Avexis (AVXS) 10-Q 9 Nov 17 2017 Q3 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	9 Months Ended
	Sep. 30, 2017	Nov. 08, 2017
Document and Entity Information [Abstract]
Entity Registrant Name	AveXis, Inc.
Entity Central Index Key	1,652,923
Document Type	10-Q
Document Period End Date	Sep. 30, 2017
Amendment Flag	false
Current Fiscal Year End Date	--12-31
Entity Current Reporting Status	Yes
Entity Filer Category	Non-accelerated Filer
Entity Common Stock, Shares Outstanding		31,976,418
Document Fiscal Year Focus	2,017
Document Fiscal Period Focus	Q3

Condensed Consolidated Balance

Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Current assets:
Cash and cash equivalents	$ 374,211	$ 240,430
Prepaid expenses and other current assets	9,092	4,750
Total current assets	383,303	245,180
Property and equipment, net	46,332	24,201
Other long-term assets	2,422	1,194
Total assets	432,057	270,575
Current liabilities:
Accounts payable	9,399	3,196
Accrued expenses	15,276	16,794
Accrued indemnification obligation	2,784	4,453
Total current liabilities	27,459	24,443
Long-term liabilities	548
Total liabilities	28,007	24,443
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	681,995	387,691
Accumulated deficit	(277,948)	(141,562)
Total stockholders’ equity	404,050	246,132
Total liabilities, redeemable common stock and stockholders’ equity	$ 432,057	$ 270,575

Condensed Consolidated Balance3

Condensed Consolidated Balance Sheets (Parenthetical) - $ / shares	Sep. 30, 2017	Dec. 31, 2016
Common stock
Common stock, par value (in dollars per share)	$ 0.0001	$ 0.0001
Common stock, shares authorized (in shares)	100,000,000	100,000,000
Common stock, shares issued (in shares)	31,972,377	27,700,054
Common stock, shares outstanding (in shares)	31,972,377	27,700,054

Condensed Consolidated Statemen

Condensed Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Operating expenses:
General and administrative	$ 16,095	$ 7,083	$ 38,898	$ 17,325
Research and development	33,425	14,098	98,946	40,542
Total operating expenses	49,520	21,181	137,844	57,867
Loss from operations	(49,520)	(21,181)	(137,844)	(57,867)
Interest income (expense)	881	99	1,458	230
Net loss	(48,639)	(21,082)	(136,386)	(57,637)
Comprehensive loss	$ (48,639)	$ (21,082)	$ (136,386)	$ (57,637)
Basic and diluted net loss per common share (in dollars per share)	$ (1.52)	$ (0.87)	$ (4.67)	$ (2.75)
Weighted-average basic and diluted common shares outstanding (in shares)	31,941,185	24,166,113	29,228,847	20,958,421

Condensed Consolidated Stateme5

Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016
Cash flows from operating activities
Net loss	$ (136,386)	$ (57,637)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	1,315	33
Stock-based compensation	23,595	22,713
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(4,355)	(2,908)
Other long-term assets	(1,228)	(848)
Accounts payable	6,624	1,051
Accrued expenses	(1,461)	4,262
Accrued indemnification obligation	(1,669)	136
Net cash used in operating activities, Total	(113,565)	(33,198)
Cash flows from investing activities
Capital expenditures	(23,364)	(11,702)
Net cash used in investing activities, Total	(23,364)	(11,702)
Net Cash Provided by (Used in) Financing Activities [Abstract]
Proceeds from issuance of common stock, net of issuance costs	269,869	247,228
Payments of deferred offering costs		(1,198)
Proceeds from exercise of stock options	1,258	218
Service-based restricted stock unit vesting	(417)
Proceeds from exercise of stock warrants		42
Net cash provided by financing activities, Total	270,710	246,290
Net (decrease) increase in cash and cash equivalents	133,781	201,390
Cash and cash equivalents, beginning of period	240,430	62,252
Cash and cash equivalents, end of period	374,211	263,642
Supplemental cash flow information:
Capital expenditures in accounts payable and accrued liabilities	$ 4,954	4,927
Deferred issuance costs for underwritten offering in accounts payable and accrued liabilities		$ 727

Background

Background	9 Months Ended
	Sep. 30, 2017
Background
Background	1. 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 gene therapy treatments for patients suffering from rare and life-threatening neurological genetic diseases. The Company's initial product candidate, AVXS-101, is a gene therapy product candidate currently in a pivotal clinical trial for the treatment of spinal muscular atrophy (“SMA”), Type 1, the leading genetic cause of infant mortality. 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 approximately $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, less underwriting discounts and commissions, resulting in additional net proceeds of approximately $9.8 million. 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 approximately $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 approximately $269.9 million.

Summary of Significant Accounti

Summary of Significant Accounting Policies	9 Months Ended
	Sep. 30, 2017
Summary of Significant Accounting Policies
Summary of Significant Accounting Policies	2. Basis of Presentation The condensed consolidated financial statements have been prepared in accordance with accounting principles generally accepted in the United States of America (“GAAP”) for interim financial reporting and the rules and regulations of the U.S. Securities and Exchange Commission (“SEC”) and, therefore, do not include all information and footnote disclosures normally included in the annual consolidated financial statements. In the opinion of management, the unaudited condensed consolidated financial statements reflect all adjustments of a normal recurring nature that are necessary for a fair statement of the results for the interim periods presented. Interim results are not necessarily indicative of results for a full year. The December 31, 2016 condensed consolidated balance sheet data contained within this Form 10-Q was derived from audited consolidated financial statements included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2016 (“Annual Report on Form 10-K”), but does not include all disclosures required by GAAP. Significant Accounting Policies There have been no material changes in the Company’s significant accounting policies as of and for the nine months ended September 30, 2017, as compared with the significant accounting policies described in the Company’s Annual Report on Form 10-K. Use of Estimates The preparation of the condensed consolidated financial statements in conformity with U.S. 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 reported amounts of revenues, if any, and expenses during the reporting period. Actual results could differ from those estimates. Recently Issued Accounting Pronouncements In February 2016, the Financial Accounting Standards Board (“FASB”) issued Accounting Standards Update (“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. Existing sale-leaseback guidance, including guidance for real estate, is replaced with a new model applicable to both lessees and lessors. ASC 842 is effective for fiscal years beginning after December 15, 2018. The Company is evaluating the adoption of ASC 842, but has not determined the effects it may have on the Company’s consolidated financial statements. In August 2016, the FASB issued ASU 2016-15, Statement of Cash Flows (Topic 230). ASU 2016-15 is intended to reduce the diversity in practice regarding how certain transactions are classified within the statement of cash flows. ASU 2016-15 is effective for public business entities for annual periods beginning after December 15, 2017, including interim periods within those fiscal years. Early adoption is permitted with retrospective application. The Company is evaluating the adoption of ASU 2016-15, but has not determined the effects it may have on the Company’s consolidated financial statements. Recently Adopted Accounting Pronouncements In March 2016, the FASB issued ASU 2016-09, Compensation – Stock Compensation (Topic 718). The new standard simplifies several aspects of the accounting for employee share-based payment transactions, including the accounting for income taxes, forfeitures, and statutory tax withholding requirements, as well as classification in the statement of cash flows. Under this guidance, a company recognizes all excess tax benefits and tax deficiencies as income tax expense or benefit in the statement of operations. This change eliminates the notion of the additional paid-in capital pool and reduces the complexity in accounting for excess tax benefits and tax deficiencies. The new standard is effective for public companies for annual reporting periods beginning after December 15, 2016, including interim periods within those annual reporting periods; however, early adoption is allowed. The Company adopted the new standard on January 1, 2017. The adoption of this standard did not have a material impact on the Company’s consolidated financial statements.

Property and Equipment, Net

Property and Equipment, Net	9 Months Ended
	Sep. 30, 2017
Property and Equipment, Net
Property and Equipment, Net	3. Property and equipment, net, consists of the following (in thousands): September 30, December 31, 2017 2016 Furniture $ 713 $ 108 Equipment 30,480 230 Leasehold improvements 4,458 777 Construction in progress 12,057 23,162 Property and equipment, gross 47,708 24,277 Less: accumulated depreciation (1,376) (76) Property and equipment, net $ 46,332 $ 24,201 Depreciation expense was $586 and $6 for the three months ended September 30, 2017 and 2016, respectively, and $1,315 and $33 for the nine months ended September 30, 2017 and 2016, respectively. 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.

Accrued Expenses and Other Curr

Accrued Expenses and Other Current Liabilities	9 Months Ended
	Sep. 30, 2017
Accrued Expenses
Accrued Expenses	4. Accrued expenses and other current liabilities consist of the following (in thousands): September 30, December 31, 2017 2016 Accrued manufacturing development costs $ 3,865 $ 7,167 Accrued payroll, bonus and deferred compensation 5,199 3,841 Accrued construction in progress 2,826 2,979 Accrued clinical trial costs 617 389 Accrued professional and consulting fees 1,143 1,588 Other 1,626 830 Accrued expenses and other current liabilities $ 15,276 $ 16,794

Accrued Indemnification Obligat

Accrued Indemnification Obligation	9 Months Ended
	Sep. 30, 2017
Accrued Indemnification Obligation
Accrued Indemnification Obligation	5. 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, see Note 6), a related party, 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 was required to reimburse Dr. Kaspar for the taxes he paid 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, 2016 and September 30, 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. On May 3, 2017, the Company paid Dr. Kaspar $1.7 million, which represents the tax liabilities owed to the U.S., State of Ohio and municipality of New Albany, Ohio pursuant to Dr. Kaspar’s 2014 amended tax returns. As a result, the Company has accrued balances of $2.8 million and $4.5 million at September 30, 2017 and December 31, 2016, respectively, representing the Company’s best estimate of the ultimate tax indemnification and gross-up payment to be made to Dr. Kaspar. The overall decrease in the accrued indemnification obligation was due to the May 3, 2017 payment to Dr. Kaspar, offset slightly by the accrual of additional interest through September 30, 2017. The Company expects to pay this entire amount in 2017.

Stock-Based Compensation

Stock-Based Compensation	9 Months Ended
	Sep. 30, 2017
Stock-Based Compensation
Stock-Based Compensation	6. 2014 Stock Plan (the “2014 Plan”) and 2016 Equity Incentive Plan (the “2016 Plan”) The following table summarizes stock option activity under the 2014 Plan and the 2016 Plan (collectively, the “Plans”) for the nine months ended September 30, 2017 (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 959 $ 75.37 Exercised (152) $ 10.42 Cancelled or forfeited (74) $ 20.27 Outstanding at September 30, 2017 3,310 $ 38.04 8.53 $ 194,310 Exercisable at September 30, 2017 1,116 $ 20.03 7.96 $ 85,622 Exercisable and expected to vest at September 30, 2017 3,310 $ 38.04 8.53 $ 194,310 (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 September 30, 2017 and December 31, 2016. For the nine months ended September 30, 2017 and 2016, the total number of stock options exercised was 152 and 34, respectively, resulting in total proceeds of $1,258 and $218, respectively. As of September 30, 2017 and December 31, 2016, there was $49.0 million and $20.6 million, respectively, of unrecognized stock-based compensation expense related to stock option awards that is expected to be recognized over a weighted-average period of 1.4 and 1.4 years, respectively. The Company has recorded total 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 follows (in thousands): Three Months Ended Nine Months Ended September 30, September 30, 2017 2016 2017 2016 Research and development $ 4,894 $ 2,007 $ 10,418 $ 14,758 General and administrative 5,274 2,616 12,490 7,253 $ 10,168 $ 4,623 $ 22,908 $ 22,011 Stock Options Granted The weighted-average grant date fair value of options granted during the three months ended September 30, 2017 and 2016 was $60.34 and $27.87, respectively, and for the nine months ended September 30, 2017 and 2016 was $54.56 and $23.98, respectively, on the date of grant using the Black-Scholes option-pricing model with the following weighted-average assumptions: Three Months Ended Nine Months Ended September 30, September 30, September 30, September 30, 2017 2016 2017 2016 Expected volatility 100.13 % 97.82 % 88.81 % 92.46 % Risk-free interest rate 1.81 % 1.10 % 1.90 % 1.42 % Expected term (in years) 6.08 6.08 6.08 6.08 Expected dividend yield 0.00 % 0.00 % 0.00 % % Options generally expire ten years following the date of grant. Options typically vest over a period of three to 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. Service-Based Restricted Stock Units As of September 30, 2017, and December 31, 2016, there were 37,934 and 57,500, respectively, outstanding service-based restricted stock units (“RSUs”) granted to employees. During the nine months ended September 30, 2017, 18,966 RSUs vested, which included 5,961 RSUs exchanged for tax-related purposes resulting in the remaining 13,005 RSUs converting to common stock. The Company recognized RSU-related stock-based compensation expense of $687 during the nine months ended September 30, 2017 , of which $613 is recorded within research and development expense and $74 within general and administrative expense. There were 57,500 RSUs issued and outstanding as of September 30, 2016. At September 30, 2017 and 2016, there was $541 and $0, respectively, of unrecognized compensation cost related to unvested RSUs that will be recognized as expense over a weighted-average period of 0.9 years. A summary of the status of the Company's RSUs at September 30, 2017 and of changes in RSUs outstanding under the 2016 Plan for the nine months ended September 30, 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 — — Vested (19) $ 34.90 Forfeited and cancelled (1) $ 34.90 Outstanding at September 30, 2017 38 $ 34.90 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. Performance-Based Restricted Stock Units On March 20, 2017, the Company granted to certain employees a total of 49,332 performance-based restricted stock units (“PSUs)”. 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 September 30, 2017, all 49,332 of these 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 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 during the nine months ended September 30, 2016. 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-based compensation expense during the year ended December 31, 2016. Warrants Granted to Non-Employees During the nine month period ended September 30, 2017, there were no warrants exercised and as a result no proceeds received by the Company. As of September 30, 2017, 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	9 Months Ended
	Sep. 30, 2017
Net Loss Per Common Share
Net Loss Per Common Share	7. 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. For the three and nine month periods ended September 30, 2017 and 2016, 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): Three Months Ended Nine Months Ended September 30, September 30, 2017 2016 2017 2016 Stock options 3,310 2,567 3,310 2,567 Stock warrants 306 310 306 310 Unvested service-based restricted stock units 38 58 38 58 Unvested performance-based restricted stock units 49 — 49 — 3,703 3,703 2,935 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): Three Months Ended Nine Months Ended September 30, September 30, 2017 2016 2017 2016 Numerator: Net loss and comprehensive loss $ (48,639) $ (21,082) $ (136,386) $ (57,637) Denominator: Weighted-average basic and diluted common shares 31,941 24,166 29,229 20,958 Basic and diluted net loss per common share $ (1.52) $ (0.87) $ (4.67) $ (2.75)

Commitments and Contingencies

Commitments and Contingencies	9 Months Ended
	Sep. 30, 2017
Commitments and Contingencies
Commitments and Contingencies	8. Operating Leases In March 2014, the Company entered into a lease agreement, which expired in April 2017, for approximately 2,418 square feet of office space in Dallas, Texas. The Company leases a 15,668 square foot facility for its corporate headquarters in Bannockburn, Illinois, pursuant to a lease that expires in July 2024. The lease agreement provides for annual escalation in rent payments during the lease term. The Company is amortizing the escalation in rental payments on a straight-line basis over the term of the lease. The Company also leases a 1,318 square foot facility in Columbus, Ohio for research and development activities, pursuant to a lease that expires in March 2019. In March 2016, the Company entered into a lease agreement, which expires in August 2026, for approximately 48,529 square feet of warehouse and office space in Libertyville, Illinois. A portion of the warehouse space is used as manufacturing space. The lease agreement provides for annual escalation in rent payments during the lease term. The Company is amortizing the escalation in rental payments on a straight‑line basis over the term of the lease. In May 2017, the Company entered into two month-to-month lease agreements to add an additional 4,582 square feet of office space in Libertyville, Illinois. This has subsequently been reduced to one month-to-month lease agreement. In September 2017, the Company entered into a lease agreement that commences in January 2018 and expires in August 2026 for approximately 12,539 square feet of warehouse and office space in Libertyville, Illinois adjacent to the Company’s manufacturing facility. In July 2017, the Company entered into a lease agreement which expires in June 2027, for approximately 16,808 square feet in San Diego, California for research and development activities. The lease agreement provides for annual escalation in rent payments during the lease term. The Company is amortizing the escalation in rental payments on a straight‑line basis over the term of the lease. Guarantees and Indemnifications The Company has accrued $2.8 million and $4.5 million at September 30, 2017 and December 31, 2016, respectively, representing the Company's best estimate of the ultimate tax indemnification and gross-up payment to be made to Dr. Kaspar pursuant to a tax indemnification granted to Dr. Kaspar in connection with a restricted common stock grant (see Note 5). Additionally, in the normal course of business, the Company has entered into agreements that contain a variety of representations and provide for general indemnification, including indemnification agreements with the Company’s officers and directors. 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 September 30, 2017 and December 31, 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 $500.0 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 two of the Company’s present executives, who were dropped from the second amended complaint. The Company believes that the second amended complaint is without merit and intends to vigorously defend itself and its present and former executives from the allegations. The Company views the possibility 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.

Taxes

Taxes	9 Months Ended
	Sep. 30, 2017
Taxes
Taxes	9. Deferred tax assets and liabilities are determined based on the difference between the financial statement and tax bases of assets and liabilities using enacted tax rates in effect for the year in which the differences are expected to reverse. Valuation allowances are provided if based upon the weight of available evidence, it is more likely than not that some or all of the deferred tax assets will not be realized. The Company has evaluated the positive and negative evidence bearing upon the realizability of its deferred tax assets, including its net operating losses. Based on its history of operating losses, the Company believes that it is more likely than not that the benefit of its deferred tax assets will not be realized. Accordingly, the Company has provided a full valuation allowance for deferred tax assets as of September 30, 2017 and December 31, 2016.

Collaboration and License Agree

Collaboration and License Agreements	9 Months Ended
	Sep. 30, 2017
Collaboration and License Agreements
Collaboration and License Agreements	10. The Company has entered into three license agreements related to its planned preclinical programs in Rett syndrome and amyotrophic lateral sclerosis (“ALS”) caused by mutations in the gene that produces the copper zinc superoxide dismutase 1 (“SOD1”) enzyme (“genetic ALS”). REGENXBIO Inc. Effective June 7, 2017, the Company entered into a License Agreement (the “REGENX Rett and ALS License”) with REGENXBIO Inc. (“REGENX”). Under the terms of the REGENX Rett and ALS License, REGENX granted the Company an exclusive worldwide license to utilize REGENX's proprietary adeno-associated virus ("AAV") gene delivery platform for the treatment of Rett syndrome and genetic ALS caused by mutations in the gene that produces the copper zinc superoxide dismutase 1 (“SOD1”) enzyme by in vivo gene therapy, using REGENX’s AAV9 gene delivery vector. Under the REGENX Rett and ALS License, REGENX 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 REGENX Rett and ALS License (the “REGENX 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 REGENX and its licensors. As consideration for the REGENX Rett and ALS License, in June 2017 the Company paid an initial fee of $6.0 million. This $6.0 million initial fee paid to REGENX was recognized as research and development expense during the three months ended September 30, 2017. Additionally, the Company agreed to pay potential future milestones of up to $36.0 million in the aggregate for the REGENX Rett and ALS licensed products, and a low double digit royalty on net sales, if any, of the REGENX Rett and ALS licensed products, 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 Company also agreed to pay an annual maintenance fee on each anniversary of the effective date of the REGENX Rett and ALS License. The REGENX 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 REGENX Rett and ALS licensed product. Upon expiration of the REGENX Rett and ALS License, the license granted to the Company becomes irrevocable, perpetual, royalty-free and fully paid-up. The Company has the right to terminate the REGENX Rett and ALS License upon a specified period of prior written notice. REGENX may terminate the REGENX Rett and ALS License if the Company or its affiliates become insolvent, if the Company is greater than a specified number of days late in paying money due under the REGENX Rett and ALS License, or, effective immediately, if the Company or its affiliates, or sublicensees commence any action against REGENX or its licensors to declare or render any claim of the licensed patent rights invalid or unenforceable. Either party may terminate the REGENX Rett and ALS License for material breach if such breach is not cured within a specified number of days. Nationwide Children’s Hospital In September 2016, the Company entered into two 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 (“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”). The Company paid an initial fee of $0.2 million to NCH in connection with the NCH License for Rett syndrome and $0.1 million in connection with the NCH License for ALS in human use. Each initial fee was recognized as research and development expense during the three months ended September 30, 2016. Additionally, the Company agreed to pay potential future development milestone-based payments and an annual maintenance fee under each NCH License. Following the first commercial sale of an NCH licensed product for Rett syndrome or ALS, as applicable, under an applicable NCH License, the Company must begin paying NCH an aggregate low-single digit royalty on net sales of NCH licensed products by the Company, its affiliates and sublicensees during the term of such license.

Summary of Significant Accoun16

Summary of Significant Accounting Policies (Policies)	9 Months Ended
	Sep. 30, 2017
Summary of Significant Accounting Policies
Basis of Presentation	Basis of Presentation The condensed consolidated financial statements have been prepared in accordance with accounting principles generally accepted in the United States of America (“GAAP”) for interim financial reporting and the rules and regulations of the U.S. Securities and Exchange Commission (“SEC”) and, therefore, do not include all information and footnote disclosures normally included in the annual consolidated financial statements. In the opinion of management, the unaudited condensed consolidated financial statements reflect all adjustments of a normal recurring nature that are necessary for a fair statement of the results for the interim periods presented. Interim results are not necessarily indicative of results for a full year. The December 31, 2016 condensed consolidated balance sheet data contained within this Form 10-Q was derived from audited consolidated financial statements included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2016 (“Annual Report on Form 10-K”), but does not include all disclosures required by GAAP.
Use of Estimates	Use of Estimates The preparation of the condensed consolidated financial statements in conformity with U.S. 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 reported amounts of revenues, if any, and expenses during the reporting period. Actual results could differ from those estimates.
Recent Accounting Pronouncements	Recently Issued Accounting Pronouncements In February 2016, the Financial Accounting Standards Board (“FASB”) issued Accounting Standards Update (“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. Existing sale-leaseback guidance, including guidance for real estate, is replaced with a new model applicable to both lessees and lessors. ASC 842 is effective for fiscal years beginning after December 15, 2018. The Company is evaluating the adoption of ASC 842, but has not determined the effects it may have on the Company’s consolidated financial statements. In August 2016, the FASB issued ASU 2016-15, Statement of Cash Flows (Topic 230). ASU 2016-15 is intended to reduce the diversity in practice regarding how certain transactions are classified within the statement of cash flows. ASU 2016-15 is effective for public business entities for annual periods beginning after December 15, 2017, including interim periods within those fiscal years. Early adoption is permitted with retrospective application. The Company is evaluating the adoption of ASU 2016-15, but has not determined the effects it may have on the Company’s consolidated financial statements. Recently Adopted Accounting Pronouncements In March 2016, the FASB issued ASU 2016-09, Compensation – Stock Compensation (Topic 718). The new standard simplifies several aspects of the accounting for employee share-based payment transactions, including the accounting for income taxes, forfeitures, and statutory tax withholding requirements, as well as classification in the statement of cash flows. Under this guidance, a company recognizes all excess tax benefits and tax deficiencies as income tax expense or benefit in the statement of operations. This change eliminates the notion of the additional paid-in capital pool and reduces the complexity in accounting for excess tax benefits and tax deficiencies. The new standard is effective for public companies for annual reporting periods beginning after December 15, 2016, including interim periods within those annual reporting periods; however, early adoption is allowed. The Company adopted the new standard on January 1, 2017. The adoption of this standard did not have a material impact on the Company’s consolidated financial statements.

Property and Equipment, Net (Ta

Property and Equipment, Net (Tables)	9 Months Ended
	Sep. 30, 2017
Property and Equipment, Net
Summary of property and equipment, net	September 30, December 31, 2017 2016 Furniture $ 713 $ 108 Equipment 30,480 230 Leasehold improvements 4,458 777 Construction in progress 12,057 23,162 Property and equipment, gross 47,708 24,277 Less: accumulated depreciation (1,376) (76) Property and equipment, net $ 46,332 $ 24,201

Accrued Expenses and Other Cu18

Accrued Expenses and Other Current Liabilities (Tables)	9 Months Ended
	Sep. 30, 2017
Accrued Expenses
Schedule of accrued expenses	September 30, December 31, 2017 2016 Accrued manufacturing development costs $ 3,865 $ 7,167 Accrued payroll, bonus and deferred compensation 5,199 3,841 Accrued construction in progress 2,826 2,979 Accrued clinical trial costs 617 389 Accrued professional and consulting fees 1,143 1,588 Other 1,626 830 Accrued expenses and other current liabilities $ 15,276 $ 16,794

Stock-Based Compensation (Table

Stock-Based Compensation (Tables)	9 Months Ended
	Sep. 30, 2017
Stock-Based Compensation
Summary of stock option activity	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 959 $ 75.37 Exercised (152) $ 10.42 Cancelled or forfeited (74) $ 20.27 Outstanding at September 30, 2017 3,310 $ 38.04 8.53 $ 194,310 Exercisable at September 30, 2017 1,116 $ 20.03 7.96 $ 85,622 Exercisable and expected to vest at September 30, 2017 3,310 $ 38.04 8.53 $ 194,310 (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 September 30, 2017 and December 31, 2016.
Summary of stock-based compensation expense by classification in the Statement of Operations and Comprehensive Loss	Three Months Ended Nine Months Ended September 30, September 30, 2017 2016 2017 2016 Research and development $ 4,894 $ 2,007 $ 10,418 $ 14,758 General and administrative 5,274 2,616 12,490 7,253 $ 10,168 $ 4,623 $ 22,908 $ 22,011
Summary of weighted-average assumptions used in determining fair value of stock options	Three Months Ended Nine Months Ended September 30, September 30, September 30, September 30, 2017 2016 2017 2016 Expected volatility 100.13 % 97.82 % 88.81 % 92.46 % Risk-free interest rate 1.81 % 1.10 % 1.90 % 1.42 % Expected term (in years) 6.08 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 — — Vested (19) $ 34.90 Forfeited and cancelled (1) $ 34.90 Outstanding at September 30, 2017 38 $ 34.90

Net Loss Per Common Share (Tabl

Net Loss Per Common Share (Tables)	9 Months Ended
	Sep. 30, 2017
Net Loss Per Common Share
Schedule of potentially dilutive securities excluded from computations of diluted weighted-average shares outstanding	Three Months Ended Nine Months Ended September 30, September 30, 2017 2016 2017 2016 Stock options 3,310 2,567 3,310 2,567 Stock warrants 306 310 306 310 Unvested service-based restricted stock units 38 58 38 58 Unvested performance-based restricted stock units 49 — 49 — 3,703 3,703 2,935
Summary of the basic and diluted net loss per common share	Three Months Ended Nine Months Ended September 30, September 30, 2017 2016 2017 2016 Numerator: Net loss and comprehensive loss $ (48,639) $ (21,082) $ (136,386) $ (57,637) Denominator: Weighted-average basic and diluted common shares 31,941 24,166 29,229 20,958 Basic and diluted net loss per common share $ (1.52) $ (0.87) $ (4.67) $ (2.75)

Background (Details)

Background (Details) - USD ($) $ / shares in Units, $ in Thousands	Jun. 26, 2017	Sep. 13, 2016	Mar. 03, 2016	Feb. 10, 2016	Sep. 30, 2017	Sep. 30, 2016
Public Offering
Net proceeds from issuance of common stock					$ 269,869	$ 247,228
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
Public Offering
Shares converted (in shares)				2,365,020
Class D Preferred Stock
Public Offering
Shares converted (in shares)				3,105,000
Common Stock
Public Offering
Number of common shares to be sold under public offering	4,111,250	4,887,500
Shares issued (in shares)	536,250	4,597,645
Share price (in dollars per share)	$ 70	$ 34.50
Net proceeds from issuance of common stock	$ 269,900	$ 149,100
Common Stock | IPO
Public Offering
Shares issued (in shares)				4,750,000
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

Property and Equipment, Net (De

Property and Equipment, Net (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Property and Equipment, Net
Property and equipment, gross	$ 47,708,000		$ 47,708,000		$ 24,277,000
Less: accumulated depreciation	(1,376,000)		(1,376,000)		(76,000)
Property and equipment, net	46,332,000		46,332,000		24,201,000
Depreciation expense
Depreciation expense	586	$ 6	1,315	$ 33
Furniture
Property and Equipment, Net
Property and equipment, gross	713,000		713,000		108,000
Equipment
Property and Equipment, Net
Property and equipment, gross	30,480,000		30,480,000		230,000
Leasehold improvement
Property and Equipment, Net
Property and equipment, gross	4,458,000		4,458,000		777,000
Construction in progress
Property and Equipment, Net
Property and equipment, gross	$ 12,057,000		$ 12,057,000		$ 23,162,000

Accrued Expenses and Other Cu23

Accrued Expenses and Other Current Liabilities (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Accrued Expenses and Other Current Liabilities
Accrued manufacturing development costs	$ 3,865	$ 7,167
Accrued payroll, bonus and deferred compensation	5,199	3,841
Accrued construction in progress	2,826	2,979
Accrued clinical trial costs	617	389
Accrued professional fees	1,143	1,588
Other	1,626	830
Accrued expenses and other current liabilities	$ 15,276	$ 16,794

Accrued Indemnification Oblig24

Accrued Indemnification Obligation (Details) - USD ($) $ / shares in Units, $ in Thousands	May 03, 2017	Jan. 28, 2014	Jan. 31, 2014	Sep. 30, 2017	Dec. 31, 2016	Dec. 31, 2014
Accrued Indemnification Obligation
Accrued indemnification obligation				$ 2,784	$ 4,453
Restricted Stock | Member of Board of Directors
Accrued Indemnification Obligation
Shares granted (in shares)			2,334,391
Grant date fair value (in dollars per share)		$ 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
Accrued indemnification obligation				$ 2,800	$ 4,500

Stock-Based Compensation - Stoc

Stock-Based Compensation - Stock Option Activity (Details) - USD ($) $ / shares in Units, $ in Thousands	9 Months Ended		12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Number of Shares
Outstanding at the beginning of the period (in shares)	2,577,000
Granted (in shares)	959,000
Exercised (in shares)	(152,000)
Cancelled or forfeited (in shares)	(74,000)
Outstanding at the end of the period (in shares)	3,310,000		2,577,000
Weighted Average Exercise Price
Outstanding at the beginning of the period (in dollars per share)	$ 22.01
Granted (in dollars per share)	75.37
Exercised (in dollars per share)	10.42
Cancelled or forfeited (in dollars per share)	20.27
Outstanding at the end of the period (in dollars per share)	$ 38.04		$ 22.01
Weighted Average
Remaining Contractual Life (Years)	8 years 6 months 11 days		8 years 9 months 15 days
Aggregate Intrinsic Value (in dollars)	$ 194,310		$ 66,466
Exercisable and Vested and Expected to Vest
Number of Shares - exercisable (in shares)	1,116,000
Weighted Average Exercise Price - exercisable (in dollars per share)	$ 20.03
Remaining Contractual Life (Years) - exercisable	7 years 11 months 16 days
Aggregate Intrinsic Value - exercisable (in dollars)	$ 85,622
Number of Shares - vested and expected to vest (in shares)	3,310,000
Weighted Average Exercise Price - vested and expected to vest (in dollars per share)	$ 38.04
Remaining Contractual Life (Years) - vested and expected to vest	8 years 6 months 11 days
Aggregate Intrinsic Value - vested and expected to vest (in dollars)	$ 194,310
Employee Stock Options
Number of Shares
Exercised (in shares)	(152)	(34)

Stock-Based Compensation - St26

Stock-Based Compensation - Stock Options Exercised (Details) - USD ($)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Stock-Based Compensation
Exercised (in shares)			152,000
Proceeds from exercise of stock options			$ 1,258,000	$ 218,000
Employee Stock Options
Stock-Based Compensation
Exercised (in shares)			152	34
Proceeds from exercise of stock options			$ 1,258	$ 218
Unrecognized stock-based compensation expense
Unrecognized stock-based compensation expense	$ 49,000,000		$ 49,000,000		$ 20,600,000
Weighted-average period expected to recognize stock-based compensation expense			1 year 4 months 24 days		1 year 4 months 24 days
Aggregate stock-based compensation expense by category
Aggregate stock-based compensation expense	10,168,000	$ 4,623,000	$ 22,908,000	22,011,000
Research and development expenses | Employee Stock Options
Aggregate stock-based compensation expense by category
Aggregate stock-based compensation expense	4,894,000	2,007,000	10,418,000	14,758,000
General and administrative expenses | Employee Stock Options
Aggregate stock-based compensation expense by category
Aggregate stock-based compensation expense	$ 5,274,000	$ 2,616,000	$ 12,490,000	$ 7,253,000

Stock-Based Compensation - Weig

Stock-Based Compensation - Weighted-average Assumptions (Details) - Employee Stock Options - $ / shares	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Weighted-average grant date fair value
Weighted-average grant date fair value of options granted during the period (in dollars per share)	$ 60.34	$ 27.87	$ 54.56	$ 23.98
Weighted-average fair value assumptions
Expected volatility (as a percent)	100.13%	97.82%	88.81%	92.46%
Risk-free interest rate (as a percent)	1.81%	1.10%	1.90%	1.42%
Expected term (in years)	6 years 29 days	6 years 29 days	6 years 29 days	6 years 29 days
Expected dividend yield (as a percent)	0.00%	0.00%	0.00%	0.00%
Vesting and expiration of stock options
Expiration period (in years)			10 years
Period during which vested options may be exercised after termination of service			3 months
Minimum
Vesting and expiration of stock options
Vesting period (in years)			3 years
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) - Service Based RSUs - USD ($)	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Stock Options Granted to Non-Employees
Service-based restricted stock units granted to employees (in shares)	37,934	57,500	57,500
Shares vested (in shares)	18,966
Shares exchanged for tax-related purposes	5,961
Vested RSUs converting to common shares after exchange of shares for taxes	13,005
Stock-based compensation expense	$ 687
Granted (in shares)		57,500
Shares outstanding (in shares)	37,934	57,500	57,500
Unrecognized compensation cost	$ 541	$ 0
Weighted-average period expected to recognize stock-based compensation expense	10 months 24 days
Vesting period (in years)	3 years
Research and development expenses
Stock Options Granted to Non-Employees
Stock-based compensation expense	$ 613
General and administrative expenses
Stock Options Granted to Non-Employees
Stock-based compensation expense	$ 74

Stock-Based Compensation - Re29

Stock-Based Compensation - Restricted Stock Units - Activity (Details) - Service Based RSUs - $ / shares	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016
Number of Shares
Outstanding at the beginning of the period (in shares)	57,500
Granted (in shares)		57,500
Vested (in shares)	18,966
Forfeited and cancelled (in shares)	(1,000)
Outstanding at the end of the period (in shares)	37,934	57,500
Weighted Average Grant Date Fair Value Per Share
Outstanding at the beginning of the period (in dollars per share)	$ 34.90
Vested (in dollars per share)	34.90
Forfeited and cancelled (in dollars per share)	34.90
Outstanding at the end of the period (in dollars per share)	$ 34.90

Stock-Based Compensation - Perf

Stock-Based Compensation - Performance-Based Restricted Stock Units (Details) - Performance Based RSU - USD ($) $ / shares in Units, $ in Millions	Mar. 20, 2017	Sep. 30, 2017	Sep. 30, 2016
Stock Options Granted to Non-Employees
Shares granted (in shares)	49,332		0
Shares outstanding (in shares)		49,332	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 - Re31

Stock-Based Compensation - Restricted Stock Granted to Non-Employees (Details) - Restricted Stock - USD ($) $ in Millions	1 Months Ended		12 Months Ended
	Jan. 31, 2016	Jan. 31, 2014	Dec. 31, 2016
Stock Options Granted to Non-Employees
Aggregate stock-based compensation expense			$ 10.4
Member of Board of Directors
Stock Options Granted to Non-Employees
Shares granted (in shares)		2,334,391
Shares vested (in shares)	1,750,794
Member of Board of Directors | Shares vesting upon grant
Stock Options Granted to Non-Employees
Shares vested (in shares)		583,597
Vesting percentage (as a percent)		25.00%
Member of Board of Directors | Shares vesting on second anniversary
Stock Options Granted to Non-Employees
Vesting percentage (as a percent)		25.00%
Member of Board of Directors | Shares vesting on third anniversary
Stock Options Granted to Non-Employees
Vesting percentage (as a percent)		25.00%
Member of Board of Directors | Shares vesting on fourth anniversary
Stock Options Granted to Non-Employees
Vesting percentage (as a percent)		25.00%

Stock-Based Compensation - Warr

Stock-Based Compensation - Warrants Granted to Non-Employees - General Information (Details) - USD ($)	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016
Warrant information
Proceeds from exercise of stock warrants		$ 42,000
Stock warrants
Warrant information
Stock warrants exercised (in shares)	0
Proceeds from exercise of stock warrants	$ 0
Stock warrants | Non-Employee
Warrant information
Stock warrants vested ( in shares)	305,775
Stock warrants outstanding (in shares)	305,775
Warrants, exercise price (in dollars per share)	$ 2.57

Net Loss Per Common Share - Dil

Net Loss Per Common Share - Dilutive Securities (Details) - shares shares in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Antidilutive securities not included:
Antidilutive securities	3,703	2,935	3,703	2,935
Employee Stock Options
Antidilutive securities not included:
Antidilutive securities	3,310	2,567	3,310	2,567
Stock warrants
Antidilutive securities not included:
Antidilutive securities	306	310	306	310
Service Based RSUs
Antidilutive securities not included:
Antidilutive securities	38	58	38	58
Performance Based RSU
Antidilutive securities not included:
Antidilutive securities	49		49

Net Loss Per Common Share - Cal

Net Loss Per Common Share - Calculation (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Numerator:
Net loss and comprehensive loss	$ (48,639)	$ (21,082)	$ (136,386)	$ (57,637)
Denominator:
Weighted-average basic and diluted common shares (in shares)	31,941,185	24,166,113	29,228,847	20,958,421
Basic and diluted net loss per common share (in dollars per share)	$ (1.52)	$ (0.87)	$ (4.67)	$ (2.75)

Commitments and Contingencies -

Commitments and Contingencies - Operating Lease - General Information (Details)	1 Months Ended	4 Months Ended
	May 31, 2017ft²agreement	Sep. 30, 2017ft²agreement	Jul. 31, 2017ft²	Mar. 31, 2016ft²	Mar. 31, 2014ft²
Office space in Dallas, Texas
Operating Leases
Approximate size of leased space (in square feet)					2,418
Facility for corporate headquarters Bannockburn, Illinois
Operating Leases
Approximate size of leased space (in square feet)		15,668
Facility in Columbus, Ohio
Operating Leases
Approximate size of leased space (in square feet)		1,318
Office and warehouse space in Libertyville, Illinois
Operating Leases
Approximate size of leased space (in square feet)				48,529
Additional square feet of leased space (in square feet)	4,582	12,539
Number of month-to-month lease agreements for additional space | agreement	2	1
Research And Development Space In San Diego California [Member]
Operating Leases
Approximate size of leased space (in square feet)			16,808

Commitments and Contingencies36

Commitments and Contingencies - Guarantees and Indemnifications (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Accrued Indemnification Obligation
Accrued indemnification obligation	$ 2,784	$ 4,453
Member of Board of Directors | Tax indemnification
Accrued Indemnification Obligation
Accrued indemnification obligation	$ 2,800	$ 4,500

Commitments and Contingencies37

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

Collaboration and License Agr38

Collaboration and License Agreements (Details) $ in Thousands	1 Months Ended		3 Months Ended		9 Months Ended
	Jun. 30, 2017USD ($)	Sep. 30, 2016USD ($)agreement	Sep. 30, 2017USD ($)agreement	Sep. 30, 2016USD ($)agreement	Sep. 30, 2017USD ($)agreement	Sep. 30, 2016USD ($)agreement
Collaboration and License Agreements
Number of license agreements | agreement			3		3
Research and development			$ 33,425	$ 14,098	$ 98,946	$ 40,542
License Agreement | Maximum
Collaboration and License Agreements
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
License Agreement | ReGenX | Maximum
Collaboration and License Agreements
Potential milestone payments to be paid	$ 36,000
License Agreement | NCH
Collaboration and License Agreements
Number of license agreements | agreement		2		2		2
Regenx Rett and ALS license
Collaboration and License Agreements
Research and development			$ 6,000
Regenx Rett and ALS license | ReGenX
Collaboration and License Agreements
Upfront fees paid	$ 6,000
NCH Rett license
Collaboration and License Agreements
Research and development				$ 200
NCH Rett license | NCH
Collaboration and License Agreements
Upfront fees paid		$ 200
NCH ALS license
Collaboration and License Agreements
Research and development				$ 100
NCH ALS license | NCH
Collaboration and License Agreements
Upfront fees paid		$ 100