Solid Biosciences (SLDB) 10-K 13 Mar 19 2018 FY Annual report Financial data

Document and Entity Information

Document and Entity Information - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Feb. 15, 2019	Jun. 29, 2018
Document And Entity Information [Abstract]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2018
Document Fiscal Year Focus	2018
Document Fiscal Period Focus	FY
Trading Symbol	sldb
Entity Registrant Name	Solid Biosciences Inc.
Entity Central Index Key	0001707502
Current Fiscal Year End Date	--12-31
Entity Filer Category	Non-accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	true
Entity Ex Transition Period	true
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		35,422,915
Entity Public Float			$ 598.7
Entity Well-known Seasoned Issuer	No
Entity Voluntary Filers	No
Entity Current Reporting Status	No

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents	$ 86,366	$ 52,080
Available-for-sale securities	36,098	17,014
Prepaid expenses and other current assets	6,175	1,499
Restricted cash		65
Total current assets	128,639	70,658
Property and equipment, net	10,422	2,429
Other non-current assets	209
Restricted cash	327
Deferred offering costs	0	3,106
Total assets	139,597	76,193
Current liabilities:
Accounts payable	3,691	5,066
Accrued expenses	8,235	5,972
Current portion of capital lease obligations	173
Other current liabilities	382	233
Total current liabilities	12,481	11,271
Capital lease obligations, net of current portion	859
Other non-current liabilities	1,074
Total liabilities	14,414	11,271
Commitments and Contingencies (Note 13)
Preferred Units
Preferred Units	0	0
Stockholders’ / Members’ Equity / (Deficit):
Series A, B, C and D Common Units, no units authorized at December 31, 2018 and 20,189,509 units authorized at December 31, 2017; no units issued and outstanding at December 31, 2018 and 19,438,552 units issued and outstanding at December 31, 2017		65,014
Preferred stock, $0.001 par value; 10,000,000 shares authorized at December 31, 2018 and no shares authorized at December 31, 2017; no shares issued and outstanding at December 31, 2018 and December 31, 2017	0	0
Common stock, $0.001 par value; 300,000,000 shares authorized at December 31, 2018 and no shares authorized at December 31, 2017; 35,432,460 shares issued and outstanding at December 31, 2018 and no shares issued and outstanding at December 31, 2017	35
Additional Paid-in Capital	324,209
Accumulated other comprehensive loss	(5)	(13)
Accumulated deficit	(199,056)	(124,258)
Total stockholders'/members’ equity/(deficit)	125,183	(59,257)
Total liabilities, preferred units and stockholders'/members’ equity	139,597	76,193
Series 2 Senior Preferred Units [Member]
Preferred Units
Preferred Units	0	55,002
Series 1 Senior Preferred Units [Member]
Preferred Units
Preferred Units	0	25,000
Junior Preferred Units [Member]
Preferred Units
Preferred Units	$ 0	$ 44,177

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2018	Dec. 31, 2017
Series A, B, C and D common units, authorized	0	20,189,509
Series A, B, C and D common units, issued	0	19,438,552
Series A, B, C and D common units, outstanding	0	19,438,552
Preferred stock, par value	$ 0.001	$ 0.001
Preferred stock, shares authorized	10,000,000	0
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	0
Common stock, shares issued	35,432,460	0
Common stock, shares outstanding	35,432,460	0
Series 2 Senior Preferred Units [Member]
Preferred units, authorized	0	4,886,000
Preferred units, issued	0	4,886,000
Preferred units, outstanding	0	4,886,000
Series 1 Senior Preferred Units [Member]
Preferred units, authorized	0	2,500,000
Preferred units, issued	0	2,500,000
Preferred units, outstanding	0	2,500,000
Junior Preferred Units [Member]
Preferred units, authorized	0	4,414,356
Preferred units, issued	0	4,414,356
Preferred units, outstanding	0	4,414,356

Consolidated Statements of Oper

Consolidated Statements of Operations - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Statement [Abstract]
Revenue	$ 0	$ 0	$ 0
Operating expenses:
Research and development	57,965	39,905	20,116
General and administrative	17,722	14,952	5,460
Total operating expenses	75,687	54,857	25,576
Loss from operations	(75,687)	(54,857)	(25,576)
Other income (expense):
Revaluation of preferred unit tranche rights		459	1,163
Interest income	619	219	369
Other income	270	1,001	271
Total other income (expense), net	889	1,679	1,803
Net loss	(74,798)	(53,178)	(23,773)
Net loss attributable to non-controlling interest	0	(1,060)	(2,234)
Net loss attributable to Solid Biosciences Inc.	(74,798)	(52,118)	(21,539)
Decretion (accretion) of preferred units to redemption value		(959)	4,309
Redemption of preferred units		15,685
Redemption of redeemable interest from non-controlling interest in Solid GT		(1,925)
Net loss attributable to common stockholders	$ (74,798)	$ (39,317)	$ (17,230)
Net loss per share attributable to common stockholders, basic and diluted	$ (2.25)	$ (2.88)	$ (10.14)
Weighted average common stock outstanding, basic and diluted	33,262,597	13,649,485	1,698,904

Consolidated Statements of Comp

Consolidated Statements of Comprehensive Loss - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Statement Of Income And Comprehensive Income [Abstract]
Net loss	$ (74,798)	$ (53,178)	$ (23,773)
Other comprehensive loss:
Unrealized gain (loss) on available-for-sale securities	8	(36)	33
Comprehensive loss	(74,790)	(53,214)	(23,740)
Comprehensive loss attributable to non-controlling interest		(1,060)	(2,234)
Comprehensive loss attributable to Solid Biosciences Inc.	$ (74,790)	$ (52,154)	$ (21,506)

Consolidated Statements of Pref

Consolidated Statements of Preferred Units and Stockholders' /Members' Equity/(Deficit) - USD ($) $ in Thousands	Total	Series A Common Units in Exchange for Redeemable Preferred Units [Member]	Series D Common Units [Member]	Series A, B, C and D Common Units [Member]	Series A, B, C and D Common Units [Member]Series B Common Units in exchange for Series A Common Units [Member]	Series A, B, C and D Common Units [Member]Series D Common Units in Exchange for Series A Common Units [Member]	Series A, B, C and D Common Units [Member]Series A Common Units in Exchange for Redeemable Preferred Units [Member]	Series A, B, C and D Common Units [Member]Series C Common Units in Exchange for Class B Non-controlling Interest in Solid GT [Member]	Series A, B, C and D Common Units [Member]Series D Common Units in Exchange for Class C Non-controlling Interest in Solid GT [Member]	Series A, B, C and D Common Units [Member]Series D Common Units [Member]	Common Stock [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Accumulated Deficit [Member]	Accumulated Deficit [Member]Series D Common Units [Member]	Additional Paid In Capital [Member]	Total Members'/ Stockholders' Equity (Deficit) [Member]	Total Members'/ Stockholders' Equity (Deficit) [Member]Series A Common Units in Exchange for Redeemable Preferred Units [Member]	Total Members'/ Stockholders' Equity (Deficit) [Member]Series D Common Units [Member]	Total Members'/ Stockholders' Equity (Deficit) [Member]Series C Common Units in Exchange for Class B Non-controlling Interest in Solid GT [Member]	Total Members'/ Stockholders' Equity (Deficit) [Member]Series D Common Units in Exchange for Class C Non-controlling Interest in Solid GT [Member]	Noncontrolling Interest [Member]	Noncontrolling Interest [Member]Series D Common Units [Member]	Noncontrolling Interest [Member]Series C Common Units in Exchange for Class B Non-controlling Interest in Solid GT [Member]	Noncontrolling Interest [Member]Series D Common Units in Exchange for Class C Non-controlling Interest in Solid GT [Member]	Redeemable Preferred Units [Member]	Redeemable Preferred Units [Member]Series A Common Units in Exchange for Redeemable Preferred Units [Member]	Series 2 Senior Preferred Units [Member]	Series 1 Senior Preferred Units [Member]	Junior Preferred Units [Member]
Beginning balance at Dec. 31, 2016	$ (37,886)			$ 558								$ 23	$ (84,941)			$ (84,360)					$ 46,474
Beginning balance, units at Dec. 31, 2016																									17,100,000
Beginning balance at Dec. 31, 2016																									$ 71,649
Beginning balance, units at Dec. 31, 2016				5,123,917
Issuance of preferred units, value																											$ 55,002	$ 24,041	$ 44,177
Issuance of preferred units, units																											4,886,000	2,500,000	4,414,356
Accretion of Series 1 senior preferred units to redemption value	(959)												(959)			(959)												$ 959
Redemption of preferred units	15,685												15,685			15,685
Redemption of preferred units																									(15,685)
Equity-based compensation	5,330			$ 5,030												5,030					300
Net loss	(53,178)												(52,118)			(52,118)					$ (1,060)
Issuance of common units in exchange for common units					(1,301,520)	(160,954)
Conversion of units into shares of common stock, value		$ 55,964					$ 55,964	$ 2,053	$ 1,409								$ 55,964		$ 2,053	$ 1,409			$ (2,053)	$ (1,409)
Issuance of Series A common units in exchange for redeemable preferred units, units																										17,100,000
Issuance of Series A common units in exchange for redeemable preferred units, value																										$ (55,964)
Conversion of units into shares of common stock, units							12,219,299	1,635,916	1,083,205
Issuance of junior preferred units in redemption of Class D non-controlling interest in Solid GT			$ (44,177)											$ (1,925)				$ (1,925)				$ (42,252)
Issuance of Series D common units										838,689
Unrealized loss on available for sale securities	(36)											(36)				(36)
Ending balance at Dec. 31, 2017	(59,257)			$ 65,014								(13)	(124,258)			(59,257)
Ending balance, units at Dec. 31, 2017																											4,886,000	2,500,000	4,414,356
Ending balance at Dec. 31, 2017	$ 0																										$ 55,002	$ 25,000	$ 44,177
Ending balance, units at Dec. 31, 2017	19,438,552			19,438,552
Ending balance at Dec. 31, 2017	$ (59,257)
Issuance of common stock upon initial public offering, net of issuance costs of $4,592	129,096										$ 9				$ 129,087	129,096
Issuance of common stock upon initial public offering, net of issuance costs of $4,592, shares											8,984,375
Equity-based compensation	5,955			$ 166											5,789	5,955
Net loss	(74,798)												(74,798)			(74,798)
Repurchase of common units/ shares of common stock				(8,932)							(50,474)
Conversion of units into shares of common stock, value	124,179			$ (65,180)							$ 26				189,333	124,179											$ (55,002)	$ (25,000)	$ (44,177)
Conversion of units into shares of common stock, units				(19,429,620)							26,498,559																(4,886,000)	(2,500,000)	(4,414,356)
Unrealized loss on available for sale securities	8											8				8
Ending balance, units at Dec. 31, 2018																											0	0	0
Ending balance at Dec. 31, 2018	$ 0																								$ 0		$ 0	$ 0	$ 0
Ending balance, units at Dec. 31, 2018	0
Ending balance at Dec. 31, 2018	$ 125,183										$ 35	$ (5)	$ (199,056)		$ 324,209	$ 125,183
Ending balance at Dec. 31, 2018											35,432,460

Consolidated Statements of Pr_2

Consolidated Statements of Preferred Units and Stockholders' /Members' Equity/(Deficit) (Parenthetical) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Net issuance costs, issuance of common stock upon initial public offering	$ 4,592
Series 1 Senior Preferred Units [Member]
Issuance costs		$ 500
Tranche right		$ 459

Condensed Consolidated Statemen

Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Cash flows from operating activities:
Net loss	$ (74,798)	$ (53,178)	$ (23,773)
Adjustments to reconcile net loss to net cash used in operating activities:
Amortization of (discount)/premium on available-for-sale securities	(90)	206	505
Equity-based compensation expense	5,955	5,330	1,470
Depreciation expense	1,566	448	56
(Gain) / loss on sale of property and equipment	4
Loss (gain) from revaluation of preferred unit tranche right		(459)	(1,163)
Changes in operating assets and liabilities:
Proceeds from landlord lease incentive for tenant improvements	823
Prepaid expenses and other current assets	(4,930)	815	(2,005)
Accounts payable	(1,610)	1,579	2,213
Accrued expenses and other current liabilities	2,883	2,035	2,577
Net cash used in operating activities	(70,197)	(43,224)	(20,120)
Cash flows from investing activities:
Purchases of property and equipment	(7,776)	(2,276)	(392)
Proceeds from sales and maturities of available for sale securities	25,335	31,621	22,035
Purchases of available for sale securities	(44,321)	(18,897)	(25,695)
Net cash (used in) provided by investing activities	(26,762)	10,448	(4,052)
Cash flows from financing activities:
Payment of deferred offering costs	(2,168)	(2,424)
Proceeds from issuance of redeemable preferred units			3,420
Proceeds from initial public offering of common stock, net of commissions and underwriting discounts	133,688
Principal payments under capital lease obligation	(13)
Net cash provided by financing activities	131,507	77,078	3,420
Net increase (decrease) in cash, cash equivalents and restricted cash	34,548	44,302	(20,752)
Cash, cash equivalents, and restricted cash at beginning of period	52,145	7,843	28,595
Cash, cash equivalents, and restricted cash at end of period	86,693	52,145	7,843
Supplemental disclosure of non-cash investing and financing activities:
Reclassification of preferred unit tranche liability to preferred units upon settlement			10,841
Decretion (accretion) of preferred units to redemption value		(959)	4,309
Redemption of preferred units		15,685
Redemption of redeemable interest from non-controlling interest in Solid GT		(1,925)
Deferred offering costs included in accounts payable and accrued expenses		682	47
Property and equipment included in accounts payable and accruals	952	265	$ 116
Property and equipment acquired through a capital lease	1,023
Series A, B, C and D Common Units [Member]
Supplemental disclosure of non-cash investing and financing activities:
Conversion of Units into common stock	65,180
Series D Common Units in Exchange for Series A Common Units [Member]
Supplemental disclosure of non-cash investing and financing activities:
Issuance of units		638
Series A Common Units in Exchange for Redeemable Preferred Units [Member]
Supplemental disclosure of non-cash investing and financing activities:
Issuance of units		55,964
Series C Common Units in Exchange for Class B Non-controlling Interest in Solid GT [Member]
Supplemental disclosure of non-cash investing and financing activities:
Issuance of units		2,053
Series D Common Units in Exchange for Class C Non-controlling Interest in Solid GT [Member]
Supplemental disclosure of non-cash investing and financing activities:
Issuance of units		1,409
Series 1 Senior Preferred Units [Member]
Cash flows from financing activities:
Proceeds from issuance of preferred units		24,500
Supplemental disclosure of non-cash investing and financing activities:
Conversion of Units into common stock	25,000
Series 2 Senior Preferred Units [Member]
Cash flows from financing activities:
Proceeds from issuance of preferred units		55,002
Supplemental disclosure of non-cash investing and financing activities:
Conversion of Units into common stock	55,002
Junior Preferred Units [Member]
Supplemental disclosure of non-cash investing and financing activities:
Conversion of Units into common stock	$ 44,177
Junior Preferred Units Upon Redemption of Class D Non-controlling Interest in Solid GT [Member]
Supplemental disclosure of non-cash investing and financing activities:
Issuance of units		$ 44,177

Nature of the Business and Basi

Nature of the Business and Basis of Presentation	12 Months Ended
	Dec. 31, 2018
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. In October 2013, the company changed its name to Solid Ventures, LLC and in June 2015, the company changed its name to Solid Biosciences, LLC. The company operated as a Delaware limited liability company under the name Solid Biosciences, LLC 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”) In addition, entities formed solely for the purpose of holding membership interests in the Company’s limited liability company were merged with and into the Company. As a result of the Corporate Conversion, all of the Series 1 and 2 Senior Preferred, Junior Preferred Units, Series A, B, C and D Common Units of Solid Biosciences, LLC converted into shares of common stock of Solid Biosciences Inc. on a one for 0.8485 basis and all of the unit holders of Solid Biosciences, LLC became holders of common stock of Solid Biosciences Inc. The Company’s mission is to cure Duchenne muscular dystrophy (“DMD”), 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 under development to restore functional dystrophin protein expression in patients’ muscles. SGT-001 has been granted Rare Pediatric Disease Designation and Fast Track 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 for SGT-001 in the United States during the fourth quarter of 2017 , which is called IGNITE DMD. 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 pre-clinical 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. Initial Public Offering in January 2018 On January 30, 2018, the Company completed its initial public offering with the sale of 8,984,375 shares of common stock, including shares of common stock issued upon the exercise in full of the underwriters’ over-allotment option, at a public offering price of $16.00 per share, resulting in net proceeds of $129,096, after deducting underwriting discounts and commissions and offering expenses. 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, 2018, the Company has funded its operations primarily with proceeds from the sale of redeemable preferred units and member units and its recently completed initial public offering. In accordance with 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, 2018, the Company had an accumulated deficit of $199,056. During the year ended December 31, 2018, the Company incurred a net loss of $74,798 and used $70,197 of cash in operations. The Company expects to continue to generate operating losses in the foreseeable future. The Company currently expects that its cash, cash equivalents and available-for-sale securities of $122,464 will be sufficient to fund its operating expenses and capital requirements, based upon its current operating plan, into early 2020. In accordance with the requirements of ASC 205-40, the Company determined that there is substantial doubt about the Company’s ability to continue as a going concern within twelve months of the issuance date of these financial statements. The financial statements do not include any adjustments that might result from the outcome of this uncertainty. Although the Company has been successful in raising capital in the past, there is no assurance that it will be successful in obtaining such additional financing on terms acceptable to the Company, if at all, nor is it considered probable under the accounting standards. As such, under the requirements of ASC 205-40, management may not consider the potential for future capital raises or management plans to reduce costs that are not considered probable in its assessment of the Company’s ability to meet its obligations for the next twelve months. If the Company is unable to obtain funding, the Company would be forced to delay, reduce or eliminate some or all of its research and development programs, pre-clinical and clinical testing or commercialization efforts, which could adversely affect its business prospects. Merger and Recapitalization in March 2017 The Company had historically owned 100% of the voting units of its wholly owned subsidiary, Solid GT, LLC (“Solid GT”), and the results of Solid GT were included in the Company’s consolidated financial statements. In November 2015, Solid GT issued voting units to new investors which decreased the Company’s voting ownership in Solid GT to 77%. The Company continued to consolidate the results of Solid GT into its financial statements as the Company owned a majority voting interest in Solid GT and directed the activities of Solid GT. However, because the Company controlled but owned less than 100% of Solid GT, the Company recorded a non-controlling ownership interest at its fair value at inception and recognizes the net loss or profit attributable to non-controlling interests in the consolidated statements of operations based on a profit and loss sharing arrangement between the Company and the non-controlling interests. The Company also presented the change in equity related to equity-based compensation issued to Solid GT employees by Solid GT, in non-controlling interest. See Note 12, Equity-Based Compensation, On March 29, 2017, the Company merged the operations of Solid GT into the Company and Solid GT ceased to exist as a legal entity. See Note 3, Merger and Recapitalization The proportionate share of the loss attributed to the non-controlling interest amounted to $0, $1,060 and $2,234, for the years ended December 31, 2018, 2017 and 2016, respectively. There was no non-controlling interest at December 31, 2018 and 2017. The accompanying consolidated financial statements have been prepared in accordance with accounting principles generally accepted in the United States of America (“GAAP”). The accompanying consolidated financial statements include the accounts of Solid Biosciences Inc. 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, 2018
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 expenses during the reporting periods. Significant estimates and assumptions reflected in these consolidated financial statements include, but are not limited to, 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. 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 $327 in separate restricted bank accounts as security deposits for the leases of the Company’s facilities as of December 31, 2018. The Company has included the restricted cash of $327 as a non-current asset as of December 31, 2018. The Company held restricted cash of $65 in a separate restricted bank account as a security deposit for the lease of the Company’s facility as of December 31, 2017. The Company has included the restricted cash of $65 as a current asset as of December 31, 2017. 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’/members’ equity/(deficit). 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 pre-clinical 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 Deferred Offering Costs The Company capitalizes certain legal and other third-party fees that are directly associated with in-process equity financings as deferred offering costs until such financings are consummated. After consummation of the equity financing, these costs are recorded as a reduction of the proceeds generated as a result of the offering. Should the planned equity financing be abandoned, the deferred offering costs will be expensed immediately as a charge to operating expense in the consolidated statements of operations. Deferred offering costs amounted to $0 at December 31, 2018 and $3,106 at December 31, 2017. 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 capital lease is stated at fair value at the inception of the lease less accumulated depreciation. Equipment held under capital lease 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. 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 pre-clinical 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. 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 In connection with the completion of the Company’s initial public offering, the Company adopted the 2018 Omnibus Incentive Plan, which provides for the issuance of share-based awards, including options to purchase common stock. The 2018 Omnibus Incentive Plan provides for the awarding of up to 5,001,000 shares of common stock for equity awards. The Company measures all stock options and other stock-based awards granted to employees and directors 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. For stock-based awards granted to non-employees, compensation expense is recognized over the period during which services are rendered by such non-employees until completed. At the end of each financial reporting period prior to the completion of the service, the fair value of these awards is remeasured using the then-current fair value of the Company’s common stock and updated assumption inputs in the Black-Scholes option-pricing model. 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. The expected term of stock options granted to non-employees is equal to the contractual term of the option award. 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 Company classifies stock-based compensation expense in its consolidated statement of operations and comprehensive loss 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 DMD. 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’/members’ equity/(deficit) 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 attributable to common stockholders is computed by dividing the net loss attributable to common stockholders by the weighted average number of shares of common stock outstanding for the period. Diluted net loss attributable to common stockholders is computed by adjusting net loss attributable to common stockholders to reallocate undistributed earnings based on the potential impact of dilutive securities. Diluted net loss per share attributable to common stockholders is computed by dividing the diluted net loss attributable to common stockholders by the weighted average number of shares of common stock outstanding for the period, including potential dilutive shares of common stock assuming the dilutive effect of common stock equivalents. The Company’s preferred stock contractually entitles the holders of such shares to participate in dividends but does not contractually 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 attributable to common stockholders, diluted net loss per share attributable to common stockholders is the same as basic net loss per share attributable to common stockholders, since dilutive shares of common stock are not assumed to have been issued if their effect is anti-dilutive. Funding from Charitable Organizations The Company has received funding from charitable organizations to perform research and development services to identify therapies for people with DMD. 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 $270, $1,001 and $271 for the years ended December 31, 2018, 2017 and 2016, 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 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 In May 2017, the Financial Accounting Standards Board (“FASB”) issued Accounting Standards Update (“ASU”) 2017-09, Compensation—Stock Compensation (Topic 718): Scope of Modification Accounting In November 2016, the FASB issued ASU 2016-18, Statement of Cash Flows A reconciliation of the amounts from the cash flow statement to the balance sheet is as follows: December 31 2018 December 31 2017 December 31 2016 Cash and cash equivalents as presented on balance sheet $ 86,366 $ 52,080 $ 7,678 Restricted cash, current, as presented on balance sheet - 65 165 Restricted cash, non-current, as presented on balance sheet 327 - - Cash and cash equivalents and restricted cash as presented on cash flow statement $ 86,693 $ 52,145 $ 7,843 In August 2016, the FASB issued ASU 2016-15, Statement of Cash Flows (Topic 230): Classification of Certain Cash Receipts and Cash Payments Recently Issued Accounting Pronouncements In February 2016, the FASB issued ASU 2016-02, Leases (Topic 842) Leases The Company will adopt the new leasing standards on January 1, 2019, using a modified retrospective transition approach to be applied to leases existing as of, or entered into after, January 1, 2019. The Company has reviewed the existing lease contract and the impact of the new leasing standards on the consolidated results of operations, financial position and disclosures. Upon adoption of the new leasing standards, the Company expects to recognize a lease liability and related right of-use asset on the consolidated balance sheet within a range of approximately $4.5 million to $6.0 million. The impact of adoption of the new leasing standards will have an immaterial impact to the consolidated statements of income. In August 2018 the FASB issued ASU No. 2018-13 , Fair Value Measurement (Topic 820): Disclosure Framework - Changes to the Disclosure Requirements for Fair Value Measurement

Merger and Recapitalization

Merger and Recapitalization	12 Months Ended
	Dec. 31, 2018
Business Combinations [Abstract]
Merger and Recapitalization	3. Merger and Recapitalization On March 29, 2017, the Company completed a series of transactions, which included the issuance of Series 1 Senior Preferred Units pursuant to the Senior Preferred Unit Purchase Agreement (the “Senior Preferred Unit Purchase Agreement”) and the merger of Solid GT into the Company pursuant to the merger agreement between the Company and Solid GT (the “Merger Agreement”), collectively referred to as the “Merger and Recapitalization.” As part of the Merger and Recapitalization, the Company (a) issued 2,500,000 Series 1 Senior Preferred Units to new investors at $10.00 per unit resulting in gross proceeds to the Company of $25,000, (b) merged operations of Solid GT into the Company, effected through the exchange of Solid GT units held by non-controlling interests of the Company into new classes of the Company units, and (c) exchanged existing Redeemable Preferred Units and Series A Common Units of the Company into new units. The details of each component of the Merger and Recapitalization are as follows: (a) Issuance of Series 1 Senior Preferred Units Pursuant to the Senior Preferred Unit Purchase Agreement, the Company issued 2,500,000 Series 1 Senior Preferred Units to new investors at $10.00 per unit resulting in gross proceeds to the Company of $25,000. See Note 10, Redeemable Preferred Units, Series 2 and Series 1 Senior Preferred Units and Junior Preferred Units (b) Merger of Solid GT into the Company Prior to the Merger and Recapitalization, the Company issued Class B Non-Voting and Class D Voting Units of Solid GT to holders which represent non-controlling interests of the Company. On March 29, 2017, in connection with the Merger and Recapitalization, the non-controlling interests were eliminated as follows: • 50,000 Class B Non-Voting Units of Solid GT (“Solid GT Class B Units”) were exchanged for 1,635,916 Series C Common Units of the Company; and • 134,920 Class D Voting Units of Solid GT (“Solid GT Class D Units”) were exchanged for 4,414,356 Junior Preferred Units of the Company. In addition, the Class C Non-Voting Units of Solid GT (“Solid GT Class C Restricted Units”) were exchanged for Series D Common Units of the Company. The Solid GT Class C Restricted Units were held by employees and consultants of Solid GT. See Note 12, Equity-Based Compensation Since there was no change in control in connection with the Solid GT merger, the exchange of Solid GT Class B Units, Class C Restricted Units and Class D Units was accounted for as an equity transaction. In addition, because Solid GT Class D Units represented preferred units with preference over the other classes of Solid GT Units, the difference between the carrying value of the Solid GT Class D Units and the fair value of Junior Preferred Units was recorded as a deemed dividend in members’ deficit, which impacts net loss attributable to common unitholders. See Note 15, Net Loss per Unit (c) Exchange of the Company’s existing Redeemable Preferred Units and Series A Common Units In connection with the Merger and Recapitalization, the Company exchanged its existing Redeemable Preferred Units and Series A Common Units as follows: • 17,100,000 Redeemable Preferred Units of the Company were exchanged for 12,219,299 Series A Common Units of the Company. See Note 10, Redeemable Preferred Units, Series 2 and Series 1 Senior Preferred Units and Junior Preferred Units • 4,560,000 Series A Common Units of the Company were exchanged for 3,258,480 Series B Common Units of the Company. See Note 11, Members’ Deficit • 563,917 Series A Common Units of the Company were exchanged for 402,963 Series D Common Units of the Company. See Note 11, Members’ Deficit The table below displays the pre-merger and post-merger capitalization structure of the Company: Pre-Merger and Recapitalization Post-Merger and Recapitalization Entity Class Issued Entity Class Issued Company Redeemable Preferred 17,100,000 Company Series A Common 12,219,299 Company Series A Common (Founders) 4,560,000 Company Series B Common 3,258,480 Company Series A Common (Others) 563,917 Company Series D Common 402,963 Solid GT Class A Voting 450,000 Ceased to exist Solid GT Class B Non-Voting 50,000 Company Series C Common 1,635,916 Solid GT Class C Non-Voting 33,107 Company Series D Common 1,083,205 Solid GT Class D Voting 134,920 Company Junior Preferred 4,414,356 Company (Total) Common Units (Series A) 5,123,917 Company (Total) Common Units (Series A, B, C and D) 18,599,863

Fair Value of Financial Assets

Fair Value of Financial Assets and Liabilities	12 Months Ended
	Dec. 31, 2018
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, 2018 Using: Level 1 Level 2 Level 3 Total Assets: Cash equivalents $ - $ 54,423 $ - $ 54,423 Available for sale securities - 36,098 - $ 36,098 $ - $ 90,521 $ - $ 90,521 Fair Value Measurements as of December 31, 2017 Using: Level 1 Level 2 Level 3 Total Assets: Available for sale securities $ - $ 17,014 $ - $ 17,014 As of December 31, 2018 and 2017, the fair values of the Company’s available-for-sale securities were determined using level two inputs. At December 31, 2018 the portfolio consisted of U.S. government agency securities, corporate bond securities and commercial paper. At December 31, 2017, the portfolio consisted of U.S. government agency securities and corporate bond securities. During the years ended December 31, 2018 and 2017, there were no transfers between Level 1, Level 2 and Level 3. The fair value of the Company’s cash, restricted cash, accounts payable, and 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, 2018
Investments Debt And Equity Securities [Abstract]
Available-for-Sale Securities	5. Available-for-Sale Securities As of December 31, 2018 and 2017, the fair value of available-for-sale debt securities by type of security was as follows: December 31, 2018 Amortized Cost Gross Unrealized Gain Gross Unrealized Loss Fair Value Investments: U.S. government agency securities $ 13,543 - $ (2 ) $ 13,541 Corporate bond securities 12,860 2 (5 ) $ 12,857 Commercial paper 9,700 - - $ 9,700 $ 36,103 $ 2 $ (7 ) $ 36,098 December 31, 2017 Amortized Cost Gross Unrealized Gain Gross Unrealized Loss Fair Value Investments: U.S. government agency securities $ 9,473 $ - $ (7 ) $ 9,466 Corporate bond securities 7,554 - (6 ) 7,548 $ 17,027 $ - $ (13 ) $ 17,014 The estimated fair value and amortized cost of the Company’s available-for-sale securities by contractual maturity are summarized as follows: December 31, 2017 December 31, 2018 Amortized Cost Fair Value Amortized Cost Fair Value Due in one year or less $ 17,027 $ 17,014 $ 36,103 $ 36,098 Total available-for-sale securities $ 17,027 $ 17,014 $ 36,103 $ 36,098 The average maturity of the Company’s available-for-sale securities as of December 31, 2018 and 2017 and was approximately 0.3 years and 0.4 years, respectively.

Prepaid Expenses and Other Curr

Prepaid Expenses and Other Current Assets	12 Months Ended
	Dec. 31, 2018
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, 2018 2017 Prepaid research and development expenses $ 4,365 $ 951 Prepaid expenses and other assets 1,810 548 $ 6,175 $ 1,499

Property and Equipment

Property and Equipment	12 Months Ended
	Dec. 31, 2018
Property Plant And Equipment [Abstract]
Property and Equipment	7. Property and Equipment Property and equipment consists of the following: December 31, 2018 2017 Furniture and fixtures $ 187 $ 61 Laboratory equipment 5,998 2,338 Leasehold improvements 4,585 68 Computer equipment 166 77 Computer software 123 23 Construction in process 1,351 366 12,410 2,933 Less accumulated depreciation 1,988 504 $ 10,422 $ 2,429 Depreciation expense was $1,566, $448 and $56 for the years ended December 31, 2018, 2017, and 2016 respectively.

Accrued Expenses

Accrued Expenses	12 Months Ended
	Dec. 31, 2018
Payables And Accruals [Abstract]
Accrued Expenses	8. Accrued Expenses Accrued expenses and other current liabilities consist of the following: December 31, 2018 December 31, 2017 Accrued research and development $ 3,529 $ 1,855 Accrued compensation 3,534 2,033 Accrued other 1,172 2,084 $ 8,235 $ 5,972

Preferred Unit Tranche Rights

Preferred Unit Tranche Rights	12 Months Ended
	Dec. 31, 2018
Equity [Abstract]
Preferred Unit Tranche Rights	9. Preferred Unit Tranche Rights Included in the terms of the Redeemable Preferred Unit Purchase Agreement and the Series 1 Senior Preferred Unit Purchase Agreement were Tranche Rights which obligated the investors to purchase additional preferred units under certain conditions. The Tranche Rights also provided the investors with the right to purchase these additional units. The Company concluded that the Tranche Rights met the definition of a freestanding financial instrument as the Tranche Rights were legally detachable and separately exercisable from the Redeemable Preferred Units and the Series 1 Senior Preferred Units. Therefore, the Company allocated the net proceeds to each Tranche Right and the Redeemable Preferred Units or the Series 1 Senior Preferred Units based on the fair value at the date of issuance with the remaining proceeds being allocated to the Redeemable Preferred Units or Series 1 Senior Preferred Units. Through the final settlement date in October 2016, the Company estimated the fair value of the Redeemable Preferred Unit Tranche Right based on the probability of closing the tranches and the estimated future value of the Redeemable Preferred Units. The Redeemable Preferred Unit Tranche Right was recorded as a liability as the purchase price of the additional Redeemable Preferred Units was less than the estimated fair value of the Redeemable Preferred Units at the expected settlement date. Upon settlement, the Redeemable Preferred Unit Tranche Right was reclassified to Redeemable Preferred Units. In October 2016, the Redeemable Preferred Unit Tranche Right was settled and no Redeemable Preferred Unit Tranche Right was outstanding subsequent to October 2016. The estimated fair value of the Series 1 Senior Preferred Unit Tranche Right was determined using a probability-weighted present value model that considered the probability of closing the tranche through achievement of the preclinical milestones, estimated to be 50% on the date of issue and the estimated future value of Series 1 Senior Preferred Units at closing. The Company converted future values to present value using a discount rate appropriate for probability adjusted cash flows. The estimates were based, in part, on subjective assumptions. Changes to these assumptions can have a significant impact on the fair value of the Series 1 Senior Preferred Unit Tranche Right. The Series 1 Tranche Right was settled in connection with the closing of the Series 2 Senior Preferred Unit financing on October 26, 2017. A roll-forward of each tranche right is as follows: Redeemable Preferred Unit Tranche Right Series 1 Senior Preferred Unit Tranche Right Balance at December 31, 2015 $ 12,004 $ - Change in fair value (1,163 ) - Reclassification to preferred units (10,841 ) - Balance at December 31, 2016 - - Issuance - 459 Change in fair value - (459 ) Balance at December 31, 2017 - - Issuance - - Change in fair value - - Balance at December 31, 2018 $ - $ -

Redeemable Preferred Units, Ser

Redeemable Preferred Units, Series 2 and Series 1 Senior Preferred Units and Junior Preferred Units	12 Months Ended
	Dec. 31, 2018
Temporary Equity Disclosure [Abstract]
Redeemable Preferred Units, Series 2 and Series 1 Senior Preferred Units and Junior Preferred Units	10. Redeemable Preferred Units, Series 2 and Series 1 Senior Preferred Units and Junior Preferred Units Redeemable Preferred Units The Company has issued redeemable preferred units (“Redeemable Preferred Units”). The Redeemable Preferred Units are classified outside of members’ deficit because the units contain redemption features that are not solely within the control of the Company. In December 2013, the Company issued 3,420,000 Redeemable Preferred Units at an issuance price of $1.00 per unit for proceeds of $3,420. In December 2014, the Company issued 3,420,000 Redeemable Preferred Units at an issuance price of $1.00 per unit for proceeds of $3,420. In October 2015, the Company issued 6,840,000 Redeemable Preferred Units at an issuance price of $1.00 per unit for proceeds of $6,840. In November and December 2016, the Company issued an aggregate of 3,420,000 Redeemable Preferred Units at $1.00 per unit for proceeds of $3,420. On March 29, 2017, the Redeemable Preferred Units were exchanged to Series A Common Units. See Note 3, Merger and Recapitalization Redeemable Preferred Units consisted of the following: Authorized Issued and Outstanding Carrying Value Liquidation Preference At December 31, 2016 60,000,000 17,100,000 $ 71,649 $ 55,746 At December 31, 2017 - - $ - $ - At December 31, 2018 - - $ - $ - The holders of the Redeemable Preferred Units had the following rights and preferences: Tranche Right The Redeemable Preferred Unit Tranche Right obligated the holders to purchase, and provided the holders with the right to purchase, additional Redeemable Preferred Units, under certain circumstances. The Redeemable Preferred unitholders purchased these additional units in 2016 and 2015. In October 2016, the Redeemable Preferred Unit Tranche Right was settled with the closing of the Redeemable Preferred Unit financing. See Note 9, Preferred Unit Tranche Rights Redemption The Redeemable Preferred Units were redeemable on or after December 27, 2022 at the option of the Redeemable Preferred unitholder. The Redeemable Preferred Units were redeemable at the fair market value on the redemption date. Conversion The Redeemable Preferred Units had no conversion rights. Voting Rights The holders of Redeemable Preferred Units were entitled to vote as a single class with the holders of the Series A Common Units on certain matters, including the election of managers, with each Redeemable Preferred Unit and Series A Common Unit carrying one vote per unit. Distributions The Company’s Board of Managers had authority to determine the amount, if any, of proceeds available for distribution to the unitholders. Prior to the conversion of the Redeemable Preferred Units on March 29, 2017, such proceeds were to be distributed in accordance with the following order of priority: • First, to the holders of Redeemable Preferred Units, pro rata in proportion to the remaining amount to be distributed to each such holder, until each such holder has received distributions in an amount equal to the cumulative capital contributions since inception in respect of the Redeemable Preferred Units. • Thereafter, to all Redeemable Preferred Unitholders, Series A Common Units held by the Company’s founders, Series A Common Units issued to non-founders between December 27, 2013 and December 26, 2014, and vested Series A Restricted Common Unitholders issued subsequent to December 26, 2014 pro rata in proportion to their percentage interest at the time of distribution. No distributions were made in 2018, 2017 or 2016. Liquidation In the event of any liquidation, dissolution, or winding-up of the Company, the assets of the Company would have been distributed in accordance with the same order of priority that applied to distributions. Series 2 Senior Preferred Units On October 26, 2017, the Company completed the sale of 4,886,000 Series 2 Senior Preferred Units at a price of $11.26 per unit resulting in net proceeds of $55,002. Series 2 Senior Preferred Units consist of the following: Authorized Issued and Outstanding Carrying Value Liquidation Preference Common Units Issuable Upon Conversion At December 31, 2017 4,886,000 4,886,000 $ 55,002 $ 55,002 4,886,000 At December 31, 2018 - - $ - $ - - Series 1 Senior Preferred Units On March 29, 2017, the Company issued 2,500,000 Series 1 Senior Preferred Units at an issuance price of $10.00 per unit for proceeds of $25,000. See Note 3, Merger and Recapitalization Series 1 Senior Preferred Units consist of the following: Authorized Issued and Outstanding Carrying Value Liquidation Preference Common Units Issuable Upon Conversion At December 31, 2017 2,500,000 2,500,000 $ 25,000 $ 25,000 2,500,000 At December 31, 2018 - - - - - Junior Preferred Units On March 29, 2017, 134,920 Solid GT Class D Units were exchanged for 4,414,356 Junior Preferred Units of the Company. See Note 3, Merger and Recapitalization Junior Preferred Units consisted of the following: Authorized Issued and Outstanding Carrying Value Liquidation Preference At December 31, 2017 4,414,356 4,414,356 $ 44,177 $ 42,500 At December 31, 2018 - - $ - $ - The holders of the Series 1 and Series 2 Senior Preferred Units and Junior Preferred Units had the following rights and preferences: Tranche Right The holders of Series 1 Senior Preferred Units were obligated to purchase 1,973,430 Series 2 Senior Preferred Units at $12.67 per unit for gross proceeds of $25,000 in the event the Company achieves certain pre-clinical milestones. In addition, the holders of a majority of the Series 1 Senior Preferred Units had the right to require the holders of the Series 1 Senior Preferred Units to purchase the Series 2 Senior Preferred Units at any time prior to September 1, 2017, which in August 2017, was extended to December 1, 2017. The Series 1 Tranche Right was subject to certain transfer rights. See Note 9, Preferred Unit Tranche Rights Redemption The Series 1 and Series 2 Senior Preferred Units were redeemable on or after March 29, 2022 at the option of the holder at a redemption price equal to the original purchase price of $10.00 and $11.26 per unit, respectively, plus any declared but unpaid distributions. The Company presented Series 1 and Series 2 Senior Preferred Units outside of permanent equity since the redemption of Series 1 and Series 2 Senior Preferred Units was outside the control of the Company. The consent of the Junior Preferred unitholders along with Series 1 and Series 2 Senior Preferred unitholders could have effected a deemed liquidation event. Therefore, the Company presented the Junior Preferred Units outside of permanent equity. Voting Rights The holders of the Series 1 and Series 2 Senior Preferred Units and Junior Preferred Units were entitled to vote together, and not as separate classes, with each Series 1 and Series 2 Senior Preferred Unit, Junior Preferred Unit, Series A Common Unit and Series B Common Unit carrying one vote per unit. Subject to maintaining certain ownership levels, the Series 1 and Series 2 Senior Preferred unitholders as a class were entitled to elect two of the nine board members while such units were outstanding. The Junior Preferred unitholders as a class are entitled to elect two of the nine board members while such units were outstanding. Dividends The holders of Series 1 and Series 2 Senior Preferred Units were entitled to an 8% annual dividend based on the Series 1 and Series 2 Senior Preferred Unit issuance price of $10.00 and $11.26 per unit, respectively, when and if declared by the Board of Managers. No dividends were declared or paid to Series 1 or Series 2 Senior Preferred unitholders. The holders of the Junior Preferred Units were entitled to an 8% annual dividend based on the Junior Preferred Unit issuance price of $9.63 per unit, when and if declared by the Board of Managers. No dividends were declared or paid to Junior Preferred unitholders. Distributions The Company’s Board of Managers had authority to determine the amount, if any, of proceeds available for distribution. Such proceeds were to be distributed in accordance with the following order of priority: • First, the Series 2 Senior Preferred unitholders were entitled to an amount distributed, on a pro rata basis, equal to the Series 2 Senior Preferred Unit price of $11.26 per unit and any declared but unpaid Series 2 Senior Preferred dividends. • Second, the Series 1 Senior Preferred and the Junior Preferred unitholders were entitled to an amount distributed, on a pro rata basis, equal to the Series 1 Senior Preferred Unit price of $10.00 per unit and any declared but unpaid Series 1 Senior Preferred dividends and the Junior Preferred Unit price of $9.63 per unit and any declared but unpaid Junior Preferred dividends, respectively. • Third, the Series A, B, C and D Common unitholders were entitled to an amount distributed, on a pro rata basis, subject to certain limitations, until the cumulative amount distributed with respect to one Series A Common Unit, Series B Common Unit, Series C Common Unit and vested Series D Common Unit equaled the cumulative amount distributed to one Junior Preferred Unit. • Fourth, the Junior Preferred unitholders and the Series A, B, C and vested D Common unitholders were entitled to an amount distributed on a pro rata basis, subject to certain limitations, until the cumulative amount distributed with respect to one Junior Preferred Unit, Series A Common Unit, Series B Common Unit, Series C Common Unit and vested Series D Common Unit equaled the cumulative amount distributed to one Series 1 Senior Preferred Unit. • Fifth, the Series 1 and Series 2 Senior Preferred, the Junior Preferred and the Series A, B, C and vested D Common unitholders were entitled to participate on a pro rata basis in cumulative distributions, subject to certain limitations, in the remaining proceeds available for distribution. As a result of the issuance of the Series 2 Senior Preferred Units on October 26, 2017, the Series 2 Senior Preferred unitholders were entitled to cumulative amounts distributed equal to the amount paid per unit for the Series 2 Senior Preferred Units and any declared but unpaid Series 2 Senior Preferred cumulative dividends, prior to and with priority over any distributions to any other unitholders. In addition, upon the issuance of the Senior Series 2 Preferred units, the holders of the Junior Preferred Units no longer shared pro rata in the order of distributions with the Senior Series 1 Preferred unitholders and were subordinate to distributions made to Series 1 Senior Preferred unitholders. No distributions were made during the years ended December 31, 2018 and 2017. Liquidation In the event of any liquidation, dissolution, or winding-up of the Company, the assets of the Company would have been distributed in accordance with the same order of priority that applied to distributions. Corporate Conversion Immediately prior to the effectiveness of the Company’s registration statement on Form S-1, which occurred on January 25, 2018, the Company completed the Corporate Conversion whereby all the Series 1 and Series 2 Senior Preferred, Junior Preferred Units converted into shares of common stock.

Members' Deficit

Members' Deficit	12 Months Ended
	Dec. 31, 2018
Members Equity [Abstract]
Members’ Deficit	11. Members’ Deficit Series A, B, C and D Common Units Series A, B, C and D Common Units consisted of the following: December 31, 2018 Authorized Issued and Outstanding Carrying Value Series A Common Units - - $ - Series B Common Units - - - Series C Common Units - - - Series D Common Units - - - - - $ - December 31, 2017 Authorized Issued and Outstanding Carrying Value Series A Common Units 12,219,299 12,219,299 $ 55,964 Series B Common Units 3,258,480 3,258,480 3,613 Series C Common Units 1,635,916 1,635,916 2,053 Series D Common Units 3,075,814 2,324,857 3,384 20,189,509 19,438,552 $ 65,014 December 31, 2016 Authorized Issued and Outstanding Carrying Value Series A Common Units 20,000,000 5,123,917 $ 558 Series A Common Units Founders Series A Common Units On December 27, 2013, the Company issued 4,560,000 restricted Series A Common Units to its founders with time-based vesting conditions. Unvested units of Series A Common Units may not be sold or transferred by the holder. These restrictions lapse according to the time-based vesting conditions of each award. At December 31, 2016 and 2015, 3,420,000 and 2,280,000 restricted Series A Common Units were vested, respectively. The aggregated intrinsic value of the restricted Series A Common Units that vested during the year ended December 31, 2016 was $3,306. There were no restricted Series A Common Units that vested during the years ended December 31, 2017 and 2018. On March 29, 2017, in connection with the Merger and Recapitalization, the 4,560,000 founders’ restricted Series A Common Units were exchanged for 3,258,480 restricted Series B Common Units. All restricted Series B Common Units continued to vest pursuant to the original vesting terms under the restricted Series A Common Units agreements and the Company continued to recognize compensation expense over the related service period. In addition, in connection with the exchange of the founders’ restricted Series A Common Units into restricted Series B Common Units, the Company recognized $2,710 of equity based compensation expense for vested units, which represents the incremental fair value of the units before and after the Merger and Recapitalization. The Company has recorded the additional compensation expense in the amount of $904 over the remaining vesting period of the Series B Common Units during the year ended December 31, 2017. Non-Founder Series A Common Units In March and November 2014, the Company issued 169,667 restricted Series A Common Units at a per unit value of $2.59 to certain employees and consultants. In September and November 2015, the Company issued 305,000 restricted Series A Common Units at per unit values between $2.39 and $2.65 to certain employees. In May and September 2016, the Company issued 60,000 restricted Series A Common Units at per unit values between $2.03 and $2.14 to certain employees. In December 2016, the Company issued 48,000 restricted Series A Common Units at a per unit value of $2.25 to certain employees. On March 29, 2017, in connection with the Merger and Recapitalization, 563,917 non-founder restricted Series A Common Units were exchanged for 402,963 restricted Series D Common Units. All restricted Series D Common Units will continue to vest pursuant to their original vesting period, which was generally four years, under the restricted Series A Common Units agreement, and the Company will continue to recognize compensation expense over the related service period. In addition, in connection with the exchange of the non-founders’ restricted Series A Common Units into restricted Series D Common Units, the Company recognized $140 of equity-based compensation expense for vested units, which represents the incremental fair value of the units before and after the Merger and Recapitalization. The Company will record additional compensation expense in the amount of $115 over the remaining vesting period of the Series D Common units of which $48 and $54 were recognized during the years ended December 31, 2017 and 2018, respectively. The holders of the Series A, B, C and D Common Units were entitled to the following rights and priorities: Voting Rights Holders of Series A and B Common Units had the right to one vote per unit held by such member. The Series A Common unitholders as a class were entitled to elect two of the eight board members while such units are outstanding. The Series B Common unitholders as a class were entitled to elect three of the eight board members while such units are outstanding. Holders of Series C and D Common Units did not have the right to vote for the election of board members. Redemption The Series A, B, C and D Common Units were not redeemable. Distributions and Liquidation Preference The holders of the Series A, B, C and D Common Units were entitled to participate in distributions after preferential distributions were made to the Series 1 and Series 2 Senior Preferred and Junior Preferred unitholders as follows: • The Series A, B, C and D Common unitholders were entitled to participate in distributions on a pro rata basis, subject to certain limitations, until the cumulative amount distributed with respect to one Series A Common Unit, Series B Common Unit, Series C Common Unit and vested Series D Common Unit equaled the cumulative amount distributed to one Junior Preferred Unit. • The Junior Preferred unitholders and the Series A, B, C and D Common unitholders were entitled to participate in distributions on a pro rata basis, subject to certain limitations, until the cumulative amount distributed with respect to one Junior Preferred Unit, Series A Common Unit, Series B Common Unit, Series C Common Unit and vested Series D Common Unit equaled the cumulative amount distributed to one Series 1 Senior Preferred Unit. • The Series 1 Senior Preferred, Junior Preferred unitholders and the Series A, B, C and D Common unitholders were entitled to participate in distributions on a pro rata basis, subject to certain limitations, until the cumulative amount distributed with respect to one Series 1 Senior Preferred Unit, Junior Preferred Unit, Series A Common Unit, Series B Common Unit, Series C Common Unit and vested Series D Common Unit equaled the cumulative amount distributed to one Series 2 Senior Preferred Unit. • All unitholders were entitled to participate on a pro rata basis in cumulative distributions, subject to certain limitations, in the remaining proceeds available for distribution. No distributions were made to the Series A, B, C or D Common unitholders during the years ended December 31, 2018, 2017 and 2016. Corporate Conversion Immediately prior to the effectiveness of the Company’s registration statement on Form S-1, which occurred on January 25, 2018, the Company completed the Corporate Conversion whereby all the Series A, B, C, and D Common Units converted into shares of common stock.

Equity-Based Compensation

Equity-Based Compensation	12 Months Ended
	Dec. 31, 2018
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Equity-Based Compensation	12. Equity-Based Compensation 2018 Omnibus Incentive Plan In connection with the closing of the Company’s initial public offering, the board of directors and stockholders approved the 2018 Omnibus Incentive Plan, which provides for the reservation of 5,001,000 shares of common stock for equity awards. The following table summarizes the Company’s stock option activity for the year ended December, 31 2018: Number of Options Weighted Average Exercise Price Outstanding at December 31, 2017 - $ - Granted 1,185,207 28.51 Forfeitures (32,009 ) 20.80 Outstanding at December 31, 2018 1,153,198 28.73 Vested and expected to vest as of December 31, 2018 1,153,198 28.73 Exercisable at December 31, 2018 - $ - At December 31, 2018, the Company had an aggregate of $19,528 of unrecognized equity-based compensation cost related to stock options outstanding. Stock options outstanding as well as options vested and expected to vest are expected to be recognized over a weighted average period of 3.2 years and have an intrinsic value of $2,260 as of December 31, 2018. 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: Year Ended December 31, 2018 Expected volatility 73.8% - 86.4% Expected dividends 0.0% Expected term (in years) 5.25 - 6.25 Risk-free rate 2.6% - 3.1% The weighted average fair value of options to purchase shares of common stock granted during the year ended December 31, 2018 was $19.94. 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, 2018: Units Weighted- Average Grant Date Fair Value Unvested restricted Series D Common Units at December 31, 2017 1,404,265 $ 4.83 Vested units prior to Corporate Conversion (66,019 ) 3.46 Forfeited units prior to Corporate Conversion (8,932 ) 2.99 Unvested Series D Common Units at January 25, 2018 prior to Corporate Conversion 1,329,314 4.91 Conversion to restricted shares of common stock upon Corporate Conversion 1,128,182 5.79 Vested shares of Common Stock (334,144 ) 4.51 Forfeited (50,474 ) 4.99 Unvested restricted Series D Common Units at December 31, 2018 743,564 $ 6.40 The aggregate intrinsic value of restricted common units that vested during the years ended December 31, 2018, 2017, and 2016 were $8,721, $3,358, and $351 respectively. At December 31, 2018, the Company had an aggregate of $4,466 of unrecognized equity-based compensation related restricted shares of common stock, which is expected to be recognized over a weighted average period of 1.3 years. The Company recorded equity-based compensation expense related to all of its share and unit-based awards to employees and non-employees in the following captions within its consolidated statements of operations for the years ended December 31, 2018, 2017 and 2016 Year Ended December 31, 2018 2017 2016 Research and development expenses $ 4,180 $ 1,206 $ 1,262 General and administrative expenses $ 1,775 $ 4,124 $ 208 $ 5,955 $ 5,330 $ 1,470

Commitments and Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2018
Commitments And Contingencies Disclosure [Abstract]
Commitments and Contingencies	13. Commitments and Contingencies Operating Lease 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 $3,387. 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 term of the lease runs through February 2022. The future minimum rent commitment for the lease term is approximately $3,922. In addition to rent, the lease requires the Company to pay additional amounts for taxes, insurance, maintenance and other operating expenses. During the years ended December 31, 2018, 2017 and 2016, the Company recognized $2,524, $1,301 and $270, respectively, of rental expense related to office and laboratory space. Future minimum lease payments for these operating leases as of December 31, 2018 amounted to $7,309. Capital Lease 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 is approximately $1,313. At December 31, 2018 the Company expects to make the following fixed and non-cancelable payments related to its operating and capital lease commitments: Year Ending December 31, 2019 $ 2,210 2020 2,293 2021 2,332 2022 1,508 2023 278 Thereafter - Total $ 8,621 Letter of Credit The Company had outstanding letters of credit in the amounts of $327 and $65 at December 31, 2018 and 2017, respectively, which were required as a condition of the Company’s office and laboratory leases. 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, 2018 and 2017. Contingencies In the first quarter of 2017, the Company terminated the development, manufacturing and testing agreement (the “Agreement”) it had entered into in January 2016 with a third-party. The Company and the third-party were in dispute regarding the remaining amounts owed by the Company to the third-party under the Agreement. The Company settled the matter in April 2018 for $1,320. Legal Proceedings On March 27, 2018, a purported stockholder of the Company, filed a putative class action complaint alleging violations of the federal securities laws, in the United States District Court for the District of Massachusetts (Case No. 18-10587), against the Company and certain of the Company’s current executive officers and underwriters in the Company’s initial public offering. The plaintiff claims to represent purchasers of the Company’s common stock during the period from January 25, 2018 to March 14, 2018 and seeks unspecified damages arising out of the alleged failure to disclose risks associated with toxicity and potential for adverse events related to the Company’s lead product candidate. On May 29, 2018, the plaintiff and another purported stockholder filed separate motions each seeking his appointment as lead plaintiff and approval of his selected lead counsel. On June 5, 2018, the plaintiff withdrew his motion. On August 14, 2018, the other purported stockholder withdrew his motion. On October 16, 2018, the plaintiff voluntarily dismissed his claims without prejudice to his ability to participate in any putative class actions as an absent class member. On March 28, 2018, a purported stockholder of the Company, filed a putative class action complaint alleging violations of the federal securities laws, in the Business Litigation Section of the Superior Court of the Commonwealth of Massachusetts (Civil Action No. 1884-00984), against the Company, Ilan Ganot, Jennifer Ziolkowski, the Company’s directors and certain of the underwriters in the Company’s initial public offering. The plaintiff in this suit claims to represent purchasers of the Company’s common stock in or traceable to the Company’s January 25, 2018 initial public offering and seeks unspecified damages arising out of the alleged failure to disclose risks associated with toxicity and potential for adverse events related to the Company’s lead product candidate. On April 30, 2018, all defendants including the Company moved to stay the proceedings in favor of the prior-filed federal court securities class action. The plaintiff filed his opposition to this motion on May 14, 2018, and defendants filed a reply in support of their motion on May 24, 2018. After oral argument on June 13, 2018, the court issued an order on June 22, 2018 allowing the motion to stay and directing the parties to advise the court of the status of the federal court action every six months. On December 21, 2018, the parties filed a joint status report informing the court of the voluntary dismissal of the federal actions and of plaintiff’s intent to move to vacate the stay. The court has scheduled a status conference for June 24, 2019. While the Company is vigorously defending against all claims asserted, this litigation could result in substantial costs to the Company and a diversion of the Company’s management’s attention and resources, which could harm its business. In addition, the uncertainty of the pending lawsuit or potential filing of additional lawsuits could lead to more volatility and a reduction in the Company’s stock price. Given the early stage of the litigation, at this time the Company is unable to reasonably estimate possible losses or form a judgment that an unfavorable outcome is either probable or remote. It is not currently possible to assess whether or not the outcome of these proceedings may have a material adverse effect on the Company.

License Agreements

License Agreements	12 Months Ended
	Dec. 31, 2018
Organization Consolidation And Presentation Of Financial Statements [Abstract]
License Agreements	14. 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 DMD 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 million 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. There were no milestones achieved during the years ended December 31, 2018 and 2016. The Company must also pay royalties of a low single digit percentage of future sales by the Company and its sublicensees of products developed under the licensed patent rights. In addition, the Company must pay an annual maintenance fee until certain milestones are achieved, at which time a minimum annual royalty requirement will replace such maintenance fee and will apply to the Company and its sublicensees. 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 $47, $135, and $0 for the years ended December 31, 2018, 2017 and 2016, respectively, under the agreement. 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 DMD 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 million 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. There were no milestones achieved during the years ended December 31, 2018 and 2016. The Company must pay a royalty of a low single digit percentage of future sales or by its sublicensees of products developed using the licensed patents. In addition, the Company must pay an annual maintenance fee until certain milestones are achieved, after which time a minimum annual royalty will replace such maintenance fee. 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 $10, $11, and $0 for the years ended December 31, 2018, 2017 and 2016, respectively, under the agreement. 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 million upon the achievement of certain milestones. There were no milestones achieved during the years ended December 31, 2018, 2017 and 2016. The Company must also pay a royalty of a low single digit percentage on future sales by the Company or its sublicensees of products developed using the licensed rights, with a minimum annual royalty after certain milestones are achieved. In addition, the Company must pay an annual maintenance fee in any year in which the minimum annual royalty is not reached. 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 $35, $4 and $145 for the years ended December 31, 2018, 2017 and 2016, respectively, under the agreement. 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 DMD. 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, 2018, 2017 and 2016. 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 renewal periods thereafter unless one of the parties provides notice of non-renewal. The Company may terminate the license agreement at any time upon providing sixty days’ written notice to Harvard College. Harvard College may terminate the agreement in the event the Company becomes bankrupt or insolvent. Both Harvard College and the Company may also terminate the agreement for an uncured material breach of the agreement by the other party. The Company recorded research and development expense in the amount of $20, $45 and $45 for the years ended December 31, 2018, 2017 and 2016, respectively, under the agreement. 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 DMD. 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, 2018 and 2017. 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 renewal periods thereafter unless one of the parties provides notice of non-renewal. The Company may terminate the license agreement at any time upon providing sixty days’ written notice to Harvard College. Harvard College may terminate the agreement in the event the Company becomes bankrupt or insolvent. Both Harvard College and the Company may also terminate the agreement for an uncured material breach of the agreement by the other party. The Company recorded research and development expense in the amount of $5 and $23 for the years ended December 31, 2018 and 2017, respectively, under the agreement. 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, 2018
Earnings Per Share [Abstract]
Net Loss per Share	15. Net Loss per Share Basic and diluted net loss per share attributable to common stockholders were calculated as follows: The numerator for basic and diluted net loss per share attributable to common stockholders is as follows: Year Ended December 31, 2018 2017 2016 Net loss $ (74,798 ) $ (53,178 ) $ (23,773 ) Net loss attributable to non-controlling interest - (1,060 ) (2,234 ) Net loss attributable to Solid Biosciences Inc. $ (74,798 ) $ (52,118 ) $ (21,539 ) Decretion (accretion) of preferred units to redemption value - (959 ) 4,309 Redemption of preferred units - 15,685 - Redemption of redeemable interest from non-controlling interest in Solid GT - (1,925 ) - Net loss attributable to common stockholders $ (74,798 ) $ (39,317 ) $ (17,230 ) The denominator is as follows: Year Ended December 31 2018 2017 2016 Weighted average common stock outstanding, basic and diluted 33,262,597 13,649,485 1,698,904 Net loss per share attributable to common stockholders, basic and diluted is as follows: Year Ended December 31, 2018 2017 2016 Net loss per share attributable to common stockholders, basic and diluted $ (2.25 ) $ (2.88 ) $ (10.14 ) 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 attributable to common stockholders for the periods indicated because including them would have had an anti-dilutive effect: Year Ended December 31, 2018 2017 2016 Options to purchase shares of common stock 1,153,198 - - Unvested shares of common stock 743,564 - - Series A common units - - 1,104,391 Series B common units - - - Series D common units - 1,404,265 - 1,896,762 1,404,265 1,104,391

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Income Taxes	16. Income Taxes The Company recorded no tax benefit for the year ended December 31, 2018 for the net operating losses incurred due to its uncertainty of realizing a benefit from those items. The Company was not subject to federal and state income taxes for the year ended December 31, 2017 as it was organized as a limited liability company which was taxed as a partnership for U.S. tax purposes. A reconciliation of income taxes computed using the U.S. federal statutory rate to that reflected in operations as of December 31, 2018 is as follows: December 31, 2018 Income tax computed at federal statutory tax rate 21.0 % State taxes, net of federal benefit 5.5 % Permanent differences (0.8 )% Tax credits 9.2 % Loss taxed as a partnership (1.7 )% Conversion to a C-Corporation 0.5 % Other (0.1 )% Valuation allowance (33.6 )% 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, 2018 are as follows: December 31 2018 Deferred tax assets: Tax loss carryforwards $ 16,417 Tax credit carryforwards 6,903 Deferred expenses 372 Accrued expenses 935 Stock Compensation 856 Other 177 Total deferred tax assets 25,660 Depreciation (406 ) Valuation allowance (25,254 ) Net deferred taxes $ - As of December 31, 2018, the Company has federal net operating loss carryforwards of $59,357 and tax credits of $6,349 which may be used to offset future federal income and tax liability, respectively. In addition, the Company has state net operating loss carryforwards of approximately $59,453 and tax credits of $702 which may be used to offset future state income and tax liability, respectively. 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 or research and development 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 had approximately $25,254 in valuation allowances recorded against its deferred tax assets as of December 31, 2018. 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 2018 tax year is 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, 2018, 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, 2018, no interest and penalties have been recorded.

Retirement Plan

Retirement Plan	12 Months Ended
	Dec. 31, 2018
Compensation And Retirement Disclosure [Abstract]
Retirement Plan	17. Retirement 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 managers. The Company made no contributions to the plan during the years ended December 31, 2016, 2017 and 2018.

Selected Quarterly Financial In

Selected Quarterly Financial Information	12 Months Ended
	Dec. 31, 2018
Quarterly Financial Information Disclosure [Abstract]
Quarterly Financial Information	18. Selected Quarterly Financial Information (Unaudited) Selected quarterly results from operations for the years ended December 31, 2018 and 2017 are as follows: 2018 Quarter Ended March 31 June 30 September 30 December 31 (in thousands, except for per share data) Revenues $ - $ - $ - $ - Operating expenses 15,973 18,178 19,150 22,386 Loss from operations (15,973 ) (18,178 ) (19,150 ) (22,386 ) Net loss (15,877 ) (17,980 ) (19,020 ) (21,921 ) Net loss attributable to Solid Biosciences Inc. (15,877 ) (17,980 ) (19,020 ) (21,921 ) Net loss attributable to common stockholders (15,877 ) (17,980 ) (19,020 ) (21,921 ) Net loss per share attributable to common stockholders $ (0.54 ) $ (0.52 ) $ (0.55 ) $ (0.63 ) 2017 Quarter Ended March 31 June 30 September 30 December 31 (in thousands, except for per unit data) Revenues $ - $ - $ - $ - Operating expenses 14,113 11,887 13,696 15,161 Loss from operations (14,113 ) (11,887 ) (13,696 ) (15,161 ) Net loss (13,875 ) (11,311 ) (13,505 ) (14,487 ) Net loss attributable to Solid Biosciences, LLC (12,815 ) (11,311 ) (13,505 ) (14,487 ) Net loss attributable to common unitholders (14 ) (11,311 ) (13,505 ) (14,487 ) Net loss per unit attributable to common unitholders $ (0.01 ) $ (0.66 ) $ (0.79 ) $ (0.84 )

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2018
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 expenses during the reporting periods. Significant estimates and assumptions reflected in these consolidated financial statements include, but are not limited to, 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.
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 $327 in separate restricted bank accounts as security deposits for the leases of the Company’s facilities as of December 31, 2018. The Company has included the restricted cash of $327 as a non-current asset as of December 31, 2018. The Company held restricted cash of $65 in a separate restricted bank account as a security deposit for the lease of the Company’s facility as of December 31, 2017. The Company has included the restricted cash of $65 as a current asset as of December 31, 2017.
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’/members’ equity/(deficit). 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 pre-clinical 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
Deferred Offering Costs	Deferred Offering Costs The Company capitalizes certain legal and other third-party fees that are directly associated with in-process equity financings as deferred offering costs until such financings are consummated. After consummation of the equity financing, these costs are recorded as a reduction of the proceeds generated as a result of the offering. Should the planned equity financing be abandoned, the deferred offering costs will be expensed immediately as a charge to operating expense in the consolidated statements of operations. Deferred offering costs amounted to $0 at December 31, 2018 and $3,106 at December 31, 2017.
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 capital lease is stated at fair value at the inception of the lease less accumulated depreciation. Equipment held under capital lease 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.
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 pre-clinical 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.
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 In connection with the completion of the Company’s initial public offering, the Company adopted the 2018 Omnibus Incentive Plan, which provides for the issuance of share-based awards, including options to purchase common stock. The 2018 Omnibus Incentive Plan provides for the awarding of up to 5,001,000 shares of common stock for equity awards. The Company measures all stock options and other stock-based awards granted to employees and directors 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. For stock-based awards granted to non-employees, compensation expense is recognized over the period during which services are rendered by such non-employees until completed. At the end of each financial reporting period prior to the completion of the service, the fair value of these awards is remeasured using the then-current fair value of the Company’s common stock and updated assumption inputs in the Black-Scholes option-pricing model. 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. The expected term of stock options granted to non-employees is equal to the contractual term of the option award. 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 Company classifies stock-based compensation expense in its consolidated statement of operations and comprehensive loss 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 DMD. 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’/members’ equity/(deficit) 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 attributable to common stockholders is computed by dividing the net loss attributable to common stockholders by the weighted average number of shares of common stock outstanding for the period. Diluted net loss attributable to common stockholders is computed by adjusting net loss attributable to common stockholders to reallocate undistributed earnings based on the potential impact of dilutive securities. Diluted net loss per share attributable to common stockholders is computed by dividing the diluted net loss attributable to common stockholders by the weighted average number of shares of common stock outstanding for the period, including potential dilutive shares of common stock assuming the dilutive effect of common stock equivalents. The Company’s preferred stock contractually entitles the holders of such shares to participate in dividends but does not contractually 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 attributable to common stockholders, diluted net loss per share attributable to common stockholders is the same as basic net loss per share attributable to common stockholders, since dilutive shares of common stock are not assumed to have been issued if their effect is anti-dilutive.
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 DMD. 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 $270, $1,001 and $271 for the years ended December 31, 2018, 2017 and 2016, 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 May 2017, the Financial Accounting Standards Board (“FASB”) issued Accounting Standards Update (“ASU”) 2017-09, Compensation—Stock Compensation (Topic 718): Scope of Modification Accounting In November 2016, the FASB issued ASU 2016-18, Statement of Cash Flows A reconciliation of the amounts from the cash flow statement to the balance sheet is as follows: December 31 2018 December 31 2017 December 31 2016 Cash and cash equivalents as presented on balance sheet $ 86,366 $ 52,080 $ 7,678 Restricted cash, current, as presented on balance sheet - 65 165 Restricted cash, non-current, as presented on balance sheet 327 - - Cash and cash equivalents and restricted cash as presented on cash flow statement $ 86,693 $ 52,145 $ 7,843 In August 2016, the FASB issued ASU 2016-15, Statement of Cash Flows (Topic 230): Classification of Certain Cash Receipts and Cash Payments
Recently Issued Accounting Pronouncements	Recently Issued Accounting Pronouncements In February 2016, the FASB issued ASU 2016-02, Leases (Topic 842) Leases The Company will adopt the new leasing standards on January 1, 2019, using a modified retrospective transition approach to be applied to leases existing as of, or entered into after, January 1, 2019. The Company has reviewed the existing lease contract and the impact of the new leasing standards on the consolidated results of operations, financial position and disclosures. Upon adoption of the new leasing standards, the Company expects to recognize a lease liability and related right of-use asset on the consolidated balance sheet within a range of approximately $4.5 million to $6.0 million. The impact of adoption of the new leasing standards will have an immaterial impact to the consolidated statements of income. In August 2018 the FASB issued ASU No. 2018-13 , Fair Value Measurement (Topic 820): Disclosure Framework - Changes to the Disclosure Requirements for Fair Value Measurement

Summary of Significant Accoun_3

Summary of Significant Accounting Policies (Tables)	12 Months Ended
	Dec. 31, 2018
Accounting Policies [Abstract]
Reconciliation of Amounts From Cash Flow Statement to Balance Sheet	A reconciliation of the amounts from the cash flow statement to the balance sheet is as follows: December 31 2018 December 31 2017 December 31 2016 Cash and cash equivalents as presented on balance sheet $ 86,366 $ 52,080 $ 7,678 Restricted cash, current, as presented on balance sheet - 65 165 Restricted cash, non-current, as presented on balance sheet 327 - - Cash and cash equivalents and restricted cash as presented on cash flow statement $ 86,693 $ 52,145 $ 7,843

Merger and Recapitalization (Ta

Merger and Recapitalization (Tables)	12 Months Ended
	Dec. 31, 2018
Business Combinations [Abstract]
Schedule of Pre-Merger and Post-Merger Capitalization Structure	The table below displays the pre-merger and post-merger capitalization structure of the Company: Pre-Merger and Recapitalization Post-Merger and Recapitalization Entity Class Issued Entity Class Issued Company Redeemable Preferred 17,100,000 Company Series A Common 12,219,299 Company Series A Common (Founders) 4,560,000 Company Series B Common 3,258,480 Company Series A Common (Others) 563,917 Company Series D Common 402,963 Solid GT Class A Voting 450,000 Ceased to exist Solid GT Class B Non-Voting 50,000 Company Series C Common 1,635,916 Solid GT Class C Non-Voting 33,107 Company Series D Common 1,083,205 Solid GT Class D Voting 134,920 Company Junior Preferred 4,414,356 Company (Total) Common Units (Series A) 5,123,917 Company (Total) Common Units (Series A, B, C and D) 18,599,863

Fair Value of Financial Asset_2

Fair Value of Financial Assets and Liabilities (Tables)	12 Months Ended
	Dec. 31, 2018
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, 2018 Using: Level 1 Level 2 Level 3 Total Assets: Cash equivalents $ - $ 54,423 $ - $ 54,423 Available for sale securities - 36,098 - $ 36,098 $ - $ 90,521 $ - $ 90,521 Fair Value Measurements as of December 31, 2017 Using: Level 1 Level 2 Level 3 Total Assets: Available for sale securities $ - $ 17,014 $ - $ 17,014

Available-for-Sale Securities (

Available-for-Sale Securities (Tables)	12 Months Ended
	Dec. 31, 2018
Investments Debt And Equity Securities [Abstract]
Schedule of Fair Value of Available-for-Sale Debt Securities by Type of Security	As of December 31, 2018 and 2017, the fair value of available-for-sale debt securities by type of security was as follows: December 31, 2018 Amortized Cost Gross Unrealized Gain Gross Unrealized Loss Fair Value Investments: U.S. government agency securities $ 13,543 - $ (2 ) $ 13,541 Corporate bond securities 12,860 2 (5 ) $ 12,857 Commercial paper 9,700 - - $ 9,700 $ 36,103 $ 2 $ (7 ) $ 36,098 December 31, 2017 Amortized Cost Gross Unrealized Gain Gross Unrealized Loss Fair Value Investments: U.S. government agency securities $ 9,473 $ - $ (7 ) $ 9,466 Corporate bond securities 7,554 - (6 ) 7,548 $ 17,027 $ - $ (13 ) $ 17,014
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, 2017 December 31, 2018 Amortized Cost Fair Value Amortized Cost Fair Value Due in one year or less $ 17,027 $ 17,014 $ 36,103 $ 36,098 Total available-for-sale securities $ 17,027 $ 17,014 $ 36,103 $ 36,098

Prepaid Expenses and Other Cu_2

Prepaid Expenses and Other Current Assets (Tables)	12 Months Ended
	Dec. 31, 2018
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, 2018 2017 Prepaid research and development expenses $ 4,365 $ 951 Prepaid expenses and other assets 1,810 548 $ 6,175 $ 1,499

Property and Equipment (Tables)

Property and Equipment (Tables)	12 Months Ended
	Dec. 31, 2018
Property Plant And Equipment [Abstract]
Property and Equipment	Property and equipment consists of the following: December 31, 2018 2017 Furniture and fixtures $ 187 $ 61 Laboratory equipment 5,998 2,338 Leasehold improvements 4,585 68 Computer equipment 166 77 Computer software 123 23 Construction in process 1,351 366 12,410 2,933 Less accumulated depreciation 1,988 504 $ 10,422 $ 2,429

Accrued Expenses and Other Curr

Accrued Expenses and Other Current Liabilities (Tables)	12 Months Ended
	Dec. 31, 2018
Payables And Accruals [Abstract]
Summary of Accrued Expenses and Other Current Liabilities	Accrued expenses and other current liabilities consist of the following: December 31, 2018 December 31, 2017 Accrued research and development $ 3,529 $ 1,855 Accrued compensation 3,534 2,033 Accrued other 1,172 2,084 $ 8,235 $ 5,972

Preferred Unit Tranche Rights (

Preferred Unit Tranche Rights (Tables)	12 Months Ended
	Dec. 31, 2018
Equity [Abstract]
Roll-Forward of Each Tranche Right	A roll-forward of each tranche right is as follows: Redeemable Preferred Unit Tranche Right Series 1 Senior Preferred Unit Tranche Right Balance at December 31, 2015 $ 12,004 $ - Change in fair value (1,163 ) - Reclassification to preferred units (10,841 ) - Balance at December 31, 2016 - - Issuance - 459 Change in fair value - (459 ) Balance at December 31, 2017 - - Issuance - - Change in fair value - - Balance at December 31, 2018 $ - $ -

Redeemable Preferred Units, S_2

Redeemable Preferred Units, Series 2 and Series 1 Senior Preferred Units and Junior Preferred Units (Tables)	12 Months Ended
	Dec. 31, 2018
Redeemable Preferred Units [Member]
Summary of Preferred Units	Redeemable Preferred Units consisted of the following: Authorized Issued and Outstanding Carrying Value Liquidation Preference At December 31, 2016 60,000,000 17,100,000 $ 71,649 $ 55,746 At December 31, 2017 - - $ - $ - At December 31, 2018 - - $ - $ -
Series 2 Senior Preferred Units [Member]
Summary of Preferred Units	Series 2 Senior Preferred Units consist of the following: Authorized Issued and Outstanding Carrying Value Liquidation Preference Common Units Issuable Upon Conversion At December 31, 2017 4,886,000 4,886,000 $ 55,002 $ 55,002 4,886,000 At December 31, 2018 - - $ - $ - -
Series 1 Senior Preferred Units [Member]
Summary of Preferred Units	Series 1 Senior Preferred Units consist of the following: Authorized Issued and Outstanding Carrying Value Liquidation Preference Common Units Issuable Upon Conversion At December 31, 2017 2,500,000 2,500,000 $ 25,000 $ 25,000 2,500,000 At December 31, 2018 - - - - -
Junior Preferred Units [Member]
Summary of Preferred Units	Junior Preferred Units consisted of the following: Authorized Issued and Outstanding Carrying Value Liquidation Preference At December 31, 2017 4,414,356 4,414,356 $ 44,177 $ 42,500 At December 31, 2018 - - $ - $ -

Members' Deficit (Tables)

Members' Deficit (Tables)	12 Months Ended
	Dec. 31, 2018
Members Equity [Abstract]
Summary of Series A, B, C and D Common Units	Series A, B, C and D Common Units consisted of the following: December 31, 2018 Authorized Issued and Outstanding Carrying Value Series A Common Units - - $ - Series B Common Units - - - Series C Common Units - - - Series D Common Units - - - - - $ - December 31, 2017 Authorized Issued and Outstanding Carrying Value Series A Common Units 12,219,299 12,219,299 $ 55,964 Series B Common Units 3,258,480 3,258,480 3,613 Series C Common Units 1,635,916 1,635,916 2,053 Series D Common Units 3,075,814 2,324,857 3,384 20,189,509 19,438,552 $ 65,014 December 31, 2016 Authorized Issued and Outstanding Carrying Value Series A Common Units 20,000,000 5,123,917 $ 558

Equity-Based Compensation (Tabl

Equity-Based Compensation (Tables)	12 Months Ended
	Dec. 31, 2018
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 2018: Number of Options Weighted Average Exercise Price Outstanding at December 31, 2017 - $ - Granted 1,185,207 28.51 Forfeitures (32,009 ) 20.80 Outstanding at December 31, 2018 1,153,198 28.73 Vested and expected to vest as of December 31, 2018 1,153,198 28.73 Exercisable at December 31, 2018 - $ -
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: Year Ended December 31, 2018 Expected volatility 73.8% - 86.4% Expected dividends 0.0% Expected term (in years) 5.25 - 6.25 Risk-free rate 2.6% - 3.1%
Schedule 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, 2018 Units Weighted- Average Grant Date Fair Value Unvested restricted Series D Common Units at December 31, 2017 1,404,265 $ 4.83 Vested units prior to Corporate Conversion (66,019 ) 3.46 Forfeited units prior to Corporate Conversion (8,932 ) 2.99 Unvested Series D Common Units at January 25, 2018 prior to Corporate Conversion 1,329,314 4.91 Conversion to restricted shares of common stock upon Corporate Conversion 1,128,182 5.79 Vested shares of Common Stock (334,144 ) 4.51 Forfeited (50,474 ) 4.99 Unvested restricted Series D Common Units at December 31, 2018 743,564 $ 6.40
Schedule of Equity-based Compensation Expense	The Company recorded equity-based compensation expense related to all of its share and unit-based awards to employees and non-employees in the following captions within its consolidated statements of operations for the years ended December 31, 2018, 2017 and 2016 Year Ended December 31, 2018 2017 2016 Research and development expenses $ 4,180 $ 1,206 $ 1,262 General and administrative expenses $ 1,775 $ 4,124 $ 208 $ 5,955 $ 5,330 $ 1,470

Commitments and Contingencies (

Commitments and Contingencies (Tables)	12 Months Ended
	Dec. 31, 2018
Commitments And Contingencies Disclosure [Abstract]
Summary of Fixed and Non-cancelable Payments Related to Operating and Capital Lease Commitments	At December 31, 2018 the Company expects to make the following fixed and non-cancelable payments related to its operating and capital lease commitments: Year Ending December 31, 2019 $ 2,210 2020 2,293 2021 2,332 2022 1,508 2023 278 Thereafter - Total $ 8,621

Net Loss per Share (Tables)

Net Loss per Share (Tables)	12 Months Ended
	Dec. 31, 2018
Earnings Per Share [Abstract]
Schedule of Basic and Diluted Net Loss per Share Attributable to Common Stockholders	The numerator for basic and diluted net loss per share attributable to common stockholders is as follows: Year Ended December 31, 2018 2017 2016 Net loss $ (74,798 ) $ (53,178 ) $ (23,773 ) Net loss attributable to non-controlling interest - (1,060 ) (2,234 ) Net loss attributable to Solid Biosciences Inc. $ (74,798 ) $ (52,118 ) $ (21,539 ) Decretion (accretion) of preferred units to redemption value - (959 ) 4,309 Redemption of preferred units - 15,685 - Redemption of redeemable interest from non-controlling interest in Solid GT - (1,925 ) - Net loss attributable to common stockholders $ (74,798 ) $ (39,317 ) $ (17,230 ) The denominator is as follows: Year Ended December 31 2018 2017 2016 Weighted average common stock outstanding, basic and diluted 33,262,597 13,649,485 1,698,904 Net loss per share attributable to common stockholders, basic and diluted is as follows: Year Ended December 31, 2018 2017 2016 Net loss per share attributable to common stockholders, basic and diluted $ (2.25 ) $ (2.88 ) $ (10.14 )
Schedule of Antidilutive Securities Excluded from Calculation of Diluted Net Loss per Share Attributable to Common Stockholders	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 attributable to common stockholders for the periods indicated because including them would have had an anti-dilutive effect: Year Ended December 31, 2018 2017 2016 Options to purchase shares of common stock 1,153,198 - - Unvested shares of common stock 743,564 - - Series A common units - - 1,104,391 Series B common units - - - Series D common units - 1,404,265 - 1,896,762 1,404,265 1,104,391

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Schedule of Reconciliation of Income Taxes Computed by U.S. Federal Statutory Rate	December 31, 2018 Income tax computed at federal statutory tax rate 21.0 % State taxes, net of federal benefit 5.5 % Permanent differences (0.8 )% Tax credits 9.2 % Loss taxed as a partnership (1.7 )% Conversion to a C-Corporation 0.5 % Other (0.1 )% Valuation allowance (33.6 )% 0.0 %
Schedule of Net Deferred Tax Assets	December 31 2018 Deferred tax assets: Tax loss carryforwards $ 16,417 Tax credit carryforwards 6,903 Deferred expenses 372 Accrued expenses 935 Stock Compensation 856 Other 177 Total deferred tax assets 25,660 Depreciation (406 ) Valuation allowance (25,254 ) Net deferred taxes $ -

Selected Quarterly Financial _2

Selected Quarterly Financial Information (Tables)	12 Months Ended
	Dec. 31, 2018
Quarterly Financial Information Disclosure [Abstract]
Schedule of Selected Quarterly Financial Information (Unaudited)	Selected quarterly results from operations for the years ended December 31, 2018 and 2017 are as follows: 2018 Quarter Ended March 31 June 30 September 30 December 31 (in thousands, except for per share data) Revenues $ - $ - $ - $ - Operating expenses 15,973 18,178 19,150 22,386 Loss from operations (15,973 ) (18,178 ) (19,150 ) (22,386 ) Net loss (15,877 ) (17,980 ) (19,020 ) (21,921 ) Net loss attributable to Solid Biosciences Inc. (15,877 ) (17,980 ) (19,020 ) (21,921 ) Net loss attributable to common stockholders (15,877 ) (17,980 ) (19,020 ) (21,921 ) Net loss per share attributable to common stockholders $ (0.54 ) $ (0.52 ) $ (0.55 ) $ (0.63 ) 2017 Quarter Ended March 31 June 30 September 30 December 31 (in thousands, except for per unit data) Revenues $ - $ - $ - $ - Operating expenses 14,113 11,887 13,696 15,161 Loss from operations (14,113 ) (11,887 ) (13,696 ) (15,161 ) Net loss (13,875 ) (11,311 ) (13,505 ) (14,487 ) Net loss attributable to Solid Biosciences, LLC (12,815 ) (11,311 ) (13,505 ) (14,487 ) Net loss attributable to common unitholders (14 ) (11,311 ) (13,505 ) (14,487 ) Net loss per unit attributable to common unitholders $ (0.01 ) $ (0.66 ) $ (0.79 ) $ (0.84 )

Nature of the Business and Ba_2

Nature of the Business and Basis of Presentation - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	Jan. 30, 2018	Nov. 30, 2015	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Jan. 25, 2018
Nature Of Business And Basis Of Presentation [Line Items]
Net loss			$ 21,921	$ 19,020	$ 17,980	$ 15,877	$ 14,487	$ 13,505	$ 11,311	$ 13,875	$ 74,798	$ 53,178	$ 23,773
Accumulated deficit			199,056				124,258				199,056	124,258
Net cash used in operating activities											70,197	43,224	20,120
Cash, cash equivalents and available-for-sale securities			122,464								122,464
Net loss attributable to non-controlling interest											0	(1,060)	(2,234)
Carrying value of non-controlling interest			$ 0				$ 0				0	0
Solid GT, LLC [Member]
Nature Of Business And Basis Of Presentation [Line Items]
Voting ownership percentage before transactions		100.00%
Voting ownership percentage after transactions		77.00%
Net loss attributable to non-controlling interest											$ 0	$ (1,060)	$ (2,234)
IPO [Member]
Nature Of Business And Basis Of Presentation [Line Items]
Number of shares sold	8,984,375
Public offering price per share	$ 16
Net proceeds from issuance	$ 129,096
Corporate Conversion [Member]
Nature Of Business And Basis Of Presentation [Line Items]
Common stock, conversion ratio														84.85%

Summary of Significant Accoun_4

Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Jan. 01, 2019
Significant Accounting Policies [Line Items]
Restricted cash non-current	$ 327,000
Restricted cash, current		$ 65,000	$ 165,000
Deferred offering costs	0	3,106,000
Other income	$ 270,000	$ 1,001,000	$ 271,000
Subsequent Event [Member] | Minimum [Member]
Significant Accounting Policies [Line Items]
Lease liability and related right-of-use asset				$ 4,500,000
Subsequent Event [Member] | Maximum [Member]
Significant Accounting Policies [Line Items]
Lease liability and related right-of-use asset				$ 6,000,000
2018 Omnibus Incentive Plan [Member]
Significant Accounting Policies [Line Items]
Number of shares authorized for common stock for equity awards	5,001,000

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, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reconciliation Of Beginning And End Of Period Amounts From Cash Flow Statement To Balance Sheet [Abstract]
Cash and cash equivalents as presented on balance sheet	$ 86,366	$ 52,080	$ 7,678
Restricted cash, current, as presented on balance sheet		65	165
Restricted cash, non-current, as presented on balance sheet	327
Cash and cash equivalents and restricted cash as presented on cash flow statement	$ 86,693	$ 52,145	$ 7,843	$ 28,595

Merger and Recapitalization - A

Merger and Recapitalization - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	Mar. 29, 2017	Oct. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2016	Dec. 31, 2017	Dec. 31, 2018
Solid GT, LLC [Member] | Series D Common Units [Member]
Schedule of Capitalization [Line Items]
Common units exchanged, units	134,920
Series C Common Units in Exchange for Class B Non-controlling Interest in Solid GT [Member]
Schedule of Capitalization [Line Items]
Common units exchanged, units	50,000
Issuance of common units in exchange, units	1,635,916
Series A Common Units in Exchange for Redeemable Preferred Units [Member]
Schedule of Capitalization [Line Items]
Issuance of common units in exchange, units	12,219,299
Series B Common Units in exchange for Series A Common Units [Member]
Schedule of Capitalization [Line Items]
Common units exchanged, units	4,560,000
Issuance of common units in exchange, units	3,258,480
Series D Common Units in Exchange for Series A Common Units [Member]
Schedule of Capitalization [Line Items]
Common units exchanged, units	563,917
Issuance of common units in exchange, units	402,963
Series 1 Senior Preferred Units [Member]
Schedule of Capitalization [Line Items]
Issuance of preferred units, units	2,500,000					2,500,000
Public offering price per share	$ 10
Proceeds from issuance of units	$ 25,000
Junior Preferred Units [Member]
Schedule of Capitalization [Line Items]
Issuance of preferred units, units	4,414,356					4,414,356
Public offering price per share							$ 9.63
Redeemable Preferred Units [Member]
Schedule of Capitalization [Line Items]
Issuance of preferred units, units		6,840,000	3,420,000	3,420,000	3,420,000
Public offering price per share		$ 1	$ 1	$ 1	$ 1
Issuance of Series A common units in exchange for redeemable preferred units, units	17,100,000

Merger and Recapitalization - S

Merger and Recapitalization - Schedule of Pre-Merger And Post-Merger Capitalization Structure (Detail)	Mar. 29, 2017shares
Pre Merger And Recapitalization | Redeemable Preferred Units [Member]
Schedule of Capitalization [Line Items]
Common units exchanged, units	17,100,000
Pre Merger And Recapitalization | Series A Common Founders
Schedule of Capitalization [Line Items]
Common units exchanged, units	4,560,000
Pre Merger And Recapitalization | Series A Common Others
Schedule of Capitalization [Line Items]
Common units exchanged, units	563,917
Pre Merger And Recapitalization | Class A Voting
Schedule of Capitalization [Line Items]
Common units exchanged, units	450,000
Pre Merger And Recapitalization | Class B Non Voting
Schedule of Capitalization [Line Items]
Common units exchanged, units	50,000
Pre Merger And Recapitalization | Class C Non Voting
Schedule of Capitalization [Line Items]
Common units exchanged, units	33,107
Pre Merger And Recapitalization | Class D Voting
Schedule of Capitalization [Line Items]
Common units exchanged, units	134,920
Pre Merger And Recapitalization | Common Units Series A
Schedule of Capitalization [Line Items]
Common units exchanged, units	5,123,917
Post Merger And Recapitalization | Series A Common Units [Member]
Schedule of Capitalization [Line Items]
Issuance of common units in exchange, units	12,219,299
Post Merger And Recapitalization | Series B Common Units
Schedule of Capitalization [Line Items]
Issuance of common units in exchange, units	3,258,480
Post Merger And Recapitalization | Series D Common Units [Member]
Schedule of Capitalization [Line Items]
Issuance of common units in exchange, units	402,963
Post Merger And Recapitalization | Series D Common Units [Member] | Solid GT, LLC [Member]
Schedule of Capitalization [Line Items]
Issuance of common units in exchange, units	1,083,205
Post Merger And Recapitalization | Series C Common Units [Member] | Solid GT, LLC [Member]
Schedule of Capitalization [Line Items]
Issuance of common units in exchange, units	1,635,916
Post Merger And Recapitalization | Junior Preferred | Solid GT, LLC [Member]
Schedule of Capitalization [Line Items]
Issuance of common units in exchange, units	4,414,356
Post Merger And Recapitalization | Series A B C D Common Units | Solid GT, LLC [Member]
Schedule of Capitalization [Line Items]
Issuance of common units in exchange, units	18,599,863

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 ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Assets:
Cash equivalents	$ 54,423
Available for sale securities	36,098	$ 17,014
Assets Fair Value Disclosure	90,521
Level 2 [Member]
Assets:
Cash equivalents	54,423
Available for sale securities	36,098	$ 17,014
Assets Fair Value Disclosure	$ 90,521

Fair Value of Financial Asset_4

Fair Value of Financial Assets and Liabilities - Additional Information (Detail) - USD ($)	Dec. 31, 2018	Dec. 31, 2017
Fair Value Disclosures [Abstract]
Fair value, assets, level 1 to level 2 transfers, amount	$ 0	$ 0
Fair value, assets, level 2 to level 1 transfers, amount	0	0
Fair value, assets, level 3 to level 1 transfers, amount	0	0
Fair value, assets, level 1 to level 3 transfers, amount	$ 0	$ 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 ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 36,103	$ 17,027
Gross Unrealized Gain	2	0
Gross Unrealized Loss	(7)	(13)
Fair Value	36,098	17,014
U.S. Government Agency Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	13,543	9,473
Gross Unrealized Gain	0	0
Gross Unrealized Loss	(2)	(7)
Fair Value	13,541	9,466
Corporate Bond Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	12,860	7,554
Gross Unrealized Gain	2	0
Gross Unrealized Loss	(5)	(6)
Fair Value	12,857	$ 7,548
Commercial Paper [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	9,700
Gross Unrealized Gain	0
Fair Value	$ 9,700

Available-for-Sale Securities_2

Available-for-Sale Securities - Summary Estimated Fair Value and Amortized Cost of Available-for-Sale Securities by Contractual Maturity (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Investments Debt And Equity Securities [Abstract]
Due in one year or less, amortized cost	$ 36,103	$ 17,027
Total available-for-sale securities, amortized cost	36,103	17,027
Due in one year or less, fair value	36,098	17,014
Total available-for-sale securities, fair value	$ 36,098	$ 17,014

Available-for-Sale Securities_3

Available-for-Sale Securities - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Investments Debt And Equity Securities [Abstract]
Available-for-sale securities weighted average maturity period	3 months 18 days	4 months 24 days

Prepaid Expenses and Other Cu_3

Prepaid Expenses and Other Current Assets - Schedule of Prepaid Expenses and Other Current Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Deferred Costs Capitalized Prepaid And Other Assets Disclosure [Abstract]
Prepaid research and development expenses	$ 4,365	$ 951
Prepaid expenses and other assets	1,810	548
Prepaid expenses and other current assets	$ 6,175	$ 1,499

Property and Equipment - Summar

Property and Equipment - Summary of Property and Equipment (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 12,410	$ 2,933
Less accumulated depreciation	1,988	504
Property and equipment, net	10,422	2,429
Furniture and Fixtures [Member]
Property Plant And Equipment [Line Items]
Property and equipment, gross	187	61
Laboratory Equipment [Member]
Property Plant And Equipment [Line Items]
Property and equipment, gross	5,998	2,338
Leasehold Improvements [Member]
Property Plant And Equipment [Line Items]
Property and equipment, gross	4,585	68
Computer Equipment [Member]
Property Plant And Equipment [Line Items]
Property and equipment, gross	166	77
Computer Software [Member]
Property Plant And Equipment [Line Items]
Property and equipment, gross	123	23
Construction in Progress [Member]
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 1,351	$ 366

Property and Equipment - Additi

Property and Equipment - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Property Plant And Equipment [Abstract]
Depreciation expense	$ 1,566	$ 448	$ 56

Accrued Expenses - Summary of A

Accrued Expenses - Summary of Accrued Expenses and Other Current Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Accrued Expenses And Other Current Liabilities [Line Items]
Accrued expenses and other current liabilities	$ 8,235	$ 5,972
Accrued Research and Development [Member]
Accrued Expenses And Other Current Liabilities [Line Items]
Accrued expenses and other current liabilities	3,529	1,855
Accrued Compensation [Member]
Accrued Expenses And Other Current Liabilities [Line Items]
Accrued expenses and other current liabilities	3,534	2,033
Accrued Other [Member]
Accrued Expenses And Other Current Liabilities [Line Items]
Accrued expenses and other current liabilities	$ 1,172	$ 2,084

Preferred Unit Tranche Rights -

Preferred Unit Tranche Rights - Roll-Forward of Each Tranche Right (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Redeemable Preferred Unit Tranche Right [Member]
Preferred Units [Line Items]
Beginning Balance	$ 0	$ 0	$ 12,004
Issuance	0	0
Change in fair value	0	0	(1,163)
Reclassification to preferred units			(10,841)
Ending Balance	0	0	0
Series 1 Senior Preferred Units [Member]
Preferred Units [Line Items]
Beginning Balance	0	0	0
Issuance	0	459
Change in fair value	0	(459)	0
Reclassification to preferred units			0
Ending Balance	$ 0	$ 0	$ 0

Redeemable Preferred Units, S_3

Redeemable Preferred Units, Series 2 and Series 1 Senior Preferred Units and Junior Preferred Units - Additional Information (Details) - USD ($) $ / shares in Units, $ in Thousands	Mar. 29, 2022	Oct. 26, 2017	Mar. 29, 2017	Oct. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Temporary Equity [Line Items]
Proceeds from issuance of redeemable preferred units										$ 3,420
Solid GT, LLC [Member] | Series D Common Units [Member]
Temporary Equity [Line Items]
Common units exchanged, units			134,920
Redeemable Preferred Units [Member]
Temporary Equity [Line Items]
Issuance of preferred units, units				6,840,000	3,420,000	3,420,000	3,420,000
Public offering price per share				$ 1	$ 1	$ 1	$ 1			$ 1
Proceeds from issuance of redeemable preferred units				$ 6,840	$ 3,420	$ 3,420	$ 3,420
Series 2 Senior Preferred Units [Member]
Temporary Equity [Line Items]
Issuance of preferred units, units		4,886,000							4,886,000
Public offering price per share		$ 11.26
Proceeds from issuance of redeemable preferred units		$ 55,002
Obligated to purchase by holder									1,973,430
Share price per unit									$ 12.67
Proceeds from issuance of units in the event of pre-clinical milestone achievement									$ 25,000
Dividends declared								$ 0
Series 2 Senior Preferred Units [Member] | Scenario, Forecast
Temporary Equity [Line Items]
Preferred stock, redemption price per share	$ 11.26
Preferred stock, dividend rate, percentage	8.00%
Series 1 Senior Preferred Units [Member]
Temporary Equity [Line Items]
Issuance of preferred units, units			2,500,000						2,500,000
Public offering price per share			$ 10
Proceeds from issuance of units			$ 25,000
Dividends declared								0
Series 1 Senior Preferred Units [Member] | Scenario, Forecast
Temporary Equity [Line Items]
Preferred stock, redemption price per share	$ 10
Preferred stock, dividend rate, percentage	8.00%
Junior Preferred Units [Member]
Temporary Equity [Line Items]
Issuance of preferred units, units			4,414,356						4,414,356
Public offering price per share								$ 9.63
Preferred stock, dividend rate, percentage								8.00%
Dividends declared								$ 0

Redeemable Preferred Units, S_4

Redeemable Preferred Units, Series 2 and Series 1 Senior Preferred Units and Junior Preferred Units - Summary of Preferred Units (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Class Of Stock [Line Items]
Carrying Value	$ 0	$ 0
Redeemable Preferred Units [Member]
Class Of Stock [Line Items]
Authorized	0		60,000,000
Issued	0		17,100,000
Outstanding	0		17,100,000
Carrying Value	$ 0		$ 71,649
Liquidation Preference	$ 0		$ 55,746
Series 2 Senior Preferred Units [Member]
Class Of Stock [Line Items]
Authorized	0	4,886,000
Issued	0	4,886,000
Outstanding	0	4,886,000
Carrying Value	$ 0	$ 55,002
Liquidation Preference	$ 0	$ 55,002
Common Units Issuable Upon Conversion	0	4,886,000
Series 1 Senior Preferred Units [Member]
Class Of Stock [Line Items]
Authorized	0	2,500,000
Issued	0	2,500,000
Outstanding	0	2,500,000
Carrying Value	$ 0	$ 25,000
Liquidation Preference	$ 0	$ 25,000
Common Units Issuable Upon Conversion	0	2,500,000
Junior Preferred Units [Member]
Class Of Stock [Line Items]
Authorized	0	4,414,356
Issued	0	4,414,356
Outstanding	0
Carrying Value	$ 0	$ 44,177
Liquidation Preference	$ 0	$ 42,500

Members Deficit - Summary of Se

Members Deficit - Summary of Series A, B, C and D Common Units (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Authorized	0	20,189,509
Issued and Outstanding		19,438,552
Carrying Value		$ 65,014
Series A Common Units [Member]
Authorized		12,219,299	20,000,000
Issued and Outstanding		12,219,299	5,123,917
Carrying Value		$ 55,964	$ 558
Series B Common Units [Member]
Authorized		3,258,480
Issued and Outstanding		3,258,480
Carrying Value		$ 3,613
Series C Common Units [Member]
Authorized		1,635,916
Issued and Outstanding		1,635,916
Carrying Value		$ 2,053
Series D Common Units [Member]
Authorized		3,075,814
Issued and Outstanding		2,324,857
Carrying Value		$ 3,384

Members Deficit - Additional In

Members Deficit - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	Mar. 29, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Sep. 30, 2016	May 31, 2016	Nov. 30, 2015	Sep. 30, 2015	Nov. 30, 2014	Mar. 31, 2014	Dec. 27, 2013
Common stock, shares issued		35,432,460	0
Equity-based compensation expense		$ 5,955	$ 5,330	$ 1,470
Common stock, par value		$ 0.001	$ 0.001
Series B Common Units in exchange for Series A Common Units [Member]
Common units exchanged, units	4,560,000
Issuance of common units in exchange, units	3,258,480
Equity-based compensation expense	$ 2,710
Additional share based compensation expense			$ 904
Series D Common Units in Exchange for Series A Common Units [Member]
Common units exchanged, units	563,917
Issuance of common units in exchange, units	402,963
Equity-based compensation expense	$ 140	$ 54	48
Additional share based compensation expense	$ 115
Vesting period		4 years
Restricted Series A Common Units [Member]
Common stock, shares issued				48,000		60,000	60,000	305,000	305,000	169,667	169,667	4,560,000
Common unit vested				3,420,000	2,280,000
Aggregated intrinsic value, common unit vested			$ 0	$ 3,306
Common stock, par value				$ 2.25						$ 2.59	$ 2.59
Restricted Series A Common Units [Member] | Minimum [Member]
Common stock, par value						$ 2.03	$ 2.03	$ 2.39	$ 2.39
Restricted Series A Common Units [Member] | Maximum [Member]
Common stock, par value						$ 2.14	$ 2.14	$ 2.65	$ 2.65

Equity-Based Compensation - Add

Equity-Based Compensation - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Stock Option [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized equity-based compensation cost related to stock option	$ 19,528
Stock options outstanding, weighted average recognition period	3 years 2 months 12 days
Stock options vested and expected to vest, weighted average recognition period	3 years 2 months 12 days
Stock options outstanding, intrinsic value	$ 2,260
Stock options vested and expected to vest, intrinsic value	$ 2,260
Weighted average fair value of options to purchase common stock granted	$ 19.94
Restricted Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized equity-based compensation cost related to common units	$ 4,466
Unrecognized equity-based compensation cost related to stock options, period for recognition	1 year 3 months 18 days
Aggregated intrinsic value, common unit vested	$ 8,721	$ 3,358	$ 351
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

Equity-Based Compensation - Sum

Equity-Based Compensation - Summary of Stock Option Activity (Detail)	12 Months Ended
	Dec. 31, 2018$ / sharesshares
Shares
Granted | shares	1,185,207
Forfeitures | shares	(32,009)
Outstanding at December 31, 2018 | shares	1,153,198
Vested and expected to vest as of December 31, 2018 | shares	1,153,198
Exercisable at December 31, 2018 | shares	0
Granted | $ / shares	$ 28.51
Forfeitures | $ / shares	20.80
Outstanding at December 31, 2018 | $ / shares	28.73
Vested and expected to vest as of December 31, 2018 | $ / shares	28.73
Exercisable at December 31, 2018 | $ / shares	$ 0

Equity-Based Compensation - Ass

Equity-Based Compensation - Assumptions for Measuring Fair Values of Stock Options (Detail)	12 Months Ended
	Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected dividends	0.00%
Minimum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected volatility	73.80%
Expected term (in years)	5 years 3 months
Risk-free rate	2.60%
Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected volatility	86.40%
Expected term (in years)	6 years 3 months
Risk-free rate	3.10%

Equity-Based Compensation - Sch

Equity-Based Compensation - Schedule of Unvested Restricted Shares of Common Stock Activity (Detail) - Restricted Stock [Member]	12 Months Ended
	Dec. 31, 2018$ / sharesshares
Number of Options
Unvested restricted Series D Common Units at December 31, 2017 | shares	1,404,265
Vested units prior to Corporate Conversion | shares	(66,019)
Forfeited units prior to Corporate Conversion | shares	(8,932)
Unvested Series D Common Units at January 25, 2018 prior to Corporate Conversion | shares	1,329,314
Conversion to restricted shares of common stock upon Corporate Conversion | shares	1,128,182
Vested shares of Common Stock | shares	(334,144)
Forfeited | shares	(50,474)
Unvested restricted Series D Common Units at December 31, 2018 | shares	743,564
Weighted-Average Grant Date Fair Value
Unvested restricted Series D Common Units at December 31, 2017 | $ / shares	$ 4.83
Vested units prior to Corporate Conversion | $ / shares	3.46
Forfeited units prior to Corporate Conversion | $ / shares	2.99
Unvested Series D Common Units at January 25, 2018 prior to Corporate Conversion | $ / shares	4.91
Conversion to restricted shares of common stock upon Corporate Conversion | $ / shares	5.79
Vested shares of Common Stock | $ / shares	4.51
Forfeited | $ / shares	4.99
Unvested restricted Series D Common Units at December 31, 2018 | $ / shares	$ 6.40

Equity-Based Compensation - S_2

Equity-Based Compensation - Schedule of Equity-based Compensation Expense (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock-based compensation expense	$ 5,955	$ 5,330	$ 1,470
Research and Development Expenses [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock-based compensation expense	4,180	1,206	1,262
General and Administrative Expenses [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock-based compensation expense	$ 1,775	$ 4,124	$ 208

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Detail) $ in Thousands	1 Months Ended		12 Months Ended
	Apr. 30, 2018USD ($)	Jan. 31, 2018USD ($)ft²	Dec. 31, 2018USD ($)	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)	Nov. 30, 2018USD ($)
Commitment And Contingencies [Line Items]
Contingency settled amount to third-party, under agreement	$ 1,320
Lab Space In Cambridge Massachusetts [Member]
Commitment And Contingencies [Line Items]
Area of lease agreement | ft²		9,500
Lease term		5 years
Lease renewal term		2 years
Future minimum rent commitment		$ 3,387
Office Space In Cambridge Massachusetts [Member]
Commitment And Contingencies [Line Items]
Area of lease agreement | ft²		16,000
Future minimum rent commitment		$ 3,922
Lease expiration period		2022-02
Office and Laboratory Lease [Member]
Commitment And Contingencies [Line Items]
Future minimum rent commitment			$ 7,309
rental expense related to office and laboratory space			2,524	$ 1,301	$ 270
Letter of credit outstanding amount			$ 327	$ 65
Lab Equipment In Cambridge Massachusetts [Member]
Commitment And Contingencies [Line Items]
Capital lease term						48 months
Future minimum lease commitment						$ 1,313

Commitments and Contingencies_2

Commitments and Contingencies - Summary of Fixed and Non-cancelable Payments Related to Operating and Capital Lease Commitments (Detail) $ in Thousands	Dec. 31, 2018USD ($)
Contractual Obligation Fiscal Year Maturity [Abstract]
2019	$ 2,210
2020	2,293
2021	2,332
2022	1,508
2023	278
Total	$ 8,621

License Agreement - Additional

License Agreement - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Indefinite Lived Intangible Assets By Major Class [Line Items]
Research and development	$ 57,965,000	$ 39,905,000	$ 20,116,000
University of Washington [Member]
Indefinite Lived Intangible Assets By Major Class [Line Items]
Maximum milestone payment to be made				$ 1,000,000
Research and development	47,000	135,000	0
University of Missouri [Member]
Indefinite Lived Intangible Assets By Major Class [Line Items]
Maximum milestone payment to be made				$ 1,000,000
Research and development	10,000	11,000	0
University of Michigan [Member]
Indefinite Lived Intangible Assets By Major Class [Line Items]
Maximum milestone payment to be made			1,000,000
Research and development	35,000	4,000	145,000
President and Fellows of Harvard College [Member]
Indefinite Lived Intangible Assets By Major Class [Line Items]
Research and development	$ 20,000	45,000	$ 45,000
License agreement initial term	15 years
License agreement renewal term	3 years
Harvard College [Member]
Indefinite Lived Intangible Assets By Major Class [Line Items]
Research and development	$ 5,000	$ 23,000
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 Attributable to Common Stockholders (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Earnings Per Share [Abstract]
Net loss	$ (21,921)	$ (19,020)	$ (17,980)	$ (15,877)	$ (14,487)	$ (13,505)	$ (11,311)	$ (13,875)	$ (74,798)	$ (53,178)	$ (23,773)
Net loss attributable to non-controlling interest									0	(1,060)	(2,234)
Net loss attributable to Solid Biosciences Inc.	(21,921)	(19,020)	(17,980)	(15,877)	(14,487)	(13,505)	(11,311)	(12,815)	(74,798)	(52,118)	(21,539)
Decretion (accretion) of preferred units to redemption value										(959)	4,309
Redemption of preferred units										15,685
Redemption of redeemable interest from non-controlling interest in Solid GT										(1,925)
Net loss attributable to common stockholders	$ (21,921)	$ (19,020)	$ (17,980)	$ (15,877)	$ (14,487)	$ (13,505)	$ (11,311)	$ (14)	$ (74,798)	$ (39,317)	$ (17,230)
Weighted average common stock outstanding, basic and diluted									33,262,597	13,649,485	1,698,904
Net loss per share attributable to common stockholders, basic and diluted	$ (0.63)	$ (0.55)	$ (0.52)	$ (0.54)	$ (0.84)	$ (0.79)	$ (0.66)	$ (0.01)	$ (2.25)	$ (2.88)	$ (10.14)

Net Loss per Share - Schedule_2

Net Loss per Share - Schedule of Antidilutive Securities Excluded from Calculation of Diluted Net Loss per Share Attributable to Common Stockholders (Detail) - shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Common stock equivalents excluded from calculation of diluted net loss per share	1,896,762	1,404,265	1,104,391
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	1,153,198
Series A Common 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			1,104,391
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	743,564
Series D Common 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		1,404,265

Income Taxes - Additional Infor

Income Taxes - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2018USD ($)
Income Taxes [Line Items]
Income tax expense (benefit)	$ 0
Uncertain tax position	0
Deferred tax assets, valuation allowances	25,254,000
Unrecognized tax benefits	0
Tax interest and penalties	0
Domestic Tax Authority [Member]
Income Taxes [Line Items]
Net operating loss carryforwards	59,357,000
Federal tax credits carryforwards	6,349,000
State and Local Jurisdiction [Member]
Income Taxes [Line Items]
Net operating loss carryforwards	59,453,000
Federal tax credits carryforwards	$ 702,000

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, 2018
Income Tax Disclosure [Abstract]
Income tax computed at federal statutory tax rate	21.00%
State taxes, net of federal benefit	5.50%
Permanent differences	(0.80%)
Tax credits	(9.20%)
Loss taxed as a partnership	(1.70%)
Conversion to a C-Corporation	0.50%
Other	(0.10%)
Valuation allowance	(33.60%)
Total	0.00%

Income Taxes - Schedule of Net

Income Taxes - Schedule of Net Deferred Tax Assets (Detail) $ in Thousands	Dec. 31, 2018USD ($)
Deferred tax assets:
Tax loss carryforwards	$ 16,417
Tax credit carryforwards	6,903
Deferred expenses	372
Accrued expenses	935
Stock Compensation	856
Other	177
Total deferred tax assets	25,660
Depreciation	(406)
Valuation allowance	(25,254)
Net deferred taxes	$ 0

Retirement Plan - Additional In

Retirement Plan - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Compensation And Retirement Disclosure [Abstract]
Company's Contribution to the Plan	$ 0	$ 0	$ 0

Selected Quarterly Financial _3

Selected Quarterly Financial Information (Unaudited) - Schedule of Unaudited Quarterly Result of Operations (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Quarterly Financial Information Disclosure [Abstract]
Revenue	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0
Operating expenses	22,386	19,150	18,178	15,973	15,161	13,696	11,887	14,113	75,687	54,857	25,576
Loss from operations	(22,386)	(19,150)	(18,178)	(15,973)	(15,161)	(13,696)	(11,887)	(14,113)	(75,687)	(54,857)	(25,576)
Net loss	(21,921)	(19,020)	(17,980)	(15,877)	(14,487)	(13,505)	(11,311)	(13,875)	(74,798)	(53,178)	(23,773)
Net loss attributable to Solid Biosciences Inc./Solid Biosciences, LLC	(21,921)	(19,020)	(17,980)	(15,877)	(14,487)	(13,505)	(11,311)	(12,815)	(74,798)	(52,118)	(21,539)
Net loss attributable to common stockholders/unitholders	$ (21,921)	$ (19,020)	$ (17,980)	$ (15,877)	$ (14,487)	$ (13,505)	$ (11,311)	$ (14)	$ (74,798)	$ (39,317)	$ (17,230)
Net loss per share/unit attributable to common stockholders/unitholders	$ (0.63)	$ (0.55)	$ (0.52)	$ (0.54)	$ (0.84)	$ (0.79)	$ (0.66)	$ (0.01)	$ (2.25)	$ (2.88)	$ (10.14)