of December 31, 2021 | $ / shares	$ 2.75

Equity-Based Compensation - S_3

Equity-Based Compensation - Summary of Unvested Restricted Shares of Common Stock Activity (Detail) - Restricted Stock [Member]	12 Months Ended
	Dec. 31, 2021$ / sharesshares
Units
Unvested at December 31, 2020 | shares	70,052
Releases | shares	(40,840)
Forfeitures | shares	(29,212)
Weighted-Average Grant Date Fair Value
Unvested at December 31, 2020 | $ / shares	$ 9.83
Releases | $ / shares	10.50
Forfeitures | $ / shares	$ 8.89

Equity-Based Compensation - Sch

Equity-Based Compensation - Schedule of Equity-based Compensation Expense (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock-based compensation expense	$ 13,373	$ 11,629	$ 14,207
Research and Development Expenses [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock-based compensation expense	6,289	5,822	8,006
General and Administrative Expenses [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock-based compensation expense	$ 7,084	4,956	$ 6,201
Restructuring Expense [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock-based compensation expense		$ 851

Leases - Additional Information

Leases - Additional Information (Detail) $ in Thousands	1 Months Ended			12 Months Ended
	Jun. 30, 2021USD ($)ft²	Jan. 31, 2019USD ($)	Jan. 31, 2018USD ($)ft²	Dec. 31, 2021USD ($)ft²	Dec. 31, 2020USD ($)	Dec. 31, 2019USD ($)
Commitment And Contingencies [Line Items]
Future minimum rent commitment				$ 1,634
Future minimum lease commitment				576
Lease liability				1,538
Operating lease, right of use asset				1,142	$ 3,579
Rent expense				2,568	$ 2,562	$ 2,500
Cambridge, Massachusetts [Member] | Lab Equipment [Member]
Commitment And Contingencies [Line Items]
Lease term of contract					48 months
Future minimum lease commitment				576
Lab Space [Member] | Cambridge, Massachusetts [Member]
Commitment And Contingencies [Line Items]
Area of lease agreement | ft²			9,500
Lease term			5 years
Lessee, Operating Lease, Existence of Option to Extend [true false]			true
Lease renewal term			2 years
Future minimum rent commitment			$ 1,090	$ 1,090
Office Space [Member] | Cambridge, Massachusetts [Member]
Commitment And Contingencies [Line Items]
Area of lease agreement | ft²			16,000	5,000
Future minimum rent commitment				$ 544
Lease expiration period		2025-10	2022-02
Operating lease, right of use asset		$ 1,233
Office Space [Member] | MOROCCO
Commitment And Contingencies [Line Items]
Lease liability		$ 1,314
Office laboratory [Member] | Hood Park LLC [Member]
Commitment And Contingencies [Line Items]
Area of lease agreement | ft²	49,869
Lease term	10 years
Lease renewal term	5 years
Lease description				The Company intends to relocate its corporate headquarters to the Premises in May 2022. The term of the lease commences on the later of (i) the date the Landlord delivers the Premises to the Company or (ii) the earlier of (a) the date the Company’s work on the Premises is substantially completed, (b) the date the Company commences business operations in the Premises, or (c) the one hundred twentieth (120th) day following the Landlord’s satisfaction of item (i) above.
Operating lease, monthly payments	$ 305
Operating lease payments annual escalation percentage	3.00%
Construction allowance	$ 10,223
Letter of credit amount				$ 1,833
Construction costs				$ 5,833

Leases - Future Minimum Payment

Leases - Future Minimum Payments for Operating and Finance Leases (Detail) $ in Thousands	Dec. 31, 2021USD ($)
Finance Leases
2022	$ 276
2023	300
Total future minimum lease payments	576
Less: Imputed Interest	51
Total Lease Liabilities	525
Operating Leases
2022	1,355
2023	279
Total future minimum lease payments	1,634
Less: Imputed Interest	96
Lease liability	$ 1,538

Leases - Supplemental Disclosur

Leases - Supplemental Disclosure of Cash Flow Information Related to Leases (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Other information
Cash paid for amounts included in the measurement of lease liabilities	$ 2,408	$ 2,408	$ 2,288
Operating lease liabilities arising from obtaining right-of-use-assets	$ 310	$ 1,629	$ 1,629
Weighted-average remaining lease term (in years)
Operating lease	1 year	2 years 9 months 18 days	3 years 6 months
Finance lease	2 years 3 months 18 days	2 years 3 months 18 days	3 years 6 months
Weighted-average discount rate
Operating lease	11.90%	12.60%	12.50%
Finance lease	10.70%	10.70%	10.70%

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Detail) - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020
Office and Laboratory Lease [Member]
Commitment And Contingencies [Line Items]
Letter of credit outstanding amount	$ 2,070	$ 327

License Agreements - Additional

License Agreements - Additional Information (Detail) - USD ($)	1 Months Ended			12 Months Ended
	Feb. 28, 2022	Feb. 28, 2021	Oct. 31, 2020	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2016	Dec. 31, 2015
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Research and development expense					$ 58,739,000	$ 64,881,000	$ 94,737,000
University of Washington [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Maximum milestone payment to be made									$ 1,000,000
Milestone payment upon achievement of certain milestones			$ 3,400,000
Amount payable in connection with execution of collaboration agreement			375,000
Research and development expense					60,000	446,000	38,000
University of Missouri [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Maximum milestone payment to be made									$ 1,000,000
Milestone payment upon achievement of certain milestones			$ 1,900,000
Research and development expense					195,000	2,111,000	23,000
Milestone payment		$ 750,000
University of Missouri [Member] | Subsequent Event [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Milestone payment	$ 1,300,000
University of Missouri [Member] | Ultragenyx [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Amount payable in connection with execution of collaboration agreement					750,000
University of Missouri [Member] | Forecast | Ultragenyx [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Amount payable in connection with execution of collaboration agreement				$ 1,300,000
University of Michigan [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Maximum milestone payment to be made								$ 1,000,000
Research and development expense					37,000	35,000	39,000
President and Fellows of Harvard College [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Research and development expense					$ 10,000	0	10,000
License agreement initial term					15 years
License agreement renewal term					3 years
Harvard College [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Research and development expense					$ 5,000	$ 0	$ 5,000
License agreement initial term					15 years
License agreement renewal term					3 years

Net Loss per Share - Schedule o

Net Loss per Share - Schedule of Basic and Diluted Net Loss per Share (Detail) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Earnings Per Share Basic [Line Items]
Net loss	$ (72,188)	$ (88,290)	$ (117,223)
Weighted average common stock outstanding, basic and diluted	106,770,372	51,937,147	40,289,290
Net loss per share, basic and diluted	$ (0.68)	$ (1.70)	$ (2.91)
Common Stock [Member]
Earnings Per Share Basic [Line Items]
Weighted average common stock outstanding, basic and diluted	104,612,043	49,675,603	39,326,983
Pre-Funded Warrants [Member]
Earnings Per Share Basic [Line Items]
Weighted average common stock outstanding, basic and diluted	2,158,329	2,261,544	962,307

Net Loss per Share - Schedule_2

Net Loss per Share - Schedule of Antidilutive Securities Excluded from Calculation of Diluted Net Loss per Share (Detail) - shares	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Common stock equivalents excluded from calculation of diluted net loss per share	6,522,330	4,106,222	3,209,027
Options to Purchase Shares of Common Stock [Member]
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Common stock equivalents excluded from calculation of diluted net loss per share	6,012,636	3,117,775	2,610,975
Unvested Restricted Stock Units [Member]
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Common stock equivalents excluded from calculation of diluted net loss per share	509,694	918,395	245,100
Unvested Shares of Common Stock [Member]
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Common stock equivalents excluded from calculation of diluted net loss per share		70,052	352,952

Income Taxes - Additional Infor

Income Taxes - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020
Income Taxes [Line Items]
Income tax expense (benefit)	$ 0	$ 0
Uncertain tax position	0
Unrecognized tax benefits	0	0
Tax interest and penalties	0	$ 0
Domestic Tax Authority [Member]
Income Taxes [Line Items]
Net operating loss carryforwards	219,928,000
Tax credits carryforwards	$ 34,606,000
Tax credit carryforwards, expiration period	2033
Percentage of annual deduction equal to annual taxable income	80.00%
State and Local Jurisdiction [Member]
Income Taxes [Line Items]
Net operating loss carryforwards	$ 218,052,000
Operating loss carryforwards, expiration period	2038
Tax credits carryforwards	$ 2,910,000
Tax credit carryforwards, expiration period	2033

Income Taxes - Schedule of Reco

Income Taxes - Schedule of Reconciliation of Income Taxes Computed by U.S. Federal Statutory Rate (Detail)	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020
Income Tax Disclosure [Abstract]
Income tax computed at federal statutory tax rate	21.00%	21.00%
State taxes, net of federal benefit	6.40%	6.00%
Permanent differences	0.50%	(0.40%)
Tax credits	(12.10%)	(9.60%)
Change in deferred tax rate	0.30%	0.00%
Stock compensation cancelations	(1.90%)	0.00%
Other	0.10%	(0.40%)
Valuation allowance	(38.50%)	(35.80%)
Total	0.00%	0.00%

Income Taxes - Schedule of Net

Income Taxes - Schedule of Net Deferred Tax Assets (Detail) - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020
Deferred tax assets:
Tax loss carryforwards	$ 60,405	$ 53,321
Tax credit carryforwards	36,905	28,154
Deferred expenses	420	1,200
Accrued expenses	815	994
Stock compensation	7,821	5,909
Intangible assets	19,919	12,034
Depreciation	224
Other	2,373	144
Total deferred tax assets	128,882	101,756
Valuation allowance	(128,570)	(100,740)
Deferred tax liabilities:
Right of use asset	(312)	(972)
Depreciation		(44)
Total deferred tax liabilities	(312)	(1,016)
Net deferred taxes	$ 0	$ 0

Defined Contribution Plan - Add

Defined Contribution Plan - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Compensation And Retirement Disclosure [Abstract]
Company's contribution to the plan	$ 241,000	$ 0	$ 0

Restructuring- Additional Infor

Restructuring- Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2021	Dec. 31, 2021	Dec. 31, 2020
Restructuring Costs [Abstract]
Restructuring charges were paid	$ 62		$ 1,882
Number of headcount reduced, percentage		30.00%
Restructuring charges related to severance payment and other employee-related costs			$ 1,944

Restructuring - Schedule of Tot

Restructuring - Schedule of Total Amount Expected to be Incurred and Liability Related to Restructuring (Detail) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2021	Dec. 31, 2021	Dec. 31, 2020
Restructuring Costs [Line Items]
Restructuring charges incurred during the period			$ 1,944
Amounts paid during the period	$ (62)		(1,882)
One-Time Employee Termination Benefits [Member]
Restructuring Costs [Line Items]
Accrued restructuring costs, beginning balance	$ 62	$ 62
Restructuring charges incurred during the period			1,944
Amounts paid during the period		$ (62)	(1,882)
Accrued restructuring costs, ending balance			$ 62