Avexis (AVXS) 10-K 28 Feb 18 2017 FY Annual report Financial data

Document and Entity Information

Document and Entity Information - USD ($)	12 Months Ended
	Dec. 31, 2017	Feb. 26, 2018	Jun. 30, 2017
Document and Entity Information [Abstract]
Entity Registrant Name	AveXis, Inc.
Entity Central Index Key	1,652,923
Document Type	10-K
Document Period End Date	Dec. 31, 2017
Amendment Flag	false
Current Fiscal Year End Date	--12-31
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Filer Category	Large Accelerated Filer
Entity Public Float			$ 2,265,553,538
Entity Common Stock, Shares Outstanding		36,725,399
Document Fiscal Year Focus	2,017
Document Fiscal Period Focus	FY

Condensed Consolidated Balance

Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Current assets:
Cash and cash equivalents	$ 324,117	$ 240,430
Prepaid expenses and other current assets	8,321	4,750
Total current assets	332,438	245,180
Property and equipment, net	56,174	24,201
Other long-term assets	2,966	1,194
Total assets	391,578	270,575
Current liabilities:
Accounts payable	10,587	3,197
Accrued expenses	44,938	16,794
Accrued indemnification obligation	2,788	4,452
Total current liabilities	58,313	24,443
Long-term liabilities	525
Total liabilities	58,838	24,443
Preferred stock
Common stock; par value $0.0001 per share, 100,000,000 shares authorized, 31,972,377 shares issued and outstanding at September 30, 2017; 27,700,054 shares issued and outstanding at December 31, 2016	3	3
Additional paid-in capital	692,350	387,691
Accumulated deficit	(359,613)	(141,562)
Total stockholders’ equity	332,740	246,132
Total liabilities, redeemable common stock and stockholders’ equity	$ 391,578	$ 270,575

Condensed Consolidated Balance3

Condensed Consolidated Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2017	Dec. 31, 2016
Preferred stock
Preferred stock, par value (in dollars per share)	$ 0.0001	$ 0.0001
Preferred stock, shares authorized (in shares)	10,000,000	10,000,000
Preferred stock, shares issued (in shares)	0	0
Common stock
Common stock, par value (in dollars per share)	$ 0.0001	$ 0.0001
Common stock, shares authorized (in shares)	100,000,000	27,700,000
Common stock, shares issued (in shares)	32,090,000	27,700,000
Common stock, shares outstanding (in shares)	32,090,000

Condensed Consolidated Statemen

Condensed Consolidated Statements of Operations and Comprehensive Loss - USD ($) shares in Thousands, $ in Thousands	3 Months Ended				12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Operating expenses:
General and administrative					$ 69,976	$ 24,523	$ 11,080
Research and development expenses					150,391	58,892	27,493
Total operating expenses					220,367	83,415	38,573
Loss from operations	$ (25,547)	$ (21,180)	$ (15,798)	$ (20,889)	(220,367)	(83,415)	(38,573)
Other income	172	99	79	53			84
Interest income (expense)					2,316	403	15
Loss before income taxes					(218,051)	(83,012)	(38,474)
Net loss	$ (25,375)	$ (21,082)	$ (15,719)	$ (20,836)	$ (218,051)	$ (83,012)	$ (38,474)
Basic and diluted net loss per common share (in dollars per share)	$ (0.92)	$ (0.87)	$ (0.68)	$ (1.24)	$ (7.28)	$ (3.67)	$ (5.43)
Weighted-average basic and diluted common shares outstanding (in shares)	27,678	24,166	23,014	16,775	29,935	22,648	7,088

Condensed Consolidated Stateme5

Condensed Consolidated Statements of Changes in Redeemable Common Stock and Stockholders’ Equity (Deficit) - USD ($) $ in Thousands	Redeemable Common Stock	Class D Preferred Stock	Class C Preferred Stock	Class B-1 Preferred Stock	Employee Stock Options	Common StockIPO	Common StockFollow-On Offering	Common StockClass B Note	Common Stock	Additional Paid-in CapitalIPO	Additional Paid-in CapitalFollow-On Offering	Additional Paid-in CapitalClass B Note	Additional Paid-in Capital	Accumulated Deficit	IPO	Follow-On Offering	Class B Note	Total
Balance at the beginning of the period at Dec. 31, 2014	$ 560
Balance at the beginning of the period (in shares) at Dec. 31, 2014	418,000		1,262,000	2,226,000
Increase (Decrease) in Redeemable Common Stock
Issuance of redeemable common stock at fair value	$ 473
Issuance of redeemable common stock at fair value (in shares)	38,000	3,093,000	1,103,000	1,012,000
Balance at the end of the period at Dec. 31, 2015	$ 1,033
Balance at the end of the period (in shares) at Dec. 31, 2015	456,000	3,093,000	2,365,000	3,238,000
Balance at the beginning of the period at Dec. 31, 2014									$ 1				$ 17,643	$ (20,076)				$ (2,432)
Balance at the beginning of the period (in shares) at Dec. 31, 2014									6,472,000
Increase (Decrease) in Stockholders' Equity (Deficit)
Stock-based compensation - restricted stock													19,322
Stock based compensation—restricted stock																		19,322
Stock-based compensation - stock options													5,250					5,250
Stock-based compensation - stock warrants													359					359
Stock-based compensation – shares exchange													64,787					64,787
Stock-based compensation – share grants													5,000					5,000
Share exchanges													2,500					2,500
Conversion of convertible debt												$ 341
Exercise of stock options													521					521
Exercise of stock options (in shares)									207,000
Exercise of stock warrant																	$ 341
Exercise of stock warrant (in shares)								138,000
Net loss														(38,474)				(38,474)
Balance at the end of the period at Dec. 31, 2015									$ 1				115,723	(58,550)				57,174
Balance at the end of the period (in shares) at Dec. 31, 2015									6,817,000
Balance at the end of the period at Dec. 31, 2016	$ 0
Balance at the end of the period (in shares) at Dec. 31, 2016	0
Increase (Decrease) in Stockholders' Equity (Deficit)
Stock-based compensation - restricted stock													10,371					10,371
Stock based compensation—restricted stock (in shares)									1,751,000
Stock-based compensation - stock options													16,415					16,415
Shares conversion amount									$ 1				(1)
Shares converted (in shares)		(3,093,000)	(2,365,000)	(3,238,000)					8,696,000
Share conversion, converted to common stock	$ (1,033)												1,033					1,033
Share conversion, converted to common stock (in shares)	(456,000)								456,000
Exercise of stock options													430					430
Exercise of stock options (in shares)									84,000
Exercise of stock warrant													53					53
Exercise of stock warrant (in shares)									20,000
Issuance of stock						$ 1				$ 95,348	$ 148,319				$ 95,349	$ 148,319
Issuance of stock (in shares)						5,278,000	4,598,000
Net loss														(83,012)				(83,012)
Balance at the end of the period at Dec. 31, 2016		$ 0	$ 0	$ 0					$ 2				387,691	(141,562)				246,132
Balance at the end of the period (in shares) at Dec. 31, 2016		0	0	0					27,700,000
Increase (Decrease) in Stockholders' Equity (Deficit)
Stock based compensation—restricted stock													659					659
Stock-based compensation - stock options													30,885					30,885
Exercise of stock options													3,689					3,689
Exercise of stock options (in shares)					(268,000)				265,000
Issuance of stock											$ 269,869		(443)			$ 269,869		(443)
Issuance of stock (in shares)							4,111,000		14,000
Net loss														(218,051)				(218,051)
Balance at the end of the period at Dec. 31, 2017									$ 2				$ 692,350	$ (359,613)				$ 332,740
Balance at the end of the period (in shares) at Dec. 31, 2017									32,090,000

Condensed Consolidated Stateme6

Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash flows from operating activities
Net loss	$ (218,051)	$ (83,012)	$ (38,474)
Adjustments to reconcile net loss to net cash used in operating activities:
Loss on common stock settlement	11,647
Depreciation and amortization	2,270	64	14
Stock-based compensation	31,544	26,786	5,250
Stock-based third-party research and development			20,154
Disposal of fixed assets	629	24
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(3,667)	(3,854)	(887)
Other long-term assets	(1,771)	(1,132)	(1,312)
Accounts payable	6,374	958	276
Accrued expenses	15,308	11,773	1,183
Accrued indemnification obligation	(1,664)	372
Net cash used in operating activities, Total	(157,381)	(48,021)	(13,796)
Cash flows from investing activities
Capital expenditures	(32,047)	(19,182)	(221)
Net cash used in investing activities, Total	(32,047)	(19,182)	(221)
Net Cash Provided by (Used in) Financing Activities [Abstract]
Proceeds from exercise of stock options	3,689	430	520
Shares surrendered for taxes	(443)
Proceeds from exercise of stock warrants		53	341
Payments of deferred IPO costs		(1,592)
Proceeds from IPO and Follow-On Offering, net of issuance costs	269,869	246,490
Net cash provided by financing activities, Total	273,115	245,381	73,149
Net (decrease) increase in cash and cash equivalents	83,687	178,178	59,132
Cash and cash equivalents, beginning of period	240,430	62,252	3,120
Cash and cash equivalents, end of period	324,117	240,430	62,252
Supplemental cash flow information:
Capital expenditures in accounts payable and accrued liabilities	7,697	$ 4,872
Deferred issuance costs for underwritten offering in accounts payable and accrued liabilities	$ 83		371
Cash paid for interest			15
Class B Preferred Stock
Net Cash Provided by (Used in) Financing Activities [Abstract]
Proceeds from issuance of common stock, net of issuance costs			2,500
Class D Preferred Stock
Net Cash Provided by (Used in) Financing Activities [Abstract]
Proceeds from issuance of common stock, net of issuance costs			64,788
Class C Preferred Stock
Net Cash Provided by (Used in) Financing Activities [Abstract]
Proceeds from issuance of common stock, net of issuance costs			$ 5,000

Background

Background	12 Months Ended
	Dec. 31, 2017
Background
Background	AveXis, Inc. Notes to Consolidated Financial Statements 1. Background AveXis, Inc. was formed on March 8, 2010 in the state of Delaware as Biolife Cell Bank, LLC. In January 2012, the Company converted from a limited liability company to a corporation, Biolife Cell Bank, Inc. In January 2014, the Company amended and restated its Certificate of Incorporation to change its name to AveXis, Inc. (“AveXis” or “the Company”). The Company is a clinical‑stage gene therapy company dedicated to developing and commercializing novel treatments for patients suffering from rare and life‑threatening neurological genetic diseases. The Company’s initial product candidate, AVXS‑101, is its proprietary gene therapy product candidate for the treatment of spinal muscular atrophy, (“SMA). SMA is a severe neuromuscular disease characterized by the loss of motor neurons, leading to progressive muscle weakness and paralysis. SMA is generally divided into sub‑categories termed SMA Type 1, 2, 3 and 4. The Company is conducting a pivotal clinical trial for AVXS‑101 for the treatment of SMA Type 1, the leading genetic cause of infant mortality, and has initiated a Phase 1 clinical trial for the treatment of SMA Type 2. Liquidity and Risks As of December 31, 2017, the Company generated an accumulated deficit of $359.6 million since inception and had cash and cash equivalents of $324.1 million. The Company believes its cash and cash equivalents as of December 31, 2017 together with the net proceeds from its January 2018 underwritten public offering of approximately $431.9 million (Note 15), are sufficient to fund its current planned operations for at least the next twelve months from the issuance of these consolidated financial statements. As the Company continues to incur losses, transition to profitability is dependent upon the successful manufacturing, development, approval, and commercialization of its product candidate and achieving a level of revenues adequate to support the Company’s cost structure. The Company may never achieve profitability, and unless and until it does, the Company will continue to need to raise additional capital. Management intends to fund future operations through additional private or public debt or equity offerings and may seek additional capital through arrangements with strategic partners or from other sources. The Company is subject to risks common to companies in the biotechnology industry, including, but not limited to, development by the Company or its competitors of technological innovations, risks of failure of clinical studies, dependence on key personnel, protection of proprietary technology, compliance with government regulations, and ability to transition from preclinical manufacturing to commercial production of products. Initial Public Offering On February 10, 2016, the Company completed an initial public offering (“IPO”), which resulted in the issuance and sale of 4,750,000 shares of its common stock at a public offering price of $20.00 per share, resulting in net proceeds of $88.4 million after deducting underwriting discounts. Upon the closing of the IPO, the 3,278,938 shares of Class B-1 preferred stock, 326,557 shares of Class B-2 preferred stock, 2,365,020 shares of Class C preferred stock and 3,105,000 of Class D preferred stock were automatically converted into shares of the Company’s common stock. On March 3, 2016, the underwriters of the Company’s IPO exercised their over-allotment option to purchase an additional 527,941 shares of the Company’s common stock at the initial public offering price of $20.00 per share, resulting in additional net proceeds of $9.8 million after deducting underwriting discounts. September 2016 Underwritten Public Offering On September 13, 2016, the Company completed an underwritten public offering of 4,887,500 shares of its common stock, 4,597,645 shares of which were issued and sold by the Company, including the exercise in full by the underwriters of their option to purchase 637,500 shares from the Company, and 289,855 shares of which were sold by PBM Capital Investments, LLC (“PBM”), an existing stockholder of the Company, each at a public offering price of $34.50 per share. After deducting the underwriting discounts and commissions, the net proceeds to the Company were $149.1 million. The Company did not receive proceeds from the sale of the common stock by PBM. June 2017 Underwritten Public Offering On June 26, 2017, the Company completed an underwritten public offering of 4,111,250 shares of its common stock, including the exercise in full by the underwriters of their option to purchase 536,250 shares from the Company, at a public offering price of $70.00 per share. After deducting the underwriting discounts and commissions, the net proceeds to the Company were $269.9 million.

Summary of Significant Accounti

Summary of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2017
Summary of Significant Accounting Policies
Summary of Significant Accounting Policies	2. Summary of Significant Accounting Policies Basis of Presentation The accompanying consolidated financial statements of the Company and its subsidiaries have been prepared in conformity with accounting principles generally accepted in the United States of America (“GAAP”). Any reference in these notes to applicable guidance is meant to refer to the authoritative GAAP as found in the Accounting Standards Codification (“ASC”) and Accounting Standards Update (“ASU”) of the Financial Accounting Standards Board (“FASB”). These consolidated financial statements are presented in U.S. Dollars. Use of Estimates The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the financial statements and the reported amounts of revenues and expenses during the reporting period. Significant estimates and assumptions reflected in these consolidated financial statements include, but are not limited to, the measuring the grant date fair value of stock options and, prior to the Company’s IPO, valuation of common stock and restricted stock. 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 such estimates. Concentrations of Credit Risk Financial instruments that potentially expose the Company to concentrations of credit risk consist primarily of cash and cash equivalents. The Company generally invests its cash equivalents in checking accounts and money market funds held at mid-sized financial institutions. The Company does not believe that it is subject to unusual credit risk beyond the normal credit risk associated with commercial banking relationships. Cash and Cash Equivalents Cash and cash equivalents consist of cash, deposits with banks and short term highly liquid money market instruments with remaining maturities at the date of purchase of 90 days or less. Property and Equipment Property and equipment consists of construction in progress associated with the Company’s investment in its manufacturing facility, office furniture and equipment and is recorded at cost less accumulated depreciation. Maintenance and repairs that do not improve or extend the lives of the respective assets are expensed to operations as incurred. Upon disposal, retirement or sale, the related cost and accumulated depreciation are removed from the accounts and any resulting gain or loss is included in the results of operations. Advances paid towards the acquisition of property and equipment, and the cost of property and equipment not ready for use before the end of the period, are both classified as construction in progress. Impairment of Long-Lived Assets Long-lived assets consist of property and equipment. Long-lived assets 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 group for recoverability, the Company compares forecasts of undiscounted cash flows expected to result from the use and eventual disposition of the long-lived asset group 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 group are less than its carrying amount. The impairment loss would be based on the excess of the carrying value of the impaired asset group over its fair value, determined based on discounted cash flows. The Company did not record any impairment charges during any of the periods presented. Segment Information 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 and commercializing gene therapy treatments for patients suffering from rare and life-threatening neurological genetic diseases. All of the Company’s tangible assets are held in the United States. Research and Development Costs Research and development costs are expensed as incurred. Research and development expenses are comprised of costs incurred in performing research and development activities, including salaries, stock-based compensation and benefits, third party license fees, and external costs of outside vendors engaged to conduct manufacturing and preclinical development activities and clinical trials. Upfront and milestone payments made to third parties who perform research and development services on the Company’s behalf are expensed as services are rendered or when they no longer have alternative future use. Costs incurred in obtaining technology licenses through asset acquisitions are charged to research and development expense if the licensed technology has not reached technological feasibility and has no alternative future use. Patent Costs All patent-related costs incurred in connection with filing and prosecuting patent applications are expensed as incurred due to the uncertainty about the recovery of the expenditure. Amounts incurred are classified as general and administrative expenses. Stock-Based Compensation The Company accounts for its stock-based compensation awards in accordance with FASB ASC Topic 718, Compensation—Stock Compensation (“ASC 718”). ASC 718 requires all stock-based payments to employees, including grants of employee stock options and restricted stock units, to be recognized in the consolidated statements of operations based on their fair values. The Company’s stock-based awards are subject to service and performance-based vesting conditions. Compensation expense related to awards to employees with only service-based vesting conditions is recognized on a straight-line basis over the requisite service period for each separately vesting portion of the award as if the award was, in substance, multiple awards (the “Graded Vesting Attribution Method”), based on the estimated grant date fair value for each separately vesting tranche. Compensation expense related to awards to non-employees with only service-based vesting conditions is recognized based on the then-current fair value at each financial reporting date prior to the measurement date over the associated service period of the award, which is generally the vesting term, using the Graded Vesting Attribution Method. Compensation expense related to awards to employees with only performance-based vesting conditions is recognized based on the estimated grant date fair value over the requisite service period using the Graded Vesting Attribution Method to the extent achievement of the performance condition is probable. Compensation expense related to awards to non-employees only with performance-based vesting conditions is recognized based on the then-current fair value at each financial reporting date prior to the measurement date over the requisite service period using the Graded Vesting Attribution Method to the extent achievement of the performance condition is probable. The Company estimates the fair value of its option awards to employees and directors using the Black-Scholes option-pricing model, which requires the input of and use of subjective assumptions, including (i) the fair value of the underlying common stock, (ii) the expected stock price volatility, (iii) the calculation of expected term of the award, (iv) the risk-free interest rate, and (v) expected dividends. Due to the lack of company-specific historical and implied volatility data of its common stock, the Company has based its estimate of expected volatility on the historical volatility of a group of similar companies that are publicly traded. When selecting these public companies on which it has based its expected stock price volatility, the Company selected companies with comparable characteristics to it, including enterprise value, risk profiles, position within the industry, and with historical share price information sufficient to meet the expected term of the stock-based awards. The Company computes historical volatility data using the daily closing prices for the selected companies’ shares during the equivalent period of the calculated expected term of the stock-based awards. The Company’s estimates of expected term used in the Black-Scholes option pricing model were based on the estimated time from the grant date to the date of exercise. The risk-free interest rates for periods within the expected term of the option are based on the U.S. Treasury securities with a maturity date commensurate with the expected term of the associated award. The Company has never paid dividends, and does not expect to pay dividends in the foreseeable future. Stock-based awards issued to non-employees, consisting of stock warrants and restricted common shares, are accounted for using the fair value method in accordance with ASC 505-50, Equity-Based Payments to Non-Employees. These stock warrants and restricted common shares have been granted in exchange for consulting services to be rendered, and vest according to certain service or performance conditions. In accordance with authoritative guidance, the fair value of non-employee stock-based awards is estimated on the date of grant, and subsequently revalued at each reporting period until the award vests or a measurement date has occurred using the Black-Scholes option-pricing model. Comprehensive Loss Comprehensive loss includes net loss as well as other changes in stockholders’ equity (deficit) that result from transactions and economic events other than those with stockholders. There was no difference between net loss and comprehensive loss for each of the periods presented in the accompanying consolidated financial statements. Income Taxes The Company is a C corporation for federal and state income tax purposes. The Company recognizes deferred tax assets and liabilities for temporary differences between the financial reporting basis and the tax basis of its assets and liabilities and the expected benefits of net operating loss carryforwards. The impact of changes in tax rates and laws on deferred taxes is applied in the period of enactment. The measurement of deferred tax assets is reduced if, based on weight of the evidence, it is more likely than not that some, or all, of the deferred tax assets will not be realized. At December 31, 2017, 2016 and 2015, the Company has concluded that as a result of the accumulated losses to date, a full valuation allowance is necessary for its net deferred tax assets. The Company accounts for uncertain tax positions in accordance with the provisions of FASB ASC Topic 740, Income Taxes. When uncertain tax positions exist, the Company recognizes the tax benefit of tax positions to the extent that the benefit will more likely than not be realized. The determination as to whether the tax benefit will more likely than not be realized is based upon the technical merits of the tax position as well as consideration of the available facts and circumstances. As of December 31, 2017 and 2016, the Company does not have any significant uncertain tax positions. The Company’s policy is to recognize interest and penalties related to income tax matters in income tax expense. Fourth Amended and Restated Certificate of Incorporation On February 1, 2016, the Company amended its certificate of incorporation such that the total authorized capital stock of the Company consisted of 30,000,000 shares of common stock, $0.0001 par value per share, 3,278,938 shares of Class B-1 preferred stock, $0.0001 par value per share, 326,557 shares of Class B-2 preferred stock, $0.0001 par value per share, 2,365,020 shares of Class C preferred stock, $0.0001 par value per share, 3,105,000 shares of Class D preferred stock, $0.0001 par value per share and 1,000,000 shares of preferred stock, $0.0001 par value per share. Additionally, the Company effected a stock split whereby each outstanding share of common stock and Class B-1, B-2, C and D preferred stock was converted into 1.38 shares of common stock and Class B-1, B-2, C and D preferred stock, respectively. All share and per share information presented in these condensed consolidated financial statements and accompanying footnotes has been retroactively adjusted to reflect the stock split. Fifth Amended and Restated Certificate of Incorporation On February 17, 2016, the Company amended its certificate of incorporation such that the total authorized capital stock of the Company consisted of 100,000,000 shares of common stock, $0.0001 par value per share, and 10,000,000 shares of preferred stock, $0.0001 par value per share. Recent Accounting Pronouncements In January 2017, the FASB issued ASU 2017-01, Business Combinations (Topic 805): Clarifying the Definition of a Business, which clarifies the definition of a business to assist entities with evaluating whether transactions should be accounted for as acquisitions (or disposals) of assets or businesses. The standard will be effective for the Company in the first quarter of 2018. Adoption of this ASU will not impact prior periods but may impact the accounting of future transactions. In February 2016, the FASB issued ASU 2016-02, Leases (“ASC 842”). The guidance requires lessees to recognize almost all leases on their balance sheet as a right-of-use asset and a lease liability. For income statement purposes, the FASB retained a dual model, requiring leases to be classified as either operating or finance. Lessor accounting is similar to the current model, but updated to align with certain changes to the lessee model and the new revenue recognition standard. ASC 842 is effective for fiscal years beginning after December 15, 2018. The Company is evaluating the adoption of ASC 842, but has not yet determined the effects it may have on the Company’s consolidated financial statements.

Collaboration and License Agree

Collaboration and License Agreements	12 Months Ended
	Dec. 31, 2017
Collaboration and License Agreements
Collaboration and License Agreements	3. Collaboration and License Agreements The Company has entered into the following license agreements: Nationwide Children’s Hospital In October 2013 (the “Effective Date”), the Company entered into an Exclusive License Agreement (the “Nationwide License”), which agreement was amended and restated in its entirety in January 2016, with Nationwide Children’s Hospital (“NCH”). Under the terms of the agreement, NCH granted the Company an exclusive, non-transferable, worldwide license to certain patents held by NCH for the therapy and treatment of SMA. The Company was also provided a license to the Investigational New Drug application (“IND”) for AVXS-101 (the “Product Candidate”) and was provided the right to become the sponsor of the IND after completion of the Phase 1 clinical trial. On October 14, 2015, the Company exercised the option and, as of November 6, 2015, the Company became the sponsor of the IND. Additionally, the Company was provided the U.S. marketing rights to the product upon receipt of regulatory approval. The Company is responsible for all clinical trial costs incurred by NCH that are not covered by third party research grants and the Company committed to spend not less than $9.4 million for the development of the Product Candidate during the first eight years of the Nationwide License. As consideration for the Nationwide License, on the Effective Date, the Company agreed to issue 331,053 shares of its common stock to NCH (the “Up-front Shares”), which represented 3% of the Company’s outstanding capital stock on a fully-diluted basis. The Nationwide License provides that for the 30 day period immediately following FDA approval of the Biologics License Application (“BLA”), NCH shall have the option (if it owns at least 50% of the shares issued to it pursuant to the agreement) to sell all, but not less than all, of the Up-front Shares back to the Company at a per share price equal to two times the price per share of preferred stock sold by the Company in its Class B Financing ($2.47 per share), with such consideration to be paid by the Company in four equal quarterly installments (the “Royalty Option”). The rights granted to the Company under the Nationwide License represent distinct components that need to be combined with other licensed intellectual property and know-how in order to complete the clinical development of AVXS-101 and have no alternative future use. Additionally, the Company did not acquire any employees in connection with the Nationwide License. As a result of the above, and the early-stage nature of the licensed technology, the Company concluded that the acquired rights did not meet the definition of a business, and therefore the Company accounted for the Nationwide License as an asset acquisition and expensed such amounts as research and development expense. In March 2015, the Company issued an additional 34,463 common shares to NCH, and in May 2015, the Company issued an additional 3,802 common shares to NCH in each case pursuant to the anti-dilution provisions of the Nationwide License. The Company recognized additional research and development expense of $473,164 in its consolidated financial statements for the year ended December 31, 2015, representing the fair value of the additional common shares that were granted. NCH’s anti-dilution protection right expired on May 29, 2015 upon achievement by the Company of a $100,000,000 market capitalization. On April 23, 2015, the Nationwide License was again amended to further extend the filing deadline for a registration statement to December 31, 2015 in exchange for a $100,000 payment by the Company to NCH. Such amount is included in research and development expense for the year ended December 31, 2015. The Nationwide License commenced on the Effective Date and terminates on the earliest of (a) the last to expire of the licensed patents or (b) 10 years from the date of first commercial sale of the Product Candidate. The Nationwide License can also be terminated (i) by the Company for convenience at any time after the first anniversary of the Effective Date upon six months prior written notice, (ii) by either party in the event of an uncured breach upon thirty days written notice, (iii) by NCH upon the bankruptcy/insolvency of the Company, and (iv) by NCH if it is sued by the Company for anything other than breach of the agreement. On January 13, 2016 (the “Amended and Restated Nationwide License Effective Date”), the Company and NCH amended and restated the Nationwide License (the “Amended and Restated Nationwide License”) in its entirety. The Amended and Restated Nationwide License grants the Company an exclusive, non-transferable (except to a transfer to an affiliate or in other specified circumstances), sublicensable, worldwide license to certain patents held by NCH for the therapy and treatment of SMA. NCH acknowledged that, as of the date of the Amended and Restated Nationwide License, the Company had fulfilled its requirement to spend not less than $9.4 million for the development of the Product Candidate in whole. The Royalty Option expired upon the effectiveness of the Amended and Restated Nationwide License. Accordingly, NCH no longer has the right to sell the Up-front Shares issued upon the Effective Date of the original NCH License back to the Company under any circumstances. Following the first commercial sale of the Product Candidate, the Company shall pay a low single digit royalty on net sales, if any, of the Product Candidate during the term of the Amended and Restated Nationwide License, subject to certain annual minimums. In addition, the Company must pay NCH a portion of sublicensing revenue received from its sublicense of the rights to the licensed technology at percentages between low-double digits and low-teens. The Amended and Restated Nationwide License commenced on the Amended and Restated Nationwide License Effective Date and terminates upon the expiration of the royalty term for the Product Candidate in each country in which it is sold. The Amended and Restated Nationwide License can also be terminated (i) by the Company for convenience at any time after the first anniversary of the Amended and Restated Nationwide License Effective Date upon six months prior written notice, (ii) by either party in the event of a material uncured breach upon thirty days written notice, (iii) by NCH upon the bankruptcy/insolvency of the Company, and (iv) by NCH if it is sued by the Company for anything other than a suit brought in response to any suit brought by NCH regarding the validity or enforceability of the NCH patents. REGENXBIO Inc. License On March 21, 2014, the Company entered into a License Agreement (the “REGENXBIO SMA License”) with ReGenX Biosciences, LLC (“ReGenX”), predecessor to REGENXBIO Inc. (“REGENXBIO”). Under the terms of the agreement, ReGenX granted the Company an exclusive, non-transferable, worldwide license to utilize ReGenX’s proprietary adeno-associated virus (“AAV”) gene delivery platform for the treatment of SMA, by in vivo gene therapy, using ReGenX’s AAV9 gene delivery vector. As consideration for the REGENXBIO SMA License, the Company paid an up-front fee of $2.0 million during the year ended December 31, 2014 which was recorded in research and development expenses. Additionally, the Company agreed to pay potential future milestones of up to $12.25 million in the aggregate (as later amended as described below), and a mid-single to low-double digit royalty on net sales, if any, of the Company’s Product Candidate using AAV9, subject to reduction in specified circumstances; and lower mid-double digit percentages of any sublicense fees the Company receives from sublicenses of the licensed intellectual property rights. The rights granted to the Company under the REGENXBIO SMA License represent distinct components that need to be combined with other licensed intellectual property and know-how in order to complete the clinical development of AVXS-101. Additionally, the Company did not acquire any employees or manufacturing capabilities in connection with the REGENXBIO SMA License. As a result, the Company accounted for the REGENXBIO SMA License as an asset acquisition. The REGENXBIO SMA License term continues until the last valid patent claim expires or lapses in all countries of the world. Additionally, the Company may terminate the REGENXBIO SMA License at any time upon a specified notice period and REGENXBIO SMA may terminate upon the breach or insolvency of the Company, if we are greater than a specified number of days late (after notice and cure periods) in paying money due under the REGENXBIO SMA License or if the Company, its affiliates, or sublicensees challenges the REGENXBIO SMA patents subject to the REGENXBIO SMA License. Either party may terminate the REGENXBIO SMA License for material breach if such breach is not cured within a specified number of days. On January 8, 2018, the Company entered into an amendment to the REGENXBIO SMA License. Under the terms of the amendment, REGENXBIO granted the Company an exclusive, worldwide commercial license, with rights to sublicense, to any recombinant AAV vector in REGENXBIO’s intellectual property portfolio during the term of the license agreement for the treatment of SMA in humans by in vivo gene therapy. As consideration for the amendment, the Company paid or is required to pay additional fees of $80 million upon entry into the amendment, $30 million on the first anniversary of the effective date of the amendment and $30 million on the second anniversary of the effective date of the amendment. In addition, pursuant to the amendment, the Company agreed to pay up to $120 million in the aggregate of potential future sales-based milestone payments for all REGENXBIO SMA licensed products. The Company also agreed to pay a mid‑single digit to low double-digit royalty percentages on net sales of REGENXBIO SMA licensed products using AAV9 and a low-double digit royalty percentage on net sales of any product candidate developed by the Company for the treatment of SMA using an AAV vector other than AAV9, in each case subject to a reduction in specified circumstances . The Company paid REGENXBIO $80 million upon entering the amendment on January 8, 2018. See Note 15 – Subsequent Events. The REGENXBIO SMA License term continues until the last valid patent claim expires or lapses in all countries of the world. Additionally, the Company may terminate the REGENXBIO SMA License at any time upon a specified notice period and REGENXBIO SMA may terminate upon the breach or insolvency of the Company, if we are greater than a specified number of days late (after notice and cure periods) in paying money due under the REGENXBIO SMA License or if the Company, its affiliates, or sublicensees challenges the REGENXBIO SMA patents subject to the REGENXBIO SMA License. Either party may terminate the REGENXBIO SMA License for material breach if such breach is not cured within a specified number of days. Asklepios Biopharmaceutical, Inc. License On May 29, 2015, the Company and Asklepios Biopharmaceutical, Inc. (“AskBio”) entered into a Non‑Exclusive License Agreement (the “AskBio License”). Under the terms of the AskBio License, AskBio granted the Company a non‑exclusive, non‑transferable, worldwide license to certain patents and know-how held by AskBio. As consideration for the AskBio License, the Company paid an up-front fee of $1.0 million and agreed to make potential future milestone payments of up to $9.6 million in the aggregate, and a tiered royalty on net sales, if any, of the Company’s Product Candidate, on a country‑by‑country basis, starting at percentages in the low‑single digits and increasing to mid‑single digits. These royalty rates are subject to potential reduction in specified circumstances, including, in the event the Company exercises its option to make a specified one‑time royalty option fee payment to AskBio. The Company must also pay AskBio a low double-digit percentage of all consideration the Company receives from any sublicense of the licensed technology. The rights granted to the Company under the AskBio License represent distinct components that need to be combined with other licensed intellectual property and know‑how in order to complete the clinical development of AVXS‑101. Additionally, the Company did not acquire any employees or manufacturing capabilities in connection with the AskBio License. As a result, the Company accounted for the AskBio License as an asset acquisition. The AskBio up‑front payment of $1.0 million is included in the research and development expense for the year ended December 31, 2015. The AskBio License term continues until the last valid patent claim expires or lapses in all countries of the world. Additionally, the Company may terminate the AskBio License at any time upon six months’ notice and AskBio may terminate upon the breach (after notice and cure periods) or insolvency of the Company. Additionally, AskBio may terminate the AskBio License in the event the Company (i) researches, develops or commercializes any AAV-based treatment for hemophilia or (ii) undergoes a change in control or is otherwise acquired by a third party that researches, develops or commercializes any AAV-based treatment for hemophilia, in each case, until April 1, 2019, unless such change in control is first approved by AskBio, such approval not to be unreasonably withheld if the party in control following such a change of control event agrees to additional restrictive measures as reasonably proposed by AskBio in its sole discretion prior to such change of control respecting the use of the AskBio licensed technology. The Company has entered into the following license agreements related to its planned new programs in Rett syndrome and a genetic form of amyotrophic lateral sclerosis (“ALS”) caused by mutations in the superoxide dismutase 1 gene (“genetic ALS”). REGENXBIO Inc. Effective June 7, 2017, the Company entered into an exclusive license agreement with REGENXBIO under certain patents and patent applications owned by the Trustees of the University of Pennsylvania and licensed to REGENXBIO, for the development and commercialization of products to treat Rett syndrome and genetic ALS using the AAV 9 vector (the “REGENXBIO Rett and ALS License.”) Under the REGENXBIO Rett and ALS License, REGENXBIO granted the Company an exclusive, worldwide license under the licensed patent rights to make, have made, use, import, sell and offer for sale any products covered by the REGENXBIO Rett and ALS License, or the REGENXBIO Rett and ALS licensed products, in the field of the treatment of (i) Rett syndrome in humans by in vivo gene therapy using AAV9 delivering the gene encoding for methyl CpG binding protein 2, and (ii) ALS caused by SOD1 mutation in humans by in vivo gene therapy using AAV9 delivering the gene encoding for SOD1, subject to certain rights reserved by REGENXBIO and its licensors. As consideration for the REGENXBIO Rett and ALS License, the Company agreed to make an up-front payment of $6 million, which was recorded within research and development expenses for the year ended December 31, 2017. Additionally, the Company agreed to pay up to $36 million in total milestone fees for products made under the REGENXBIO Rett and ALS License (“REGENXBIO Rett and ALS licensed products”), a low-double digit royalty percentage on net sales of products made under the REGENXBIO Rett and ALS licensed products, subject to reduction in specified circumstances, and a lower mid-double digit percentage of any sublicense fees the Company receives from sublicenses of the licensed intellectual property rights. The REGENXBIO Rett and ALS License will expire upon the later of (i) the expiration, lapse, abandonment or invalidation of the last valid claim of the licensed intellectual property to expire, lapse or become abandoned or unenforceable in all the countries of the world or (ii) seven years from the first commercial sale of each REGENXBIO Rett and ALS licensed product. Upon expiration of the REGENXBIO Rett and ALS License, the license granted to us becomes irrevocable, perpetual, royalty-free and fully paid-up. The Company has the right to terminate the REGENXBIO Rett and ALS License upon a specified period of prior written notice. Nationwide Children’s Hospital In September 2016, the Company entered into exclusive license agreements with NCH, pursuant to which NCH granted the Company exclusive, worldwide licenses under certain patent rights to make, have made, use, sell, offer for sale and import any products covered by each license, or NCH licensed products, and a non-exclusive, worldwide license under certain technical information to develop and manufacture the NCH licensed products, in the field of therapies and treatments of Rett syndrome and ALS in human use, respectively (each, an “NCH License” and together, the “NCH Licenses”). As consideration for the NCH Licenses, the Company paid an aggregate up-front fee of $0.3 million, which was recorded within research and development expenses during the year ended December 31, 2016. Following the first commercial sale of a licensed product under an applicable NCH License, the Company must begin paying NCH an aggregate low-single digit royalty on net sales of such licensed products. With certain exceptions, the Company is required to make certain development milestone-based payments to NCH under each NCH License and must also pay NCH a portion of sublicensing revenue received from the Company’s sublicense of the rights to licensed technology at a mid-single digit percentage. Unless terminated earlier, each NCH License will expire on a NCH licensed product-by-NCH licensed product and country-by-country basis upon the expiration of the royalty term for such NCH licensed product in such country. The royalty term will expire on the later of (i) the date on which the last relevant patent underlying the relevant NCH licensed product expires or (ii) the expiration of any orphan drug-based exclusive marketing rights conferred by any regulatory authority with respect to a NCH licensed product in a licensed territory. Each NCH License may be terminated prior to its expiration: · By the Company at any time after the second anniversary of the effective date of the applicable NCH License by providing six months' prior written notice to NCH; · By either party upon the other party's material breach of the applicable NCH License that is not cured within 90 days after receiving written notice of such breach, except in certain cases in which we may request a longer cure period; · By NCH in the event of the Company’s bankruptcy, insolvency or certain similar occurrences; or · By NCH if the Company or any of its affiliates bring any action or proceeding against NCH, other than a suit brought in response to any suit brought by NCH.

Property and Equipment, Net

Property and Equipment, Net	12 Months Ended
	Dec. 31, 2017
Property and Equipment, Net
Property and Equipment, Net	4. Property and Equipment, Net Property and equipment, net, consists of the following (in thousands): December 31, December 31, 2017 2016 Furniture $ 919 $ 108 Equipment 34,777 230 Leasehold improvements 7,123 777 Construction in progress 15,656 23,162 Property and equipment, gross 58,475 24,277 Less: accumulated depreciation (2,301) (76) Property and equipment, net $ 56,174 $ 24,201 Depreciation expense was $2.3 million for the year ended December 31, 2017. Depreciation expense for the years ended December 31, 2016 and 2015 was not material. Furniture and equipment are depreciated on a straight-line basis over their estimated useful lives. The Company uses a life of five to ten years for office furniture and equipment. The Company uses a life of twenty years for its manufacturing pods. Leasehold improvements are depreciated over the shorter of their useful lives or the remaining term of the lease. Construction in progress decreased by $7.5 million for the year ended December 31, 2017. This decrease is due to items previously classified as construction in progress being placed into service in the Company’s manufacturing facility during 2017 at which time the cost of the asset is transferred to the appropriate property and equipment account.

Accrued Expenses and Other Curr

Accrued Expenses and Other Current Liabilities	12 Months Ended
	Dec. 31, 2017
Accrued Expenses
Accrued Expenses	5. Accrued Expenses and Other Current Liabilities Accrued expenses and other current liabilities consist of the following (in thousands): December 31, December 31, 2017 2016 Accrued manufacturing development costs $ 10,356 $ 7,167 Accrued payroll, bonus and deferred compensation 9,589 3,841 Accrued construction in progress 4,156 2,979 Accrued clinical trial costs 864 389 Accrued professional and consulting fees 5,406 1,588 Accrued loss on settlement (see Note 7) 11,647 — Other 2,920 830 Accrued expenses and other current liabilities $ 44,938 $ 16,794

Accrued Indemnification Obligat

Accrued Indemnification Obligation	12 Months Ended
	Dec. 31, 2017
Accrued Indemnification Obligation
Accrued Indemnification Obligation	6. Accrued Indemnification Obligation In January 2014, the Company issued 2,334,391 shares of restricted common stock to a member of its Board of Directors (Dr. Brian Kaspar) pursuant to a consulting agreement for scientific advisory services to be performed by the director on behalf of the Company. In connection with the restricted stock purchase agreement, the Company agreed to indemnify Dr. Kaspar for any taxes, interest, fines, penalties or other costs and expenses that Dr. Kaspar may incur in the future should the Internal Revenue Service succeed in a tax determination that the stock price paid by Dr. Kaspar (which was par value) was lower than the fair market value of the stock on the date of grant. The indemnification term is in effect for six years after the due date of the tax return for the year in which the stock was issued. In January 2016, the Company entered into an employment agreement with Dr. Kaspar. In connection with the preparation of the Company’s audited consolidated financial statements for the year ended December 31, 2014, the Company determined that the per share fair value of the Company’s common stock on January 28, 2014, the grant date, was $1.51. As a result, the Company issued Dr. Kaspar an amended Form 1099 for the 2014 tax year reflecting an aggregate fair value of the restricted stock grant of $3.5 million. Due to the indemnity obligation contained in Dr. Kaspar’s restricted stock purchase agreement, the Company will ultimately be required to reimburse Dr. Kaspar for the taxes he will pay following the amendment of Dr. Kaspar’s 2014 personal income tax return. As a result, the Company has concluded that payment of such indemnity is probable as of December 31, 2017. Additionally, the Company intends to gross-up such indemnification payment for the tax that will be payable by Dr. Kaspar on the indemnity payment. As a result, the Company has accrued balances of $2.8 million and $4.5 million at December 31, 2017 and 2016, respectively, representing the Company’s best estimate of the ultimate tax indemnification and gross-up payment to be made to Dr. Kaspar. On May 3, 2017, the Company paid Dr. Kaspar $1.7 million, which represents the tax liabilities owed to the U.S. federal government, State of Ohio and municipality of New Albany, Ohio pursuant to Dr. Kaspar’s 2014 amended tax returns. The Company expects to pay this entire amount in 2018.

Capitalization

Capitalization	12 Months Ended
	Dec. 31, 2017
Capitalization
Capitalization	7. Capitalization On February 1, 2016, the Company amended its certificate of incorporation to effect a stock split whereby each issued and outstanding share of common stock and Class B-1, B-2, C and D preferred stock was converted into 1.38 shares of common stock, Class B-1, B-2, C and D preferred stock, respectively. The par value per share was not adjusted as a result of this stock split. All share information presented in these consolidated financial statements and accompanying footnotes has been retroactively adjusted to reflect the increased number of shares resulting from this action. All classes of preferred stock were automatically converted into shares of the Company’s common stock upon the closing of the IPO. Class B‑1 and B‑2 Preferred Stock On January 30, 2014, the Company entered into a Convertible Note and Class B Stock Purchase Agreement (the “Class B Purchase Agreement”) with PBM Capital Investments, LLC (“PBM”). Under the Class B Purchase Agreement, the Company sold to PBM a $0.5 million convertible promissory note (the “Class B Note”) (see Note 9) and granted PBM an option (the “Class B Option”) to purchase 809,385 shares of Class B‑1 preferred stock at a purchase price of $2.47 per share and a warrant (the “Class B‑2 Warrant”) to purchase 130,623 shares of Class B‑2 preferred stock at $2.57 per share (the “Class B Stock Closing”). To exercise the Class B Option, PBM was required to provide written notice of its intent on or before February 28, 2014, after which date the Class B Option would terminate. After the Class B Stock Closing, the Company also agreed to sell and PBM agreed to purchase, on the same terms and conditions, 1,011,721 additional shares of Class B‑1 preferred stock (the “Class B‑1 Milestone Shares”) and a warrant to purchase 163,278 additional shares of Class B‑2 preferred stock (the “Milestone Warrant”) on the same terms and conditions. The Class B‑1 Milestone Shares and Milestone Warrant were issued to PBM on May 4, 2015 in exchange for $2.5 million of cash upon the achievement of the defined milestone. The Class B‑2 Warrant and the Milestone Warrant may be exercised by the holder, in whole or in part, upon the payment of the exercise price in cash. The Class B‑2 Warrant and the Milestone Warrant terminate upon the earliest to occur of (a) the 10-year anniversary of their issuance date or (b) a liquidation event, as defined therein. Class C Preferred Stock On August 11, 2014, the Company entered into a Class C Stock Purchase Agreement with certain investors. At closing, one of these investors purchased 504,478 shares of Class C preferred stock at a price of $3.96 per share. After the initial closing, the Company agreed to sell, and these investors each agreed to purchase, an aggregate of 551,472 additional shares of Class C preferred stock at a purchase price of $4.53 per share (the “Class C Milestone Shares”) upon certain conditions which were met in March 2015, resulting in the Class C Milestone Shares being purchased by these investors in exchange for an aggregate purchase price of $5.0 million in cash. Class D Preferred Stock Issuance On September 3, 2015, the Company entered into a Class D Stock Purchase Agreement with an investor pursuant to which the Company issued and sold an aggregate of 3,093,092 shares of Class D preferred stock at a price per share of $21.01 for an aggregate of $64.8 million, net of issuance costs. Common Stock Settlement and Issuance On December 29, 2017, the Company entered into a settlement agreement and release with a stockholder

Stock-Based Compensation

Stock-Based Compensation	12 Months Ended
	Dec. 31, 2017
Stock-Based Compensation
Stock-Based Compensation	8. Stock-based Compensation 2014 Stock Plan (the “2014 Plan”) and 2016 Equity Incentive Plan (the “2016 Plan") In May 2014, the Company’s Board of Directors adopted the 2014 Stock Plan (the “2014 Plan”), and in January 2016, the Company adopted the 2016 Equity Incentive Plan (the “2016 Plan” and, together with the 2014 Plan, the “Plans”). The 2016 Plan became effective on February 10, 2016, or the IPO Date. On and after the IPO Date, no additional stock awards may be granted under the 2014 Plan. The Board may amend or suspend the 2016 Plan at any time, although no such action may materially impair the rights under any then-outstanding award without the holder’s consent. The Company will obtain stockholder approval for any amendments to the 2016 Plan as required by law. No incentive stock options may be granted under the 2016 Plan after the tenth anniversary of the effective date of the 2016 Plan. Initially, the aggregate number of shares of the Company’s common stock that was available to be issued pursuant to stock awards under the 2016 Plan was 4,339,451 shares, which was the sum of (1) 2,400,000 new shares, plus (2) the number of shares reserved for issuance under the 2014 Plan on the IPO Date, plus (3) any shares subject to outstanding stock awards that would have otherwise been returned to the 2014 Plan. Additionally, the number of shares of the Company’s common stock reserved for issuance under the 2016 Plan will automatically increase on January 1 of each year, beginning on January 1, 2017 and continuing through and including January 1, 2026, by 4.0% of the total number of shares of the Company’s capital stock outstanding on December 31 of the preceding calendar year, or a lesser number of shares determined by the Board. The maximum number of shares that may be issued pursuant to the exercise of incentive stock options under the 2016 Plan is 8,678,902 shares. The Company has not granted stock appreciation rights to directors, officers or employees under the Plans since their inception. The Company granted 57,500 service-based restricted stock awards to several employees in August 2016 and 49,332 performance-based restricted stock units to several employees in March 2017. Options generally expire ten years following the date of grant. Options typically vest over a period of four years, but vesting provisions can vary by award based on the discretion of the Board of Directors. Options to purchase common stock carry an exercise price equal to the estimated fair value of the Company’s common stock on the date of grant. Options to purchase shares of the Company’s common stock may be exercised by payment of the exercise price in cash, by the delivery of previously acquired shares of common stock having a fair value equal to the exercise price payable or the withholding of common shares equal to the fair value of the aggregate exercise price. Upon the termination of service of a holder of stock options awarded under the Plans, all unvested options are immediately forfeited and vested options may be exercised within three months of termination. Shares of common stock underlying awards previously issued under the 2016 Plan which are reacquired by the Company, withheld by the Company in payment of the purchase price, exercise price, or withholding taxes, expired, cancelled due to forfeiture, or otherwise terminated other than by exercise, are added to the number of shares of common stock available for issuance under the 2016 Plan. Shares available for issuance under the 2016 Plan may be authorized but unissued shares of the Company’s common stock or common stock reacquired by the Company and held in treasury. The 2016 Plan expires ten years from the date it was approved by the Board of Directors. The following table summarizes stock option activity (in thousands, except per share data): Weighted Weighted Average Average Remaining Aggregate Number of Exercise Contractual Intrinsic Shares Price Life (Years) Value (a) Outstanding at December 31, 2016 2,577 $ 22.01 8.79 $ 66,466 Granted 1,103 $ 78.59 Exercised (268) $ 14.93 Cancelled or forfeited (245) $ 36.02 Outstanding at December 31, 2017 3,167 $ 41.30 8.35 $ 219,656 Exercisable at December 31, 2017 1,158 $ 20.96 7.70 $ 103,843 Exercisable and expected to vest at December 31, 2017 3,167 $ 41.30 8.35 $ 219,656 (a) The aggregate intrinsic value is calculated as the difference between the exercise price of the underlying options and the estimated fair value of the common stock for the options that were in-the-money at December 31, 2016 and 2017. As of December 31, 2017, 2,204,082 shares of common stock, were available for future grants under the 2016 Plan. The total intrinsic value of options exercised during the year ended December 31, 2017, 2016 and 2015 was $19.0 million, $3.4 million and $3.6 million, respectively. As of December 31, 2017, there was $46.5 million of unrecognized stock-based compensation expense related to employees’ awards that is expected to be recognized over a weighted-average period of 1.3 years. The Company has recorded aggregate stock-based compensation expense related to the issuance of stock option awards under the Plans in the consolidated statements of operations and comprehensive loss as follow (in thousands): Year ended Year ended Year ended December 31, December 31, December 31, 2017 2016 2015 Research and development $ 15,383 $ 5,988 $ 1,294 General and administrative 15,502 9,668 3,956 $ 30,885 $ 15,656 $ 5,250 Stock Options Granted to Employees The weighted-average grant date fair value of options granted during the years ended December 31, 2017, 2016 and 2015 was $57.13, $24.50 and $16.44, respectively, on the date of grant using the Black-Scholes option-pricing model with the following weighted-average assumptions: Year ended Year ended Year ended December 31, December 31, December 31, 2017 2016 2015 Expected volatility 86.34 % 91.84 % 90.00 % Risk-free interest rate 1.94 % 1.45 % 1.57 % Expected term (in years) 6.08 6.08 6.08 Expected dividend yield 0.00 % 0.00 % 0.00 % Restricted Stock Units (RSUs) The Company granted RSUs with service-based vesting terms. The outstanding RSUs vest over a period of three years. For awards that vest subject to the satisfaction of service requirements, compensation expense is measured based on the fair value of the RSUs on the date of grant and is recognized as expense on a straight-line basis, net of estimated forfeitures, over the requisite service period. All RSUs issued vest over time as stipulated in the individual RSU award agreements. The Company recognized RSU-related stock-based compensation expense of $0.7 million and $0.8 million during the years ended December 31, 2017 and 2016, respectively, of which the majority is recorded within research and development expense. At December 31, 2017, there was $0.4 million of unrecognized compensation cost related to unvested RSUs that will be recognized as expense over a weighted-average period of 0.7 years. A summary of the status of the Company's RSUs at December 31, 2017 and of changes in RSUs outstanding under the 2016 Plan for the year ended December 31, 2017 is as follows (in thousands, except per share data): Weighted Average Grant Date Number Fair Value of Shares Per Share Outstanding at December 31, 2016 58 $ 34.90 Granted 2 99.26 Vested (20) $ 37.60 Forfeited and cancelled (13) $ 34.90 Outstanding at December 31, 2017 27 $ 38.79 Performance-Based Restricted Stock Units (PSUs) On March 20, 2017, the Company granted 49,332 PSUs to certain employees. These PSUs vest upon the achievement of certain regulatory and manufacturing milestones. If the milestones do not occur on or before the three-year anniversary of the grant date, all unvested PSUs will be cancelled. As of December 31, 2017, all 49,332 PSUs were outstanding, none had vested and the weighted-average grant date fair value of all shares was $79.75 per share. The Company has not yet recognized any PSU-related stock-based compensation expense as regulatory and manufacturing milestones have not yet been met; however, in the event the performance conditions are met, $3.9 million of research and development compensation expense will be recognized by the Company. There were no PSUs issued and outstanding as of December 31, 2016. Valuation of Common Stock Prior to the IPO, the Company estimated the fair value of common stock underlying stock option awards at the grant date of the award. Valuation estimates were prepared by management in accordance with the framework of the American Institute of Certified Public Accountants Practice Guide, with the assistance of independent third party valuations, and approved by the Company’s Board of Directors. The Company’s valuations of its common stock were based on a number of objective and subjective factors, including external market conditions affecting the Company’s industry sector, the prices at which the Company sold shares of its common and preferred stock, and the likelihood of achieving a liquidity event such as an initial public offering. Restricted Stock Granted to Non-Employees In January 2014, the Company issued 2,334,391 shares of restricted common stock to Dr. Brian Kaspar pursuant to a consulting agreement for scientific advisory services. Of these shares, 583,597 common shares were vested at the time of grant and the remaining restricted shares were scheduled to vest in the amount of 25% per year on the second, third and fourth anniversary of the grant date pursuant to a restricted stock purchase agreement, which became effective upon the effectiveness of the consulting agreement. In January 2016, the Company entered into an employment agreement with Dr. Kaspar. Upon the effectiveness of the employment agreement, Dr. Kaspar’s 1,750,794 unvested shares granted pursuant to the restricted stock purchase agreement vested in full. As a result of the vesting of the remainder of this award the Company recorded $10.4 million of additional stock compensation expense for the year ended December 31, 2016, in research and development expense, as compared to $19.3 million of stock compensation expense recognized related to the grant for the year ended December 31, 2015. Warrants Granted to Non-Employees As of December 31, 2017 and 2016, there were 305,775 common stock warrants vested and outstanding issued to non-employees, with a weighted-average exercise price of $2.57.

Net Loss Per Common Share

Net Loss Per Common Share	12 Months Ended
	Dec. 31, 2017
Net Loss Per Common Share
Net Loss Per Common Share	9. Net Loss Per Common Share Net Loss Per Common Share Basic net loss per common share is computed using the weighted average number of common shares outstanding during the period. Diluted net loss per common share is computed using the sum of the weighted average number of common shares outstanding during the period and, if dilutive, the weighted average number of potential shares of common stock, including the assumed exercise of stock options, stock warrants and unvested restricted common stock. The Company applies the two-class method to calculate its basic and diluted net loss per share attributable to common stockholders, as its preferred stock and common stock are participating securities. The two-class method is an earnings allocation formula that treats a participating security as having rights to earnings that otherwise would have been available to common stockholders. However, the two-class method does not impact the net loss per share of common stock as the Company was in a net loss position for each of the periods presented and preferred stockholders do not participate in losses. For the years ended December 31, 2017, 2016 and 2015, the following potentially dilutive securities have been excluded from the computations of diluted weighted-average shares outstanding because the effect would be anti-dilutive (in thousands): Year Ended Year Ended Year Ended December 31, December 31, December 31, 2017 2016 2015 Stock options 3,167 2,577 1,749 Stock warrants 306 306 326 Unvested service-based restricted stock units 27 58 — Unvested performance-based restricted stock units 49 — 1,751 Amounts in the table above reflect the common stock equivalents of the noted instruments. The following table summarizes the calculation of the basic and diluted net loss per common share (in thousands, except per share data): Year Ended Year Ended Year Ended December 31, December 31, December 31, 2017 2016 2015 Numerator: Net loss and comprehensive loss $ (218,051) $ (83,012) $ (38,474) Denominator: Weighted-average basic and diluted common shares 29,935 22,648 7,088 Basic and diluted net loss per common share $ (7.28) $ (3.67) $ (5.43)

Separation Agreement

Separation Agreement	12 Months Ended
	Dec. 31, 2017
Separation Agreement
Separation Agreement	10. Separation Agreements On October 9, 2017, Thomas J. Dee ceased to be an employee of the Company. In connection with the termination of his employment, the Company paid Mr. Dee $0.5 million, which was recorded within general and administrative expenses for the year ended December 31, 2017. On April 22, 2015, John Carbona ceased to be an employee of the Company. In connection with the termination of his employment, the Company agreed to pay Mr. Carbona $0.5 million which was recorded within general and administrative expenses for the year ended December 31, 2015. Additionally, the Company agreed to fully accelerate the vesting of 53,820 unvested stock options held by Mr. Carbona at the time of the termination of his employment. The Company determined that the acceleration of vesting was a Type III modification pursuant to ASC 718. Therefore, the Company recognized the incremental fair value of the awards as of the modification date and recognized the amount immediately since the awards did not require further service. In connection with this modification, the Company recorded a charge of $0.5 million within general and administrative expenses for the year ended December 31, 2015.

Commitments and Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2017
Commitments and Contingencies
Commitments and Contingencies	11. Commitments and Contingencies Operating Leases The Company leases certain office space and other facilities under operating leases. Most leases contain renewal options for varying periods, and provide for annual escalation in rent payments during the lease term. The Company amortizes the escalation in rental payments on a straight-line basis over the term of the lease. Future minimum lease payments under all of the Company’s operating leases with initial non-cancellable lease terms in excess of one year were due as follows at December 31, 2017 (in thousands): Year ending December 31, 2018 $ 1,414 2019 1,969 2020 2,075 2021 2,126 2022 2,179 Thereafter 7,543 Total $ 17,306 Rent expense on all operating leases amounted to $1.2 million, $0.3 million and $0.1 million for the years ended December 31, 2017, 2016 and 2015, respectively. License Agreements See Note 3 for information regarding licenses entered into by the Company. Guarantees and Indemnifications See Note 6 for information regarding guarantees and indemnifications. Additionally, in the normal course of business, the Company has entered into agreements that contain a variety of representations and provide for general indemnification. The Company's exposure under these agreements is unknown because it involves claims that may be made against the Company in the future. To date, the Company has not paid any claims or been required to defend any action related to these indemnification obligations. As of December 31, 2017 and 2016, the Company did not have any material indemnification claims related to these agreements that were probable or reasonably possible and consequently has not recorded any related liabilities. Litigation On September 8, 2016, Sophia's Cure Foundation (“SCF”), a non-profit 501(c)(3) public charity, filed a complaint in U.S. District Court, Southern District of Ohio, naming as defendants Nationwide Children’s Hospital (“NCH”) and other entities affiliated with NCH, the Company and certain of the Company's present and former executives (the “Complaint”). According to the complaint, in 2012, SCF and Nationwide Children’s Hospital Foundation (“NCH Foundation”) entered into a donation agreement under which SCF provided NCH a gift of $550,000 to fund clinical work associated with the study of the product candidate that the Company now refers to as AVXS-101 for SMA Type 1 patients, and NCH Foundation agreed in such donation agreement to reference SCF as the “primary sponsor” of such clinical work in all publications issued by NCH Foundation. The complaint also alleges that NCH breached the donation agreement by not naming SCF as the sponsor of the investigational new drug application (the “IND”) that it filed for AVXS-101. Additionally, the complaint alleges that the Company and the named Company executives tortiously interfered with SCF’s rights under the donation agreement by assuming sponsorship of the IND under the NCH License. There is no contractual relationship between the Company and SCF. The complaint seeks, among other relief, monetary damages of $500 million and equitable relief, including taking steps to designate SCF as the sponsor of the IND. On October 10, 2017, the Court granted all of the defendants’ motions to dismiss. On October 31, 2017, SCF filed a second amended complaint against all of the defendants except one current and one former executive of the Company, who were dropped from the second amended complaint. The Company believes that the complaint is without merit and intends to vigorously defend itself and its current executives from the allegations. The Company views the probability of loss in this matter to be remote. Lawsuits may be asserted against the Company in the normal course of business. Based on information currently available, management believes that the disposition of any matters, including the matter involving SCF described above, will not have a material adverse effect on the financial position, results of operations or cash flows of the Company.

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2017
Taxes
Taxes	12. Income Taxes On December 22, 2017, the President of the United States signed into law the Tax Cuts and Jobs Act, or TCJA. The TCJA makes significant changes in U.S. tax law including a reduction in the corporate tax rates, changes to net operating loss carryforwards and carrybacks, and a reduction of the amount of the orphan drug credit. As a result of the TCJA, the Company remeasured its ending deferred tax assets and liabilities at December 31, 2017 to the newly enacted U.S. federal corporate tax rate of 21%. This remeasurement resulted in a provision of $37.2 million to income tax expense and an offsetting income tax benefit for the reduction in the valuation allowance. As a result, there was no impact to the Company’s consolidated statements of operations and comprehensive loss. The other provisions of the TCJA did not have a material impact on the Company’s consolidated financial statements. The Company has recognized the provisional tax impacts related to the remeasurement of the deferred tax assets and liabilities and included these amounts in its consolidated financial statements for the year ended December 31, 2017. The ultimate impact may differ from these provisional amounts due to, among other things, additional analysis, changes in interpretations and assumptions the Company has made, additional regulatory guidance that may be issued, and actions the Company may take as a result of TCJA. A reconciliation of the difference between the federal statutory rate and the effective income tax rate as a percentage of income before taxes for the years ended December 31, 2017, 2016 and 2015, respectively, is as follows (in thousands): Year ended Year ended Year ended December 31, 2017 December 31, 2016 December 31, 2015 Amount Tax Rate Amount Tax Rate Amount Tax Rate Federal statutory rate $ (74,137) 34.00 % $ (28,224) 34.00 % $ (13,081) 34.00 % Permanent differences (1,203) 0.55 % 3,991 (4.73) % 6,077 (15.80) % State taxes (11,498) 5.27 % (2,453) 2.97 % (1,275) 3.31 % Change in statutory rate 37,162 (17.04) % — — % — — % Research and development credit (4,989) 2.29 % (8,891) 10.55 % (220) 0.57 % Valuation allowance 56,049 (25.70) % 35,502 (42.70) % 8,472 (21.38) % Other (1,384) 0.63 % 75 (0.09) % 27 (0.70) % Effective income tax rate $ — — % $ — — % $ — — % Deferred income taxes reflect the net tax effects of temporary differences between the carrying amount of assets and liabilities for financial reporting purposes and the amounts used for income tax purposes. Significant components of the Company’s deferred taxes at December 31, 2017 and 2016 were as follows (in thousands): December 31, 2017 2016 Assets Liabilities Assets Liabilities Current: Accrued Expenses $ 2,950 $ — $ 2,995 $ — Other — (32) 45 — Subtotal 2,950 (32) 3,040 — Valuation allowance (2,918) — (3,040) — Total current deferred taxes 32 (32) — — Net current deferred taxes $ — $ — $ — $ — Long Term: Net operating loss carryforwards $ 31,971 $ — $ 13,063 $ — Capital loss carryforward 77 — 113 — Research and development credit 23,391 — 13,733 — State research and development credit 1,994 — — — Capitalized research and development 34,472 — 11,797 — Amortization 2,746 — 1,423 — Stock-based compensation 9,370 — 6,264 — Fixed assets — (1,474) — (17) Subtotal 104,021 (1,474) 46,393 (17) Valuation allowance (102,547) — (46,376) — Total long-term deferred taxes $ 1,474 $ (1,474) $ 17 $ (17) Net deferred tax assets/(liabilities) $ — $ — $ — $ — As of December 31, 2017, the Company had federal and state net operating loss carryforwards available to reduce future taxable income of approximately $119.8 million and $111.0 million, respectively, which expire beginning in 2032. These net operating loss carryforwards may be subject to limitation under Internal Revenue Code Section 382, or IRC Section 382, should there be a greater than 50% ownership change as determined under the regulations. During 2016, the Company performed a detailed analysis of historical and current IRC Section 382 ownership changes that may limit the utilization of net operating loss carryovers. The entire federal net operating loss carryforward of $119.8 million is available for immediate use based on the IRC Section 382 analysis performed. As of December 31, 2017 and 2016, the Company has unused federal research and development carryforwards of approximately $23.4 million and $13.7 million, respectively, which will begin to expire in 2034. A portion of these carryforwards and tax credits may expire before becoming available to reduce future income tax liabilities. The Company files income tax returns in the U.S. federal jurisdiction, and various state jurisdictions. Tax years 2014 and forward remain open for examination for federal tax purposes and tax years 2013 and forward remain open for examination for the Company’s more significant state tax jurisdictions. To the extent utilized in future years' tax returns, net operating loss carryforwards at December 31, 2017 will remain subject to examination until the respective tax year is closed. The table below details the changes to the Company’s valuation allowance for the years ended December 31, 2017 and 2016 (in thousands): Valuation allowance at December 31, 2015 $ 13,914 Additions for 2016 35,502 Change in tax rates — Valuation allowance at December 31, 2016 49,416 Additions for 2017 93,211 Change in tax rates (37,162) Valuation allowance at December 31, 2017 $ 105,465

Retirement Plan

Retirement Plan	12 Months Ended
	Dec. 31, 2017
Retirement Plan
Retirement Plan	13. Retirement Plan The Company initiated a discretionary Simple IRA Plan offered to all of its employees. Under the Simple IRA Plan, employees may defer income on a tax-exempt basis, subject to limitations under the Internal Revenue Code of 1986, as amended. Under the Simple IRA Plan, the Company may make discretionary matching contributions. Company contributions expensed totaled $0.4 million and $0.1 million during the years ended December 31, 2017 and 2016.

Quarterly Financial Information

Quarterly Financial Information (Unaudited)	12 Months Ended
	Dec. 31, 2017
Quarterly Financial Information (Unaudited)
Quarterly Financial Information (Unaudited)	14. Quarterly Financial Information (Unaudited) Summarized quarterly financial information for each of the years ended December 31, 2017 and 2016 are as follows (in thousands, except per share data): Quarter Ended December 31, September 30, June 30, March 31, 2017 2017 2017 2017 Revenue $ — $ — $ — $ — Loss from operations $ (82,523) $ (49,520) $ (58,360) $ (29,964) Interest income $ 858 $ 881 $ 331 $ 246 Net loss and comprehensive loss $ (81,665) $ (48,639) $ (58,029) $ (29,718) Basic and diluted net loss per common share (1) $ (2.55) $ (1.52) $ (2.07) $ (1.07) Weighted-average basic and diluted common shares outstanding 32,030 31,941 27,972 27,734 Quarter Ended December 31, September 30, June 30, March 31, 2016 2016 2016 2016 Revenue $ — $ — $ — $ — Loss from operations $ (25,547) $ (21,180) $ (15,798) $ (20,889) Interest income $ 172 $ 99 $ 79 $ 53 Net loss and comprehensive loss $ (25,375) $ (21,082) $ (15,719) $ (20,836) Basic and diluted net loss per common share (1) $ (0.92) $ (0.87) $ (0.68) $ (1.24) Weighted-average basic and diluted common shares outstanding 27,678 24,166 23,014 16,775 (1) The sum of the four quarters of net loss per share for 2017 and 2016 may not add to the full year net loss per share amount due to rounding and/or the use of quarter-to-date weighted average shares to calculate the net loss per share amount in each respective quarter.

Subsequent Events

Subsequent Events	12 Months Ended
	Dec. 31, 2017
Subsequent Event
Subsequent Events [Text Block]	15. Subsequent Events REGENXBIO License Amendment On January 8, 2018, the Company entered into an amendment which expanded upon the exclusive, worldwide license agreement it entered into in March 2014 (2014 License Agreement) for the development and commercialization of products to treat SMA. Under the terms of the amendment, the Company was granted exclusive, worldwide rights to all vectors in REGENXBIO’s NAV Technology Platform for the treatment of SMA in addition to adding and amending certain terms of the 2014 License Agreement, including the modification of the assignment provision to now permit assignment in the event of a change of control by the Company without REGENXBIO’s consent. The Company paid REGENXBIO an upfront payment of $80 million in January 2018 in connection with the amendment and agreed to make an additional payment of $30 million on the first anniversary of the amendment and an additional payment of $30 million on the second anniversary of the amendment. REGENXBIO is eligible to receive potential sales-based milestone payments of up to $120 million. For any product developed for the treatment of SMA using the NAV AAV9 vector, the Company will continue to pay mid-single to low double-digit royalties on net sales as defined in the original 2014 License Agreement, and for any product developed for the treatment of SMA using a NAV vector, other than NAV AAV9, the Company will pay REGENXBIO a low double-digit royalty on net sales. January 2018 Underwritten Public Offering On January 22, 2018, the Company completed an underwritten public offering of 4,509,840 shares of its common stock, including the exercise in full by the underwriters of their option to purchase 588,240 shares from the Company, at a public offering price of $102.00 per share. After deducting the underwriting discounts and commissions, the net proceeds to the Company were approximately $431.9 million.

Summary of Significant Accoun22

Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2017
Summary of Significant Accounting Policies
Basis of Presentation	Basis of Presentation The accompanying consolidated financial statements of the Company and its subsidiaries have been prepared in conformity with accounting principles generally accepted in the United States of America (“GAAP”). Any reference in these notes to applicable guidance is meant to refer to the authoritative GAAP as found in the Accounting Standards Codification (“ASC”) and Accounting Standards Update (“ASU”) of the Financial Accounting Standards Board (“FASB”). These consolidated financial statements are presented in U.S. Dollars.
Use of Estimates	Use of Estimates The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the financial statements and the reported amounts of revenues and expenses during the reporting period. Significant estimates and assumptions reflected in these consolidated financial statements include, but are not limited to, the measuring the grant date fair value of stock options and, prior to the Company’s IPO, valuation of common stock and restricted stock. 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 such estimates.
Concentrations of Credit Risk	Concentrations of Credit Risk Financial instruments that potentially expose the Company to concentrations of credit risk consist primarily of cash and cash equivalents. The Company generally invests its cash equivalents in checking accounts and money market funds held at mid-sized financial institutions. The Company does not believe that it is subject to unusual credit risk beyond the normal credit risk associated with commercial banking relationships.
Cash and Cash Equivalents	Cash and Cash Equivalents Cash and cash equivalents consist of cash, deposits with banks and short term highly liquid money market instruments with remaining maturities at the date of purchase of 90 days or less.
Property and Equipment	Property and Equipment Property and equipment consists of construction in progress associated with the Company’s investment in its manufacturing facility, office furniture and equipment and is recorded at cost less accumulated depreciation. Maintenance and repairs that do not improve or extend the lives of the respective assets are expensed to operations as incurred. Upon disposal, retirement or sale, the related cost and accumulated depreciation are removed from the accounts and any resulting gain or loss is included in the results of operations. Advances paid towards the acquisition of property and equipment, and the cost of property and equipment not ready for use before the end of the period, are both classified as construction in progress.
Impairment of Long-Lived Assets	Impairment of Long-Lived Assets Long-lived assets consist of property and equipment. Long-lived assets 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 group for recoverability, the Company compares forecasts of undiscounted cash flows expected to result from the use and eventual disposition of the long-lived asset group 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 group are less than its carrying amount. The impairment loss would be based on the excess of the carrying value of the impaired asset group over its fair value, determined based on discounted cash flows. The Company did not record any impairment charges during any of the periods presented.
Segment Information	Segment Information 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 and commercializing gene therapy treatments for patients suffering from rare and life-threatening neurological genetic diseases. All of the Company’s tangible assets are held in the United States.
Research and Development Costs	Research and Development Costs Research and development costs are expensed as incurred. Research and development expenses are comprised of costs incurred in performing research and development activities, including salaries, stock-based compensation and benefits, third party license fees, and external costs of outside vendors engaged to conduct manufacturing and preclinical development activities and clinical trials. Upfront and milestone payments made to third parties who perform research and development services on the Company’s behalf are expensed as services are rendered or when they no longer have alternative future use. Costs incurred in obtaining technology licenses through asset acquisitions are charged to research and development expense if the licensed technology has not reached technological feasibility and has no alternative future use.
Patent Costs	Patent Costs All patent-related costs incurred in connection with filing and prosecuting patent applications are expensed as incurred due to the uncertainty about the recovery of the expenditure. Amounts incurred are classified as general and administrative expenses.
Stock-Based Compensation	Stock-Based Compensation The Company accounts for its stock-based compensation awards in accordance with FASB ASC Topic 718, Compensation—Stock Compensation (“ASC 718”). ASC 718 requires all stock-based payments to employees, including grants of employee stock options and restricted stock units, to be recognized in the consolidated statements of operations based on their fair values. The Company’s stock-based awards are subject to service and performance-based vesting conditions. Compensation expense related to awards to employees with only service-based vesting conditions is recognized on a straight-line basis over the requisite service period for each separately vesting portion of the award as if the award was, in substance, multiple awards (the “Graded Vesting Attribution Method”), based on the estimated grant date fair value for each separately vesting tranche. Compensation expense related to awards to non-employees with only service-based vesting conditions is recognized based on the then-current fair value at each financial reporting date prior to the measurement date over the associated service period of the award, which is generally the vesting term, using the Graded Vesting Attribution Method. Compensation expense related to awards to employees with only performance-based vesting conditions is recognized based on the estimated grant date fair value over the requisite service period using the Graded Vesting Attribution Method to the extent achievement of the performance condition is probable. Compensation expense related to awards to non-employees only with performance-based vesting conditions is recognized based on the then-current fair value at each financial reporting date prior to the measurement date over the requisite service period using the Graded Vesting Attribution Method to the extent achievement of the performance condition is probable. The Company estimates the fair value of its option awards to employees and directors using the Black-Scholes option-pricing model, which requires the input of and use of subjective assumptions, including (i) the fair value of the underlying common stock, (ii) the expected stock price volatility, (iii) the calculation of expected term of the award, (iv) the risk-free interest rate, and (v) expected dividends. Due to the lack of company-specific historical and implied volatility data of its common stock, the Company has based its estimate of expected volatility on the historical volatility of a group of similar companies that are publicly traded. When selecting these public companies on which it has based its expected stock price volatility, the Company selected companies with comparable characteristics to it, including enterprise value, risk profiles, position within the industry, and with historical share price information sufficient to meet the expected term of the stock-based awards. The Company computes historical volatility data using the daily closing prices for the selected companies’ shares during the equivalent period of the calculated expected term of the stock-based awards. The Company’s estimates of expected term used in the Black-Scholes option pricing model were based on the estimated time from the grant date to the date of exercise. The risk-free interest rates for periods within the expected term of the option are based on the U.S. Treasury securities with a maturity date commensurate with the expected term of the associated award. The Company has never paid dividends, and does not expect to pay dividends in the foreseeable future. Stock-based awards issued to non-employees, consisting of stock warrants and restricted common shares, are accounted for using the fair value method in accordance with ASC 505-50, Equity-Based Payments to Non-Employees. These stock warrants and restricted common shares have been granted in exchange for consulting services to be rendered, and vest according to certain service or performance conditions. In accordance with authoritative guidance, the fair value of non-employee stock-based awards is estimated on the date of grant, and subsequently revalued at each reporting period until the award vests or a measurement date has occurred using the Black-Scholes option-pricing model.
Comprehensive Loss	Comprehensive Loss Comprehensive loss includes net loss as well as other changes in stockholders’ equity (deficit) that result from transactions and economic events other than those with stockholders. There was no difference between net loss and comprehensive loss for each of the periods presented in the accompanying consolidated financial statements.
Income Taxes	Income Taxes The Company is a C corporation for federal and state income tax purposes. The Company recognizes deferred tax assets and liabilities for temporary differences between the financial reporting basis and the tax basis of its assets and liabilities and the expected benefits of net operating loss carryforwards. The impact of changes in tax rates and laws on deferred taxes is applied in the period of enactment. The measurement of deferred tax assets is reduced if, based on weight of the evidence, it is more likely than not that some, or all, of the deferred tax assets will not be realized. At December 31, 2017, 2016 and 2015, the Company has concluded that as a result of the accumulated losses to date, a full valuation allowance is necessary for its net deferred tax assets. The Company accounts for uncertain tax positions in accordance with the provisions of FASB ASC Topic 740, Income Taxes. When uncertain tax positions exist, the Company recognizes the tax benefit of tax positions to the extent that the benefit will more likely than not be realized. The determination as to whether the tax benefit will more likely than not be realized is based upon the technical merits of the tax position as well as consideration of the available facts and circumstances. As of December 31, 2017 and 2016, the Company does not have any significant uncertain tax positions. The Company’s policy is to recognize interest and penalties related to income tax matters in income tax expense.
Fourth Amended and Restated Certificate of Incorporation	Fourth Amended and Restated Certificate of Incorporation On February 1, 2016, the Company amended its certificate of incorporation such that the total authorized capital stock of the Company consisted of 30,000,000 shares of common stock, $0.0001 par value per share, 3,278,938 shares of Class B-1 preferred stock, $0.0001 par value per share, 326,557 shares of Class B-2 preferred stock, $0.0001 par value per share, 2,365,020 shares of Class C preferred stock, $0.0001 par value per share, 3,105,000 shares of Class D preferred stock, $0.0001 par value per share and 1,000,000 shares of preferred stock, $0.0001 par value per share. Additionally, the Company effected a stock split whereby each outstanding share of common stock and Class B-1, B-2, C and D preferred stock was converted into 1.38 shares of common stock and Class B-1, B-2, C and D preferred stock, respectively. All share and per share information presented in these condensed consolidated financial statements and accompanying footnotes has been retroactively adjusted to reflect the stock split.
Fifth Amended and Restated Certificate of Incorporation	Fifth Amended and Restated Certificate of Incorporation On February 17, 2016, the Company amended its certificate of incorporation such that the total authorized capital stock of the Company consisted of 100,000,000 shares of common stock, $0.0001 par value per share, and 10,000,000 shares of preferred stock, $0.0001 par value per share.
Recent Accounting Pronouncements	Recent Accounting Pronouncements In January 2017, the FASB issued ASU 2017-01, Business Combinations (Topic 805): Clarifying the Definition of a Business, which clarifies the definition of a business to assist entities with evaluating whether transactions should be accounted for as acquisitions (or disposals) of assets or businesses. The standard will be effective for the Company in the first quarter of 2018. Adoption of this ASU will not impact prior periods but may impact the accounting of future transactions. In February 2016, the FASB issued ASU 2016-02, Leases (“ASC 842”). The guidance requires lessees to recognize almost all leases on their balance sheet as a right-of-use asset and a lease liability. For income statement purposes, the FASB retained a dual model, requiring leases to be classified as either operating or finance. Lessor accounting is similar to the current model, but updated to align with certain changes to the lessee model and the new revenue recognition standard. ASC 842 is effective for fiscal years beginning after December 15, 2018. The Company is evaluating the adoption of ASC 842, but has not yet determined the effects it may have on the Company’s consolidated financial statements.

Property and Equipment, Net (Ta

Property and Equipment, Net (Tables)	12 Months Ended
	Dec. 31, 2017
Property and Equipment, Net
Summary of property and equipment, net	December 31, December 31, 2017 2016 Furniture $ 919 $ 108 Equipment 34,777 230 Leasehold improvements 7,123 777 Construction in progress 15,656 23,162 Property and equipment, gross 58,475 24,277 Less: accumulated depreciation (2,301) (76) Property and equipment, net $ 56,174 $ 24,201

Accrued Expenses and Other Cu24

Accrued Expenses and Other Current Liabilities (Tables)	12 Months Ended
	Dec. 31, 2017
Accrued Expenses
Schedule of accrued expenses	December 31, December 31, 2017 2016 Accrued manufacturing development costs $ 10,356 $ 7,167 Accrued payroll, bonus and deferred compensation 9,589 3,841 Accrued construction in progress 4,156 2,979 Accrued clinical trial costs 864 389 Accrued professional and consulting fees 5,406 1,588 Accrued loss on settlement (see Note 7) 11,647 — Other 2,920 830 Accrued expenses and other current liabilities $ 44,938 $ 16,794

Stock-Based Compensation (Table

Stock-Based Compensation (Tables)	12 Months Ended
	Dec. 31, 2017
Stock-Based Compensation
Summary of stock option activity	The following table summarizes stock option activity (in thousands, except per share data): Weighted Weighted Average Average Remaining Aggregate Number of Exercise Contractual Intrinsic Shares Price Life (Years) Value (a) Outstanding at December 31, 2016 2,577 $ 22.01 8.79 $ 66,466 Granted 1,103 $ 78.59 Exercised (268) $ 14.93 Cancelled or forfeited (245) $ 36.02 Outstanding at December 31, 2017 3,167 $ 41.30 8.35 $ 219,656 Exercisable at December 31, 2017 1,158 $ 20.96 7.70 $ 103,843 Exercisable and expected to vest at December 31, 2017 3,167 $ 41.30 8.35 $ 219,656 (a) The aggregate intrinsic value is calculated as the difference between the exercise price of the underlying options and the estimated fair value of the common stock for the options that were in-the-money at December 31, 2016 and 2017.
Summary of stock-based compensation expense by classification in the Statement of Operations and Comprehensive Loss	Year ended Year ended Year ended December 31, December 31, December 31, 2017 2016 2015 Research and development $ 15,383 $ 5,988 $ 1,294 General and administrative 15,502 9,668 3,956 $ 30,885 $ 15,656 $ 5,250
Summary of weighted-average assumptions used in determining fair value of stock options	Year ended Year ended Year ended December 31, December 31, December 31, 2017 2016 2015 Expected volatility 86.34 % 91.84 % 90.00 % Risk-free interest rate 1.94 % 1.45 % 1.57 % Expected term (in years) 6.08 6.08 6.08 Expected dividend yield 0.00 % 0.00 % 0.00 %
Summary of service-based restricted stock units activity	Weighted Average Grant Date Number Fair Value of Shares Per Share Outstanding at December 31, 2016 58 $ 34.90 Granted 2 99.26 Vested (20) $ 37.60 Forfeited and cancelled (13) $ 34.90 Outstanding at December 31, 2017 27 $ 38.79

Net Loss Per Common Share (Tabl

Net Loss Per Common Share (Tables)	12 Months Ended
	Dec. 31, 2017
Net Loss Per Common Share
Schedule of potentially dilutive securities excluded from computations of diluted weighted-average shares outstanding	Year Ended Year Ended Year Ended December 31, December 31, December 31, 2017 2016 2015 Stock options 3,167 2,577 1,749 Stock warrants 306 306 326 Unvested service-based restricted stock units 27 58 — Unvested performance-based restricted stock units 49 — 1,751
Summary of the basic and diluted net loss per common share	Year Ended Year Ended Year Ended December 31, December 31, December 31, 2017 2016 2015 Numerator: Net loss and comprehensive loss $ (218,051) $ (83,012) $ (38,474) Denominator: Weighted-average basic and diluted common shares 29,935 22,648 7,088 Basic and diluted net loss per common share $ (7.28) $ (3.67) $ (5.43)

Commitments and Contingencies (

Commitments and Contingencies (Tables)	12 Months Ended
	Dec. 31, 2017
Commitments and Contingencies
Schedule of future minimum lease payments	Year ending December 31, 2018 $ 1,414 2019 1,969 2020 2,075 2021 2,126 2022 2,179 Thereafter 7,543 Total $ 17,306

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2017
Taxes
Schedule of reconciliation of difference between federal statutory rate and effective income tax rate	Year ended Year ended Year ended December 31, 2017 December 31, 2016 December 31, 2015 Amount Tax Rate Amount Tax Rate Amount Tax Rate Federal statutory rate $ (74,137) 34.00 % $ (28,224) 34.00 % $ (13,081) 34.00 % Permanent differences (1,203) 0.55 % 3,991 (4.73) % 6,077 (15.80) % State taxes (11,498) 5.27 % (2,453) 2.97 % (1,275) 3.31 % Change in statutory rate 37,162 (17.04) % — — % — — % Research and development credit (4,989) 2.29 % (8,891) 10.55 % (220) 0.57 % Valuation allowance 56,049 (25.70) % 35,502 (42.70) % 8,472 (21.38) % Other (1,384) 0.63 % 75 (0.09) % 27 (0.70) % Effective income tax rate $ — — % $ — — % $ — — %
Schedule of significant components of deferred taxes	December 31, 2017 2016 Assets Liabilities Assets Liabilities Current: Accrued Expenses $ 2,950 $ — $ 2,995 $ — Other — (32) 45 — Subtotal 2,950 (32) 3,040 — Valuation allowance (2,918) — (3,040) — Total current deferred taxes 32 (32) — — Net current deferred taxes $ — $ — $ — $ — Long Term: Net operating loss carryforwards $ 31,971 $ — $ 13,063 $ — Capital loss carryforward 77 — 113 — Research and development credit 23,391 — 13,733 — State research and development credit 1,994 — — — Capitalized research and development 34,472 — 11,797 — Amortization 2,746 — 1,423 — Stock-based compensation 9,370 — 6,264 — Fixed assets — (1,474) — (17) Subtotal 104,021 (1,474) 46,393 (17) Valuation allowance (102,547) — (46,376) — Total long-term deferred taxes $ 1,474 $ (1,474) $ 17 $ (17) Net deferred tax assets/(liabilities) $ — $ — $ — $ —
Summary of changes to the valuation allowance	Valuation allowance at December 31, 2015 $ 13,914 Additions for 2016 35,502 Change in tax rates — Valuation allowance at December 31, 2016 49,416 Additions for 2017 93,211 Change in tax rates (37,162) Valuation allowance at December 31, 2017 $ 105,465

Quarterly Financial Informati29

Quarterly Financial Information (Unaudited) (Tables)	12 Months Ended
	Dec. 31, 2017
Quarterly Financial Information (Unaudited)
Summary of quarterly financial information	Quarter Ended December 31, September 30, June 30, March 31, 2017 2017 2017 2017 Revenue $ — $ — $ — $ — Loss from operations $ (82,523) $ (49,520) $ (58,360) $ (29,964) Interest income $ 858 $ 881 $ 331 $ 246 Net loss and comprehensive loss $ (81,665) $ (48,639) $ (58,029) $ (29,718) Basic and diluted net loss per common share (1) $ (2.55) $ (1.52) $ (2.07) $ (1.07) Weighted-average basic and diluted common shares outstanding 32,030 31,941 27,972 27,734 Quarter Ended December 31, September 30, June 30, March 31, 2016 2016 2016 2016 Revenue $ — $ — $ — $ — Loss from operations $ (25,547) $ (21,180) $ (15,798) $ (20,889) Interest income $ 172 $ 99 $ 79 $ 53 Net loss and comprehensive loss $ (25,375) $ (21,082) $ (15,719) $ (20,836) Basic and diluted net loss per common share (1) $ (0.92) $ (0.87) $ (0.68) $ (1.24) Weighted-average basic and diluted common shares outstanding 27,678 24,166 23,014 16,775 (1) The sum of the four quarters of net loss per share for 2017 and 2016 may not add to the full year net loss per share amount due to rounding and/or the use of quarter-to-date weighted average shares to calculate the net loss per share amount in each respective quarter.

Background (Details)

Background (Details) - USD ($) $ / shares in Units, $ in Thousands	Jan. 22, 2018	Jun. 26, 2017	Sep. 13, 2016	Mar. 03, 2016	Feb. 10, 2016	Jan. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Liquidity and Risks
Retained Earnings (Accumulated Deficit)							$ (359,613)	$ (141,562)
Cash and cash equivalents							324,117	240,430	$ 62,252	$ 3,120
Net proceeds from issuance of common stock		$ 269,900
Public Offering
Shares issued (in shares)		4,111,250
Share price (in dollars per share)		$ 70
Net Proceeds from Initial Public Offering							$ 269,869	$ 246,490
Subsequent Event [Member]
Liquidity and Risks
Net proceeds from issuance of common stock	$ 431,900					$ 431,900
Public Offering
Shares issued (in shares)	4,509,840
Share price (in dollars per share)	$ 102
Over-Allotment Option
Public Offering
Shares issued (in shares)		536,250
Over-Allotment Option | Subsequent Event [Member]
Public Offering
Shares issued (in shares)	588,240
Class B-1 Preferred Stock
Public Offering
Shares converted (in shares)					3,278,938
Class B-2 Preferred Stock
Public Offering
Shares converted (in shares)					326,557
Class C Preferred Stock
Liquidity and Risks
Net proceeds from issuance of common stock									5,000
Public Offering
Shares converted (in shares)					2,365,020
Class D Preferred Stock
Liquidity and Risks
Net proceeds from issuance of common stock									$ 64,788
Public Offering
Shares converted (in shares)					3,105,000
Common Stock
Public Offering
Shares issued (in shares)							14,000
Shares converted (in shares)								456,000
Common Stock | IPO
Public Offering
Shares issued (in shares)					4,750,000			5,278,000
Share price (in dollars per share)					$ 20
Net Proceeds from Initial Public Offering					$ 88,400
Common Stock | Over-Allotment Option
Public Offering
Shares issued (in shares)			637,500	527,941
Share price (in dollars per share)				$ 20
Net Proceeds from Initial Public Offering				$ 9,800
Common Stock | Stock Sold by Selling Stockholder
Public Offering
Shares issued (in shares)			289,855
Common Stock | Follow-On Offering
Liquidity and Risks
Net proceeds from issuance of common stock			$ 149,100
Public Offering
Number of common shares to be sold under public offering			4,887,500
Shares issued (in shares)			4,597,645
Share price (in dollars per share)			$ 34.50

Summary of Significant Accoun31

Summary of Significant Accounting Policies (Details)	Feb. 01, 2016$ / sharesshares	Dec. 31, 2017$ / sharesshares	Dec. 31, 2016$ / sharesshares	Feb. 17, 2016$ / sharesshares
Restated Certificate of Incorporation
Preferred stock, shares authorized (in shares) | shares		10,000,000	10,000,000	10,000,000
Preferred stock, par value (in dollars per share) | $ / shares		$ 0.0001	$ 0.0001	$ 0.0001
Common stock, shares authorized (in shares) | shares		100,000,000	27,700,000	100,000,000
Common stock, par value (in dollars per share) | $ / shares		$ 0.0001	$ 0.0001	$ 0.0001
Class B-1 Preferred Stock
Restated Certificate of Incorporation
Preferred stock, shares authorized (in shares) | shares	3,278,938
Preferred stock, par value (in dollars per share) | $ / shares	$ 0.0001
Stock split ratio	1.38
Class B-2 Preferred Stock
Restated Certificate of Incorporation
Preferred stock, shares authorized (in shares) | shares	326,557
Preferred stock, par value (in dollars per share) | $ / shares	$ 0.0001
Stock split ratio	1.38
Class C Preferred Stock
Restated Certificate of Incorporation
Preferred stock, shares authorized (in shares) | shares	2,365,020
Preferred stock, par value (in dollars per share) | $ / shares	$ 0.0001
Stock split ratio	1.38
Class D Preferred Stock
Restated Certificate of Incorporation
Preferred stock, shares authorized (in shares) | shares	3,105,000
Preferred stock, par value (in dollars per share) | $ / shares	$ 0.0001
Stock split ratio	1.38
Undesignated Preferred Stock
Restated Certificate of Incorporation
Preferred stock, shares authorized (in shares) | shares	1,000,000
Preferred stock, par value (in dollars per share) | $ / shares	$ 0.0001
Common Stock
Restated Certificate of Incorporation
Common stock, shares authorized (in shares) | shares	30,000,000
Common stock, par value (in dollars per share) | $ / shares	$ 0.0001
Stock split ratio	1.38
Preferred Stock | Class B-1 Preferred Stock
Restated Certificate of Incorporation
Stock split ratio	1.38
Preferred Stock | Class B-2 Preferred Stock
Restated Certificate of Incorporation
Stock split ratio	1.38
Preferred Stock | Class C Preferred Stock
Restated Certificate of Incorporation
Stock split ratio	1.38
Preferred Stock | Class D Preferred Stock
Restated Certificate of Incorporation
Stock split ratio	1.38

Collaboration and License Agr32

Collaboration and License Agreements - Nationwide Children's Hospital (Details)	1 Months Ended			12 Months Ended
	May 31, 2015shares	Mar. 31, 2015shares	Oct. 31, 2013USD ($)installment$ / sharesshares	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)	Dec. 31, 2015USD ($)shares	Jun. 26, 2017$ / shares	May 29, 2015USD ($)
Collaboration and License Agreements
Share price (in dollars per share) | $ / shares							$ 70
Research and Development Expense				$ 150,391,000	$ 58,892,000	$ 27,493,000
Collaborative Arrangement | NCH
Collaboration and License Agreements
Minimum commitment to spend			$ 9,400,000
Minimum commitment to spend, period ( in years)			8 years
Shares issued (in shares) | shares	3,802	34,463	331,053
Ownership percentage (as a percent)			3.00%
Period following FDA approval of BLA to sell shares (in days)			30 days
Minimum ownership percentage of shares issued pursuant to the agreement (as a percent)			50.00%
Quarterly installments, number | installment			4
License patent expiration period (in years)			10 years
Market capitalization, achieved								$ 100,000,000
Period of prior written notice to terminate agreement			6 months
Termination, event of a material uncured breach, notice period (in days)			30 days
Research and development expenses | Collaborative Arrangement | NCH
Collaboration and License Agreements
Fair value of shares granted						473,164
Research and Development Expense						$ 100,000
Class B-1 Preferred Stock
Collaboration and License Agreements
Shares issued (in shares) | shares						1,012,000
Class B-1 Preferred Stock | Collaborative Arrangement | NCH
Collaboration and License Agreements
Ratio of the per share sale price to the price per share of preferred stock sold in Class B Financing			2
Share price (in dollars per share) | $ / shares			$ 2.47

Collaboration and License Agr33

Collaboration and License Agreements - RegenxBio License (Details) - Regenx Bio SMA License - ReGenX - USD ($) $ in Thousands	Mar. 21, 2014	Dec. 31, 2014	Jan. 08, 2018
Maximum
Collaboration and License Agreements
Potential milestone payments to be paid	$ 12,250
Subsequent Event [Member]
Collaboration and License Agreements
Up-front payment, agreed-upon amount			$ 80,000
Research and development expenses | Up-front Payment Arrangement [Member]
Collaboration and License Agreements
Up-front payment made		$ 2,000

Collaboration and License Agr34

Collaboration and License Agreements - Asklepios Biopharmaceutical, Inc. License 10-K (Details) - USD ($) $ in Thousands	May 29, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Collaboration and License Agreements
Research and Development Expense		$ 150,391	$ 58,892	$ 27,493
License Agreement | Asklepios Biopharmaceutical, Inc.
Collaboration and License Agreements
Up-front payment made	$ 1,000
Potential milestone payments to be paid	$ 9,600
Research and Development Expense				$ 1,000
Period of prior written notice to terminate agreement	6 months
Up-front Payment Arrangement [Member] | License Agreement | Asklepios Biopharmaceutical, Inc.
Collaboration and License Agreements
Up-front payment made	$ 1,000

Collaboration and License Agr35

Collaboration and License Agreements - Rhett and ALS (Details) - USD ($) $ in Thousands	Jun. 07, 2017	Sep. 30, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
NCH Rett and ALS
Research and development expenses			$ 150,391	$ 58,892	$ 27,493
Regenx Rett and ALS license | ReGenX
RegenXBio Rett and ALS
Period of time from the first commercial sale of each licensed product that the license will expire if that date is later than the expiration, lapse, abandonment or invalidation of the last valid claim of the licensed intellectual property to expire, lapse or become abandoned or unenforceable in all countries of the world	7 years
Regenx Rett and ALS license | ReGenX | Maximum
RegenXBio Rett and ALS
Potential milestone payments to be paid	$ 36,000
Regenx Rett and ALS license | Up-front Payment Arrangement [Member] | ReGenX
RegenXBio Rett and ALS
Up-front payment made	$ 6,000
NCH Licenses | NCH
NCH Rett and ALS
Research and development expenses				$ 300
NCH Licenses | NCH | After Second Anniversary Of License Effective Date
NCH Rett and ALS
Period of prior written notice to terminate agreement		6 months
NCH Licenses | NCH | Upon Other Party's Material Breach of License Not Cured Within Specified Period
NCH Rett and ALS
Period of prior written notice to terminate agreement		90 days

Property and Equipment, Net (De

Property and Equipment, Net (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Property and Equipment, Net
Property and equipment, gross	$ 58,475	$ 24,277
Less: accumulated depreciation	(2,301)	(76)
Property and equipment, net	56,174	24,201
Depreciation expense
Depreciation and amortization	2,270	64	$ 14
Furniture
Property and Equipment, Net
Property and equipment, gross	$ 919	108
Furniture | Minimum
Depreciation expense
Useful life	5 years
Furniture | Maximum
Depreciation expense
Useful life	10 years
Equipment
Property and Equipment, Net
Property and equipment, gross	$ 34,777	230
Equipment | Minimum
Depreciation expense
Useful life	5 years
Equipment | Maximum
Depreciation expense
Useful life	10 years
Leasehold improvement
Property and Equipment, Net
Property and equipment, gross	$ 7,123	777
Construction in progress
Property and Equipment, Net
Property and equipment, gross	15,656	$ 23,162
Construction in progress
Decrease in construction in progress	$ (7,500)
Manufacturing pods
Depreciation expense
Useful life	20 years

Accrued Expenses and Other Cu37

Accrued Expenses and Other Current Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Accrued Expenses and Other Current Liabilities
Accrued manufacturing development costs	$ 10,356	$ 7,167
Accrued payroll, bonus and deferred compensation	9,589	3,841
Accrued construction in progress	4,156	2,979
Accrued clinical trial costs	864	389
Accrued professional fees	5,406	1,588
Accrued loss on settlement	11,647
Other	2,920	830
Accrued expenses and other current liabilities	$ 44,938	$ 16,794

Accrued Indemnification Oblig38

Accrued Indemnification Obligation (Details) - USD ($) $ / shares in Units, $ in Thousands	May 03, 2017	Jan. 31, 2014	Dec. 31, 2017	Jun. 26, 2017	Dec. 31, 2016	Dec. 31, 2015	Jan. 28, 2014
Accrued Indemnification Obligation
Share Price				$ 70
Accrued indemnification obligation			$ 2,788		$ 4,452
Restricted Stock | Member of Board of Directors
Accrued Indemnification Obligation
Shares granted (in shares)		2,334,391
Share Price							$ 1.51
Aggregate fair value						$ 3,500
Tax indemnification | Member of Board of Directors
Accrued Indemnification Obligation
Indemnification term (in years)			6 years
Cash paid representing tax liabilities per agreement	$ 1,700

Capitalization - Stock Split (D

Capitalization - Stock Split (Details)	Feb. 01, 2016
Common Stock
Capitalization
Stock split ratio	1.38
Class B-1 Preferred Stock
Capitalization
Stock split ratio	1.38
Class B-1 Preferred Stock | Preferred Stock
Capitalization
Stock split ratio	1.38
Class B-2 Preferred Stock
Capitalization
Stock split ratio	1.38
Class B-2 Preferred Stock | Preferred Stock
Capitalization
Stock split ratio	1.38
Class C Preferred Stock
Capitalization
Stock split ratio	1.38
Class C Preferred Stock | Preferred Stock
Capitalization
Stock split ratio	1.38
Class D Preferred Stock
Capitalization
Stock split ratio	1.38
Class D Preferred Stock | Preferred Stock
Capitalization
Stock split ratio	1.38

Capitalization - Class B-1 and

Capitalization - Class B-1 and B-2 Preferred Stock (Details) - USD ($) $ / shares in Units, $ in Millions	Jun. 26, 2017	May 04, 2015	Jan. 30, 2014
Capitalization
Shares issued (in shares)	4,111,250
PBM Capital Investments, LLC | Class B-2 Warrant and Class B-2 Milestone Warrant
Capitalization
Warrants, term (in years)		10 years
PBM Capital Investments, LLC | Class B Note
Capitalization
Face amount			$ 0.5
Class B-1 Preferred Stock | PBM Capital Investments, LLC
Capitalization
Option to purchase shares, number (in shares)			809,385
Stock option, exercise price (in dollars per share)			$ 2.47
Class B-2 Preferred Stock | PBM Capital Investments, LLC
Capitalization
Stock option, exercise price (in dollars per share)			$ 2.57
Warrant to purchase shares, number (in shares)			130,623
Class B-2 Preferred Stock | PBM Capital Investments, LLC | Class B-2 Milestone Warrant
Capitalization
Warrant to purchase shares, number (in shares)		163,278
Preferred Stock | Class B-1 Preferred Stock | PBM Capital Investments, LLC
Capitalization
Shares issued (in shares)		1,011,721
Cash proceeds		$ 2.5

Capitalization - Class C Prefer

Capitalization - Class C Preferred Stock (Details) - USD ($) $ / shares in Units, $ in Millions	Jun. 26, 2017	Aug. 11, 2015	Mar. 31, 2015
Capitalization
Shares issued (in shares)	4,111,250
Share price (in dollars per share)	$ 70
Preferred Stock | Class C Preferred Stock
Capitalization
Shares issued (in shares)		504,478	551,472
Share price (in dollars per share)		$ 3.96	$ 4.53
Cash proceeds			$ 5

Capitalization - Class D Prefer

Capitalization - Class D Preferred Stock Issuance (Details) - USD ($) $ / shares in Units, $ in Millions	Jun. 26, 2017	Sep. 03, 2015
Capitalization
Shares issued (in shares)	4,111,250
Share price (in dollars per share)	$ 70
Preferred Stock | Class D Preferred Stock
Capitalization
Shares issued (in shares)		3,093,092
Share price (in dollars per share)		$ 21.01
Cash proceeds		$ 64.8

Capitalization - Common Stock S

Capitalization - Common Stock Settlement and Issuance (Details) - USD ($) $ in Millions	Dec. 29, 2017	Jun. 26, 2017	Dec. 31, 2017
Public Offering
Shares issued (in shares)		4,111,250
General and Administrative Expense			$ 11.6
Common Stock
Public Offering
Shares issued (in shares)			14,000
Shares to be issued per settlement agreement	105,237

Stock-Based Compensation - Gene

Stock-Based Compensation - General Information (Details)	Mar. 20, 2017shares	Feb. 10, 2016shares	Mar. 31, 2017shares	Aug. 31, 2016shares	Dec. 31, 2017Optionshares	Dec. 31, 2016shares
2014 Stock Plan
Stock-Based Compensation
Number of shares available for grant (in shares)		0
2016 Equity Incentive Plan
Stock-Based Compensation
Number of shares available for grant (in shares)					2,204,082
Number of incentive stock options that may be granted after the 10th anniversary of the effective date of the plan | Option					0
Common stock that may be issued pursuant to stock awards (in shares)		4,339,451
Common stock that may be issued pursuant to stock awards, new shares (in shares)		2,400,000
Annual automatic increase percentage (as a percent)		4.00%
Vesting period (in years)					10 years
Expiration period (in years)					10 years
Maximum | 2016 Equity Incentive Plan
Stock-Based Compensation
Vesting period (in years)					4 years
Employee Stock Options | 2016 Equity Incentive Plan
Stock-Based Compensation
Common stock that may be issued pursuant to stock awards (in shares)		8,678,902
Service Based RSUs
Stock-Based Compensation
Shares granted (in shares)					2,000
Vesting period (in years)					3 years
Service Based RSUs | 2016 Equity Incentive Plan
Stock-Based Compensation
Shares granted (in shares)				57,500
Performance Based RSU
Stock-Based Compensation
Shares granted (in shares)	49,332,000					0
Performance Based RSU | 2016 Equity Incentive Plan
Stock-Based Compensation
Shares granted (in shares)			49,332

Stock-Based Compensation - Stoc

Stock-Based Compensation - Stock Option Activity (Details) - Employee Stock Options - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Number of Shares
Outstanding at the beginning of the period (in shares)	2,577
Granted (in shares)	1,103
Exercised (in shares)	268
Cancelled or forfeited (in shares)	245
Outstanding at the end of the period (in shares)	3,167	2,577
Weighted Average Exercise Price
Outstanding at the beginning of the period (in dollars per share)	$ 22.01
Granted (in dollars per share)	78.59
Exercised (in dollars per share)	14.93
Cancelled or forfeited (in dollars per share)	36.02
Outstanding at the end of the period (in dollars per share)	$ 41.30	$ 22.01
Weighted Average
Remaining Contractual Life (Years)	8 years 4 months 6 days	8 years 9 months 15 days
Aggregate Intrinsic Value (in dollars)	$ 219,656	$ 66,466
Exercisable and Vested and Expected to Vest
Number of Shares - exercisable (in shares)		1,158
Weighted Average Exercise Price - exercisable (in dollars per share)		$ 20.96
Remaining Contractual Life (Years) - exercisable		7 years 8 months 12 days
Aggregate Intrinsic Value - exercisable (in dollars)		$ 103,843
Number of Shares - vested and expected to vest (in shares)	3,167
Weighted Average Exercise Price - vested and expected to vest (in dollars per share)	$ 41.30
Remaining Contractual Life (Years) - vested and expected to vest	8 years 4 months 6 days
Aggregate Intrinsic Value - vested and expected to vest (in dollars)	$ 219,656

Stock-Based Compensation - St46

Stock-Based Compensation - Stock Options Exercised (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Aggregate stock-based compensation expense by category
Aggregate stock-based compensation expense	$ 30,885	$ 15,656	$ 5,250
Employee Stock Options
Stock-Based Compensation
Total intrinsic value of option exercised	19,000	3,400	3,600
Unrecognized stock-based compensation expense
Unrecognized stock-based compensation expense	$ 46,500
Weighted-average period expected to recognize stock-based compensation expense	1 year 3 months 18 days
Research and development expenses
Aggregate stock-based compensation expense by category
Aggregate stock-based compensation expense	$ 15,383	5,988	1,294
General and administrative expenses
Aggregate stock-based compensation expense by category
Aggregate stock-based compensation expense	$ 15,502	$ 9,668	$ 3,956
2016 Equity Incentive Plan
Stock-Based Compensation
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Available for Grant	2,204,082

Stock-Based Compensation - Weig

Stock-Based Compensation - Weighted-average Assumptions (Details) - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Employee Stock Options
Weighted-average grant date fair value
Weighted-average grant date fair value of options granted during the period (in dollars per share)	$ 57.13	$ 24.50	$ 16.44
Weighted-average fair value assumptions
Expected volatility (as a percent)	86.34%	91.84%	90.00%
Risk-free interest rate (as a percent)	1.94%	1.45%	1.57%
Expected term (in years)	6 years 29 days	6 years 29 days	6 years 29 days
Expected dividend yield (as a percent)	0.00%	0.00%	0.00%
2016 Equity Incentive Plan
Vesting and expiration of stock options
Expiration period (in years)	10 years
Vesting period (in years)	10 years
Period during which vested options may be exercised after termination of service	3 months
2016 Equity Incentive Plan | Maximum
Vesting and expiration of stock options
Vesting period (in years)	4 years

Stock-Based Compensation - Rest

Stock-Based Compensation - Restricted Stock Units - General Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Stock Options Granted to Non-Employees
Stock-based compensation expense	$ 30,885	$ 15,656	$ 5,250
Service Based RSUs
Stock Options Granted to Non-Employees
Vesting period (in years)	3 years
Stock-based compensation expense	$ 700	800
Unrecognized compensation cost	$ 400
Weighted-average period expected to recognize stock-based compensation expense	8 months 12 days
Research and development expenses
Stock Options Granted to Non-Employees
Stock-based compensation expense	$ 15,383	5,988	1,294
General and administrative expenses
Stock Options Granted to Non-Employees
Stock-based compensation expense	$ 15,502	$ 9,668	$ 3,956

Stock-Based Compensation - Re49

Stock-Based Compensation - Restricted Stock Units - Activity (Details) - Service Based RSUs shares in Thousands	12 Months Ended
	Dec. 31, 2017$ / sharesshares
Number of Shares
Outstanding at the beginning of the period (in shares) | shares	58
Granted (in shares) | shares	2
Vested (in shares) | shares	(20)
Forfeited and cancelled (in shares) | shares	13
Outstanding at the end of the period (in shares) | shares	27
Weighted Average Grant Date Fair Value Per Share
Outstanding at the beginning of the period (in dollars per share) | $ / shares	$ 34.90
Granted (in dollars per share) | $ / shares	99.26
Vested (in dollars per share) | $ / shares	37.60
Forfeited and cancelled (in dollars per share) | $ / shares	34.90
Outstanding at the end of the period (in dollars per share) | $ / shares	$ 38.79

Stock-Based Compensation - Perf

Stock-Based Compensation - Performance-Based Restricted Stock Units (Details) - USD ($) $ / shares in Units, $ in Millions	Mar. 20, 2017	Dec. 31, 2017	Dec. 31, 2016
Performance Based RSU
Stock Options Granted to Non-Employees
Shares granted (in shares)	49,332,000		0
Period from grant date during which unvested PSUs will be cancelled if milestones do not occur	3 years
Shares outstanding (in shares)		49,332,000	0
Shares vested (in shares)		0
Weighted average grant date fair value		$ 79.75
Research and development expenses
Stock Options Granted to Non-Employees
Amount of share based compensation expense to be recognized if performance conditions are met		$ 3.9

Stock-Based Compensation - Re51

Stock-Based Compensation - Restricted Stock Granted to Non-Employees (Details) - USD ($) $ in Thousands	1 Months Ended		12 Months Ended
	Jan. 31, 2016	Jan. 31, 2014	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Stock Options Granted to Non-Employees
Aggregate stock-based compensation expense			$ 30,885	$ 15,656	$ 5,250
Member of Board of Directors | Restricted Stock
Stock Options Granted to Non-Employees
Shares granted (in shares)		2,334,391
Shares vested (in shares)		583,597
Member of Board of Directors | Restricted Stock | Shares vesting on second anniversary
Stock Options Granted to Non-Employees
Vesting percentage (as a percent)		25.00%
Member of Board of Directors | Restricted Stock | Shares vesting on third anniversary
Stock Options Granted to Non-Employees
Vesting percentage (as a percent)		25.00%
Member of Board of Directors | Restricted Stock | Shares vesting on fourth anniversary
Stock Options Granted to Non-Employees
Vesting percentage (as a percent)		25.00%
Employee | Restricted Stock
Stock Options Granted to Non-Employees
Shares vested (in shares)	1,750,794
Aggregate stock-based compensation expense					19,300
Research and development expenses
Stock Options Granted to Non-Employees
Aggregate stock-based compensation expense			15,383	5,988	1,294
Research and development expenses | Employee | Restricted Stock
Stock Options Granted to Non-Employees
Aggregate stock-based compensation expense				10,400
General and administrative expenses
Stock Options Granted to Non-Employees
Aggregate stock-based compensation expense			$ 15,502	$ 9,668	$ 3,956

Stock-based Compensation - Warr

Stock-based Compensation - Warrants Granted to Non-Employees - General Information 10-K (Details) - Stock warrants - Non-Employee - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Warrant information
Class of Warrant or Right, Vested	305,775	305,775
Class of Warrant or Right, Outstanding	305,775	305,775
Warrants, exercise price (in dollars per share)	$ 2.57	$ 2.57

Net Loss Per Common Share - Dil

Net Loss Per Common Share - Dilutive Securities (Details) - shares shares in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Antidilutive securities not included:
Antidilutive securities	3,549	2,941	3,826
Employee Stock Options
Antidilutive securities not included:
Antidilutive securities	3,167	2,577	1,749
Stock warrants
Antidilutive securities not included:
Antidilutive securities	306	306	326
Service Based RSUs
Antidilutive securities not included:
Antidilutive securities	27	58
Restricted Stock
Antidilutive securities not included:
Antidilutive securities	49		1,751

Net Loss Per Common Share - Cal

Net Loss Per Common Share - Calculation (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Numerator:
Net loss and comprehensive loss	$ (81,665)	$ (48,639)	$ (58,029)	$ (29,718)	$ (25,375)	$ (21,082)	$ (15,719)	$ (20,836)	$ (218,051)	$ (83,012)	$ (38,474)
Denominator:
Weighted-average basic and diluted common shares (in shares)	32,030	31,941	27,972	27,734	27,678	24,166	23,014	16,775	29,935	22,648	7,088
Basic and diluted net loss per common share (in dollars per share)	$ (2.55)	$ (1.52)	$ (2.07)	$ (1.07)	$ (0.92)	$ (0.87)	$ (0.68)	$ (1.24)	$ (7.28)	$ (3.67)	$ (5.43)

Separation Agreement - Severanc

Separation Agreement - Severance and Vacation Pay (Details) - USD ($) $ in Millions	Apr. 22, 2015	Dec. 31, 2017	Dec. 31, 2015
General and administrative expenses
Separation Agreement
Accelerated vesting charge (in dollars)			$ 0.5
General and administrative expenses | Chief Financial Officer
Separation Agreement
Severance Costs		$ 0.5
General and administrative expenses | Chief Executive Officer
Separation Agreement
Severance Costs			$ 0.5
Employee Stock Options | Chief Executive Officer
Separation Agreement
Accelerated vesting (in shares)	53,820

Commitments and Contingencies -

Commitments and Contingencies - Operating Lease - Future Minimum Lease Payments (Details) $ in Thousands	Dec. 31, 2017USD ($)
Operating Leases
2,018	$ 1,414
2,019	1,969
2,020	2,075
2,021	2,126
2,022	2,179
Thereafter	7,543
Total	$ 17,306

Commitments and Contingencies57

Commitments and Contingencies - Operating Lease - Rent Expense (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Operating Leases
Rent expense	$ 1.2	$ 0.3	$ 0.1

Commitments and Contingencies58

Commitments and Contingencies - Litigation (Details) - Sophia's Cure Foundation - NCH - Breach of donation agreement - USD ($)	Sep. 08, 2016	Dec. 31, 2012
Litigation
Gift received under donation agreement		$ 550,000
Monetary damages sought	$ 500,000,000

Income Taxes - Tax Cuts nd Jobs

Income Taxes - Tax Cuts nd Jobs Act (Details) - USD ($) $ in Millions	Jan. 01, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Effect of Tax Cuts and Jobs Act
Federal statutory rate (as a percent)		34.00%	34.00%	34.00%
Provision for remeasurement of deferred tax assets and liabilities due to Tax Cuts and Jobs Act		$ 37.2
Tax Cut and Jobs Act remeasurement offset to valuation allowance		$ 37.2
Forecast
Effect of Tax Cuts and Jobs Act
Federal statutory rate (as a percent)	21.00%

Income Taxes - Reconciliation o

Income Taxes - Reconciliation of statutory rate (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Effective income tax rate, Amount
Federal statutory rate	$ (74,137)	$ (28,224)	$ (13,081)
Permanent differences	(1,203)	3,991	6,077
State taxes	(11,498)	(2,453)	(1,275)
Change in statutory rate	37,162
Research and development credit	(4,989)	(8,891)	(220)
Valuation allowance	56,049	35,502	8,472
Other	$ (1,384)	$ 75	$ 27
Tax Rate (as a percent)
Federal statutory rate (as a percent)	34.00%	34.00%	34.00%
Permanent differences (as a percent)	0.55%	(4.73%)	(15.80%)
State taxes (as a percent)	5.27%	2.97%	3.31%
Change in statutory rate	(17.04%)
Research and development credit (as a percent)	2.29%	10.55%	0.57%
Valuation allowance (as a percent)	(25.70%)	(42.70%)	(21.38%)
Other (as a percent)	0.63%	(0.09%)	(0.70%)

Income Taxes - Deferred taxes (

Income Taxes - Deferred taxes (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Current, Assets
Accrued Expenses	$ 2,950	$ 2,995
Other		45
Subtotal	2,950	3,040
Valuation allowance	(2,918)	(3,040)
Total current deferred taxes	32
Net current deferred taxes
Current, Liabilities
Other	(32)
Net current deferred taxes	(32)
Long Term, Assets
Net operating loss carryforwards	31,971	13,063
Capital loss carryforward	77	113
Research & development credit	23,391	13,733
State research and development credit	1,994
Capitalized research and development	34,472	11,797
Amortization	2,746	1,423
Stock options and warrants	9,370	6,264
Subtotal	104,021	46,393
Valuation allowance	(102,547)	(46,376)
Total long-term deferred taxes	1,474	17
Long Term, Liabilities
Fixed assets	(1,474)	(17)
Subtotal	(1,474)	(17)
Total long-term deferred taxes	$ (1,474)	$ (17)

Income Taxes - Operating loss c

Income Taxes - Operating loss carryforwards (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Federal
Operating loss carryforwards
Net operating loss carryforwards	$ 119.8
Federal | Research and development carryforwards
Operating loss carryforwards
Research and development tax credit carryforwards	23.4	$ 13.7
State
Operating loss carryforwards
Net operating loss carryforwards	$ 111

Income Taxes - Valuation Allowa

Income Taxes - Valuation Allowance (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Taxes
Valuation allowance	$ 49,416	$ 13,914
Additions	93,211	35,502
Change in tax rates	(37,162)
Valuation allowance	$ 105,465	$ 49,416

Retirement Plan (Details)

Retirement Plan (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Retirement Plan
Contributions expenses	$ 0.4	$ 0.1

Quarterly Financial Informati65

Quarterly Financial Information (Unaudited) (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Quarterly financial information
Loss from operations	$ (82,523)	$ (49,520)	$ (58,360)	$ (29,964)	$ (25,547)	$ (21,180)	$ (15,798)	$ (20,889)	$ (220,367)	$ (83,415)	$ (38,573)
Interest income	858	881	331	246					2,316	403	15
Net loss	$ (81,665)	$ (48,639)	$ (58,029)	$ (29,718)	$ (25,375)	$ (21,082)	$ (15,719)	$ (20,836)	$ (218,051)	$ (83,012)	$ (38,474)
Basic and diluted net loss per common share (in dollars per share)	$ (2.55)	$ (1.52)	$ (2.07)	$ (1.07)	$ (0.92)	$ (0.87)	$ (0.68)	$ (1.24)	$ (7.28)	$ (3.67)	$ (5.43)
Weighted-average basic and diluted common shares outstanding (in shares)	32,030	31,941	27,972	27,734	27,678	24,166	23,014	16,775	29,935	22,648	7,088

Subsequent Events (Details)

Subsequent Events (Details) - USD ($) $ / shares in Units, $ in Thousands	Jan. 22, 2018	Jan. 08, 2018	Jun. 26, 2017	Mar. 21, 2014	Jan. 31, 2018
Public Offering
Shares issued (in shares)			4,111,250
Share price (in dollars per share)			$ 70
Net proceeds from issuance of common stock			$ 269,900
Regenx Bio SMA License | ReGenX | Maximum
License agreements
Potential milestone payments to be paid				$ 12,250
Subsequent Event [Member]
Public Offering
Shares issued (in shares)	4,509,840
Share price (in dollars per share)	$ 102
Net proceeds from issuance of common stock	$ 431,900				$ 431,900
Subsequent Event [Member] | Regenx Bio SMA License | ReGenX
License agreements
Up-front payment, paid		$ 80,000
Subsequent Event [Member] | First Anniversary Of Amendment | Regenx Bio SMA License | ReGenX
License agreements
Potential milestone payments to be paid		30,000
Subsequent Event [Member] | Second Anniversary Of Amendment | Regenx Bio SMA License | ReGenX
License agreements
Potential milestone payments to be paid		30,000
Subsequent Event [Member] | Achievement Of Sales Based Milestones | Regenx Bio SMA License | ReGenX | Maximum
License agreements
Potential milestone payments to be paid		$ 120,000
Over-Allotment Option
Public Offering
Shares issued (in shares)			536,250
Over-Allotment Option | Subsequent Event [Member]
Public Offering
Shares issued (in shares)	588,240