bluebird bio (BLUE) 10-Q 3 May 17 2017 Q1 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	3 Months Ended
	Mar. 31, 2017	Apr. 28, 2017
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Mar. 31, 2017
Document Fiscal Year Focus	2,017
Document Fiscal Period Focus	Q1
Trading Symbol	BLUE
Entity Registrant Name	bluebird bio, Inc.
Entity Central Index Key	1,293,971
Current Fiscal Year End Date	--12-31
Entity Filer Category	Large Accelerated Filer
Entity Common Stock, Shares Outstanding		40,950,146

Condensed Consolidated Balance

Condensed Consolidated Balance Sheets (Unaudited) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Current assets:
Cash and cash equivalents	$ 216,001	$ 278,887
Marketable securities	418,067	425,491
Tenant improvements receivable	21,228	8,542
Prepaid expenses	8,730	8,209
Other current assets	3,285	3,085
Total current assets	667,311	724,214
Marketable securities	165,801	180,452
Property and equipment, net	181,720	156,955
Intangible assets, net	19,753	20,694
Goodwill	13,128	13,128
Restricted cash and other non-current assets	24,857	22,679
Total assets	1,072,570	1,118,122
Current liabilities:
Accounts payable	8,769	13,664
Accrued expenses and other current liabilities	44,307	54,660
Deferred revenue, current portion	19,560	6,209
Total current liabilities	72,636	74,533
Deferred rent, net of current portion	10,882	10,408
Deferred revenue, net of current portion	22,883	40,204
Contingent consideration, net of current portion	3,130	3,277
Financing lease obligation, net of current portion	147,555	120,140
Other non-current liabilities	105	120
Total liabilities	257,191	248,682
Commitments and contingencies (Note 7)
Stockholders’ equity:
Preferred stock, $0.01 par value, 5,000 shares authorized; 0 shares issued and outstanding at March 31, 2017 and December 31, 2016
Common stock, $0.01 par value, 125,000 shares authorized; 40,923 and 40,691 shares issued and outstanding at March 31, 2017 and December 31, 2016, respectively	409	407
Additional paid-in capital	1,463,098	1,447,856
Accumulated other comprehensive loss	(1,252)	(1,149)
Accumulated deficit	(646,876)	(577,674)
Total stockholders’ equity	815,379	869,440
Total liabilities and stockholders’ equity	$ 1,072,570	$ 1,118,122

Condensed Consolidated Balance3

Condensed Consolidated Balance Sheets (Unaudited) (Parenthetical) - $ / shares	Mar. 31, 2017	Dec. 31, 2016
Statement Of Financial Position [Abstract]
Preferred stock, par value	$ 0.01	$ 0.01
Preferred stock, shares authorized	5,000,000	5,000,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value	$ 0.01	$ 0.01
Common stock, shares authorized	125,000,000	125,000,000
Common stock, shares issued	40,923,000	40,691,000
Common stock, shares outstanding	40,923,000	40,691,000

Condensed Consolidated Statemen

Condensed Consolidated Statements of Operations and Comprehensive Loss (Unaudited) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Revenue:
Collaboration revenue	$ 6,832	$ 1,499
Total revenue	6,832	1,499
Operating expenses:
Research and development	55,028	41,911
General and administrative	20,284	15,955
Change in fair value of contingent consideration	1,433	1,013
Total operating expenses	76,745	58,879
Loss from operations	(69,913)	(57,380)
Other income, net	1,201	961
Loss before income taxes	(68,712)	(56,419)
Income tax benefit		145
Net loss	$ (68,712)	$ (56,274)
Net loss per share - basic and diluted:	$ (1.68)	$ (1.52)
Weighted-average number of common shares used in computing net loss per share - basic and diluted:	40,836	36,920
Other comprehensive (loss) income:
Unrealized (loss) gain on available-for-sale securities, net of tax expense of $0.0 million and $0.7 million for the three months ended March 31, 2017 and 2016, respectively	$ (101)	$ 1,319
Comprehensive loss	$ (68,813)	$ (54,955)

Condensed Consolidated Stateme5

Condensed Consolidated Statements of Operations and Comprehensive Loss (Unaudited) (Parenthetical) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Income Statement [Abstract]
Unrealized (loss) gain on available-for-sale securities tax expense	$ 0	$ 0.7

Condensed Consolidated Stateme6

Condensed Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Operating activities
Net loss	$ (68,712)	$ (56,274)
Adjustments to reconcile net loss to net cash used in operating activities:
Change in fair value of contingent consideration	1,433	1,013
Depreciation and amortization	2,844	2,283
Stock-based compensation expense	11,481	10,143
Other non-cash items	838	868
Changes in operating assets and liabilities:
Prepaid expenses and other assets	(3,105)	(2,271)
Accounts payable	(3,257)	(2,195)
Accrued expenses and other liabilities	(13,318)	211
Deferred revenue	(3,969)	9,222
Deferred rent	462	234
Net cash used in operating activities	(75,303)	(36,766)
Investing activities
Purchase of property and equipment	(26,024)	(3,894)
Purchases of marketable securities	(82,940)	(28,175)
Proceeds from maturities of marketable securities	104,510	96,515
Net cash (used in) provided by investing activities	(4,454)	64,446
Financing activities
Reimbursement of tenant improvements for financing lease obligation	13,831
Proceeds from issuance of common stock	3,040	639
Net cash provided by financing activities	16,871	639
(Decrease) increase in cash and cash equivalents	(62,886)	28,319
Cash and cash equivalents at beginning of period	278,887	164,269
Cash and cash equivalents at end of period	216,001	192,588
Non-cash investing and financing activities:
Assets acquired under financing lease obligation	1,269	12,513
Purchases of property and equipment included in accounts payable and accrued expenses	5,971	$ 1,086
Tenant improvements under financing lease included in tenant improvements receivable	$ 12,687

Description of the business

Description of the business	3 Months Ended
	Mar. 31, 2017
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Description of the business	1. Description of the business bluebird bio, Inc. (the “Company” or “bluebird”) was incorporated in Delaware on April 16, 1992, and is headquartered in Cambridge, Massachusetts. The Company researches, develops, manufactures and plans to commercialize gene therapies for the treatment of severe genetic diseases and cancer. Since its inception, the Company has devoted substantially all of its resources to its research and development efforts relating to its product candidates, including activities to manufacture product candidates, conduct clinical studies of its product candidates, perform preclinical research to identify new product candidates and provide general and administrative support for these operations.

Summary of significant accounti

Summary of significant accounting policies and basis of presentation	3 Months Ended
	Mar. 31, 2017
Accounting Policies [Abstract]
Summary of significant accounting policies and basis of presentation	2. Summary of significant accounting policies and basis of presentation Basis of presentation and principles of consolidation The accompanying condensed consolidated financial statements are unaudited and have been prepared by the Company in accordance with accounting principles generally accepted in the United States (“GAAP”) as found in the Accounting Standards Codification (“ASC”) and Accounting Standards Update (“ASU”) of the Financial Accounting Standards Board (“FASB”). Certain information and footnote disclosures normally included in the Company’s annual financial statements have been condensed or omitted. These interim condensed consolidated financial statements, in the opinion of management, reflect all normal recurring adjustments necessary for a fair presentation of the Company’s financial position and results of operations for the interim periods ended March 31, 2017 and 2016. The results of operations for the interim periods are not necessarily indicative of the results of operations to be expected for the full year. These interim financial statements should be read in conjunction with the audited financial statements as of and for the year ended December 31, 2016, and the notes thereto, which are included in the Company’s Annual Report on Form 10-K filed with the Securities and Exchange Commission (the “SEC”) on February 22, 2017. The accompanying condensed consolidated financial statements include the accounts of the Company and its wholly-owned subsidiaries: Precision Genome Engineering, Inc. (“Pregenen”), bluebird bio France – SARL, bluebird bio Australia Pty Ltd., bluebird bio (UK) Ltd., bluebird bio (Bermuda) Ltd. Certain aggregations of prior year amounts have been made to conform to current year presentation. In the prior year balance sheet, tenant improvements receivable, prepaid expenses and other current assets are included within prepaid expenses and other current assets. Summary of accounting policies The significant accounting policies and estimates used in the preparation of the condensed consolidated financial statements are described in the Company’s audited financial statements as of and for the year ended December 31, 2016, and the notes thereto, which are included in the Company’s Annual Report on Form 10-K. There have been no material changes in the Company’s significant accounting policies during the three months ended March 31, 2017. Net loss per share Basic net loss per share is calculated by dividing net loss attributable to common stockholders by the weighted average number of common shares outstanding during the period. Diluted net income per share is calculated by dividing the net income attributable to common stockholders by the weighted-average number of common stock equivalent shares outstanding for the period, including any dilutive effect from outstanding stock options, unvested restricted stock, restricted stock units, employee stock purchase plan, and warrants using the treasury stock method. Property and equipment Property and equipment is stated at cost. Maintenance and repairs that do not improve or extend the lives of the respective assets are expensed to operations as incurred. Upon disposal, the related cost and accumulated depreciation is removed from the accounts and any resulting gain or loss is included in the results of operations. Depreciation and amortization is calculated using the straight-line method over the estimated useful lives of the assets, which are as follows: Asset Estimated useful life Building 40 years Computer equipment and software 3 years Office and laboratory equipment 2 -5 years Leasehold improvements Shorter of the useful life or remaining lease term Leases In September 2015, the Company entered into a lease agreement for additional office and laboratory space located at 60 Binney Street, Cambridge, Massachusetts, which was built between 2015 and March 2017, at which time the building became the Company’s corporate headquarters. This lease expires in 2027, subject to the Company’s right to extend the lease for an additional 10 years. Because the Company was involved in the construction project, it was deemed for accounting purposes to be the owner of the building during the construction period. Accordingly, the Company recorded project construction costs incurred by the landlord as an asset in “Property and equipment, net” and a related financing obligation in “Accrued expenses and other current liabilities” and “Financing lease obligation, net of current portion” on its condensed consolidated balance sheets. Upon completion of the construction of the building, the Company evaluated the lease and determined that it did not meet the criteria for “sale-leaseback” treatment. Accordingly, the Company depreciates the building and incurs interest expense related to the financing obligation recorded on its condensed consolidated balance sheet. The Company bifurcates its lease payments pursuant to the lease into (i) a portion that is allocated to the financing obligation related to the building and (ii) a portion that is allocated to the land on which the building was constructed. The portion of the lease obligation allocated to the land is treated for accounting purposes as an operating lease that commenced on September 2015 and is recorded on a straight-line basis over the initial lease term. See Note 7, “Commitments and contingencies,” for additional information. 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 and modifications to existing stock options, to be recognized in the consolidated statements of operations and comprehensive income (loss) based on their fair values. The Company uses the Black-Scholes option pricing model to determine the fair value of options granted. The Company’s stock-based awards are subject to either service or performance-based vesting conditions. Compensation expense related to awards to employees and directors with service-based vesting conditions is recognized on a straight-line basis based on the grant date fair value over the associated service period of the award, which is generally the vesting term. Compensation expense related to awards to non-employees with 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 accelerated attribution method. Compensation expense related to awards to employees with performance-based vesting conditions is recognized based on the grant date fair value over the requisite service period using the accelerated attribution method to the extent achievement of the performance condition is probable. Compensation expense related to awards to non-employees 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 accelerated attribution method to the extent achievement of the performance condition is probable. The Company expenses restricted stock unit awards to employees based on the fair value of the award on a straight-line basis over the associated service period of the award. Awards of restricted stock units to non-employees are adjusted through stock-based compensation expense at each reporting period end to reflect the current fair value of such awards and expensed using an accelerated attribution model. 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 subjective assumptions, including (i) the expected stock price volatility, (ii) the calculation of expected term of the award, (iii) the risk-free interest rate, and (iv) expected dividends. Due to the lack of company specific historical and implied volatility data of its common stock, the Company uses a weighted-average of expected volatility based on the estimated expected volatilities of a representative group of publicly traded companies; this representative group includes the Company’s data effective January 2017. The other public companies on which the Company has based its expected stock price volatility are 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 has estimated the expected term of its employee stock options using the “simplified” method, whereby, the expected term equals the arithmetic average of the vesting term and the original contractual term of the option due to its lack of sufficient historical data. 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, and does not expect to pay dividends in the foreseeable future. As a result of the adoption of ASU 2016-09, Improvements to Employee Share-Based Payment Accounting, effective January 1, 2017, the Company accounts for forfeitures as they occur instead of estimating forfeitures at the time of grant and revising those estimates in subsequent periods if actual forfeitures differ from its estimates. Stock-based compensation expense recognized in the financial statements is based on awards for which performance or service conditions are expected to be satisfied. Consistent with the guidance in FASB ASC Topic 505-50, Equity-Based Payments to Non-Employees, the fair value of each non-employee stock option and warrant award is estimated at the date of grant using the Black-Scholes option pricing model with assumptions generally consistent with those used for employee stock options, with the exception of expected term, which is over the contractual life. Use of estimates The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts in the financial statements and accompanying notes. Actual results could materially differ from those estimates. Management considers many factors in selecting appropriate financial accounting policies and controls, and in developing the estimates and assumptions that are used in the preparation of these financial statements. Management must apply significant judgment in this process. In addition, other factors may affect estimates, including: expected business and operational changes, sensitivity and volatility associated with the assumptions used in developing estimates, and whether historical trends are expected to be representative of future trends. The estimation process often may yield a range of potentially reasonable estimates of the ultimate future outcomes and management must select an amount that falls within that range of reasonable estimates. This process may result in actual results differing materially Recent accounting pronouncements In May 2014, the FASB issued ASU No. 2014-09, Revenue from Contracts with Customers (“Topic 606”), which supersedes all existing revenue recognition requirements, including most industry-specific guidance. The new standard requires a company to recognize revenue when it transfers goods or services to customers in an amount that reflects the consideration that the company expects to receive for those goods or services. The new standard will be effective on January 1, 2018 and earlier application is permitted only for annual reporting periods beginning after December 15, 2016, including interim reporting periods within that reporting period. Topic 606 allows for either a full retrospective adoption, in which the standard is applied to all of the periods presented, or a modified retrospective application, in which the standard is applied to the most current period presented in the financial statements. As of March 31, 2017, revenue is generated exclusively from the Company’s collaboration arrangement with Celgene. The Company is currently evaluating the potential impact that Topic 606 may have on its financial position and results of operations as it relates to this single arrangement, and expects to elect the modified retrospective application as its transition method. The Company is continuing to assess the impact of ASC 606 to its collaboration with Celgene, it is expected that the evaluation of variable consideration, and in particular, milestone payments due from Celgene will require further judgement to assess whether to include them in the transaction price, which could accelerate revenue recognition compared to ASC 605. In February 2016, the FASB issued ASU 2016-02, Leases , (“ASU 2016-02”) , which requires a lessee to recognize assets and liabilities on the balance sheet for operating leases and changes many key definitions, including the definition of a lease. The new standard includes a short-term lease exception for leases with a term of 12 months or less, as part of which a lessee can make an accounting policy election not to recognize lease assets and lease liabilities. Lessees will continue to differentiate between finance leases (previously referred to as capital leases) and operating leases using classification criteria that are substantially similar to the previous guidance. The new standard will be effective beginning January 1, 2019, and early adoption is permitted for public entities. The Company is currently evaluating the potential impact ASU 2016-02 may have on its financial position and results of operations. In March 2016, the FASB issued ASU 2016-09, Improvements to Employee Share-Based Payment Accounting, which simplifies share-based payment accounting through a variety of amendments. The amendments are effective for annual periods beginning after December 15, 2016, and interim periods within those annual periods. The Company adopted this standard effective January 1, 2017. The adoption of this standard impacted the income tax footnote disclosure and did not have a material impact on the Company’s condensed consolidated financial statements as of March 31, 2017. Upon adoption of the new standard, all excess tax benefits and tax deficiencies (including tax benefits of dividends on share-based payment awards) are recognized as income tax expense or benefit in the income statement. The tax effects of exercised or vested awards are treated as discrete items in the reporting period in which they occur. The Company also recognizes excess tax benefits regardless of whether the benefit reduces taxes payable in the current period. The Company has applied the modified retrospective adoption approach beginning in 2017 and prior periods have not been adjusted. As a result, the Company established a net operating loss deferred tax asset of $76.7 million to account for prior period excess tax benefits through retained earnings, however an offsetting valuation allowance of $76.7 million will also be established through retained earnings because it is not more likely than not that the deferred tax asset will be realized due to historical and expected future losses, such that there is no impact on the Company’s condensed consolidated financial statements. The Company also elected to account for forfeitures as they occur, and recorded a cumulative catch up of $0.5 million within additional paid-in capital and retained earnings upon adoption. In August 2016, the FASB issued ASU 2016-15, Statement of Cash Flows: Classification of Certain Cash Receipts and Cash Payments (“Topic 230”). The new standard clarifies certain aspects of the statement of cash flows, including the classification of contingent consideration payments made after a business combination and several other clarifications not currently applicable to the Company. The new standard also clarifies that an entity should determine each separately identifiable source or use within the cash receipts and cash payments on the basis of the nature of the underlying cash flows. In situations in which cash receipts and payments have aspects of more than one class of cash flows and cannot be separated by source or use, the appropriate classification should depend on the activity that is likely to be the predominant source or use of cash flows for the item. The new standard will be effective for the Company on January 1, 2018. The adoption of this standard is not expected to have a material impact on the Company’s condensed consolidated statements of cash flows upon adoption. In November 2016, the FASB issued ASU 2016-18, Statement of Cash Flows: Restricted Cash (“ASU 2016-18”). The amendments in this update require that amounts generally described as restricted cash and restricted cash equivalents be included within cash and cash equivalents when reconciling the beginning-of-period and end-of-period total amounts shown on the statement of cash flows. ASU 2016-18 will be effective January 1, 2018, with early adoption permitted. As of March 31, 2017, the Company has not elected to early adopt this guidance, but expects the adoption to have an impact on its consolidated statement of cash flows as, upon adoption, it will include the Company’s restricted cash balance in the cash and cash equivalents reconciliation of operating, investing and financing activities. In April 2017, the FASB issued ASU 2017-08, Receivables – Nonrefundable Fees and Other Costs (“Subtopic 310-20”). The new standard amends the amortization period for certain purchased callable debt securities held at a premium by shortening the amortization period for the premium to the earliest call date. Subtopic 310-20 calls for a modified retrospective application under which . The new standard will be effective beginning January 1, 2019, and early adoption is permitted for public entities. financial position or results of operations upon adoption.

Marketable Securities

Marketable Securities	3 Months Ended
	Mar. 31, 2017
Investments Debt And Equity Securities [Abstract]
Marketable securities	3. Marketable securities The following table summarizes the available-for-sale securities held at March 31, 2017 and December 31, 2016 (in thousands): Description Amortized Cost Unrealized Gains Unrealized Losses Fair Value March 31, 2017 U.S. government agency securities and treasuries $ 580,667 $ 11 $ (656 ) $ 580,022 Certificates of deposit 3,840 6 — 3,846 Total $ 584,507 $ 17 $ (656 ) $ 583,868 December 31, 2016 U.S. government agency securities and treasuries $ 600,001 $ 34 $ (575 ) $ 599,460 Certificates of deposit 6,480 6 (3 ) 6,483 Total $ 606,481 $ 40 $ (578 ) $ 605,943 No available-for-sale securities held as of March 31, 2017 or December 31, 2016 had remaining maturities greater than three years.

Fair value measurements

Fair value measurements	3 Months Ended
	Mar. 31, 2017
Fair Value Disclosures [Abstract]
Fair value measurements	4. Fair value measurements The following table sets forth the Company’s assets and liabilities that are measured at fair value on a recurring basis as of March 31, 2017 and December 31, 2016 (in thousands): Description Total Quoted prices in active markets (Level 1) Significant other observable inputs (Level 2) Significant unobservable inputs (Level 3) March 31, 2017 Assets: Cash and cash equivalents $ 216,001 $ 209,309 $ 6,692 $ — Marketable securities: U.S. government agency securities and treasuries 580,022 — 580,022 — Certificates of deposit 3,846 — 3,846 — Total assets $ 799,869 $ 209,309 $ 590,560 $ — Liabilities: Contingent consideration $ 9,189 $ — $ — $ 9,189 Total liabilities $ 9,189 $ — $ — $ 9,189 December 31, 2016 Assets: Cash and cash equivalents $ 278,887 $ 278,887 $ — $ — Marketable securities: U.S. government agency securities and treasuries 599,460 — 599,460 — Certificates of deposit 6,483 — 6,483 — Total assets $ 884,830 $ 278,887 $ 605,943 $ — Liabilities: Contingent consideration $ 7,756 $ — $ — $ 7,756 Total liabilities $ 7,756 $ — $ — $ 7,756 Cash and cash equivalents The Company considers all highly liquid securities with original final maturities of three months or less from the date of purchase to be cash equivalents. As of March 31, 2017, cash and cash equivalents comprise funds in cash, money market accounts, federally insured deposits and U.S. Treasury securities. As of December 31, 2016, cash and cash equivalents comprise funds in cash, money market accounts and federally insured deposits Marketable securities The amortized cost of available-for-sale securities is adjusted for amortization of premiums and accretion of discounts to maturity. At March 31, 2017 and December 31, 2016, the balance in the Company’s accumulated other comprehensive loss was composed primarily of activity related to the Company’s available-for-sale marketable securities. There were no realized gains The aggregate fair value of securities held by the Company in an unrealized loss position for less than twelve months as of March 31, 2017 and December 31, 2016 was $494.2 million and $376.1 million, respectively. As of March 31, 2017 and December 31, 2016, there were $53.3 million and $95.5 million in securities held by the Company in an unrealized loss position for more than twelve months. Contingent consideration On June 30, 2014, the Company acquired Pregenen. In connection with the acquisition, the Company recorded contingent consideration pertaining to the amounts potentially payable to Pregenen’s former equityholders pursuant to the Stock Purchase Agreement (the “Stock Purchase Agreement”) by and among the Company, Pregenen and Pregenen’s former equityholders. Contingent consideration is measured at fair value and is based on significant inputs not observable in the market, which represents a Level 3 measurement within the fair value hierarchy. The valuation of contingent consideration uses assumptions the Company believes would be made by a market participant. The Company assesses these estimates on an on-going basis as additional data impacting the assumptions is obtained. Future changes in the fair value of contingent consideration related to updated assumptions and estimates are recognized within the condensed consolidated statements of operations and comprehensive loss. Contingent consideration may change significantly as development progresses and additional data are obtained, impacting the Company’s assumptions regarding probabilities of successful achievement of related milestones used to estimate the fair value of the liability and the timing in which they are expected to be achieved. In evaluating the fair value information, considerable judgment is required to interpret the market and internal data used to develop the estimates. The estimates of fair value may not be indicative of the amounts that could be realized in a current market exchange. Accordingly, the use of different market and internal assumptions and/or different valuation techniques could result in materially different fair value estimates. The significant unobservable inputs used in the measurement of fair value of the Company’s contingent consideration are probabilities of successful achievement of preclinical, clinical and commercial milestones, the period in which these milestones are expected to be achieved ranging from The table below provides a roll-forward of fair value of the Company’s contingent consideration obligations, which include Level 3 inputs (in thousands): Three Months Ended March 31, 2017 Beginning balance $ 7,756 Additions — Changes in fair value 1,433 Payments — Ending balance $ 9,189 The Company may be required to make up to $129.0 million in remaining future contingent cash payments to the former equityholders of Pregenen upon the achievement of certain milestones related to the Pregenen technology, of which $9.0 million relates to preclinical milestones, $20.1 million relates to clinical milestones, and $99.9 million relates to commercial milestones. The milestone achievement end date for the remaining preclinical milestones is June 30, 2017, with other milestone achievement end dates occurring between 2021 and 2028. As of March 31, 2017, $6.1 million of the fair value of the Company’s total contingent consideration obligations was reflected as a component of accrued expenses and other current liabilities within the condensed consolidated balance sheets, with the remaining balance of $3.1 million reflected as a non-current liability.

Property and equipment, net

Property and equipment, net	3 Months Ended
	Mar. 31, 2017
Property Plant And Equipment [Abstract]
Property and equipment, net	5. Property and equipment, net Property and equipment, net, consists of the following (in thousands): March 31, December 31, Building 122,451 — Computer equipment and software 5,528 1,655 Office equipment 2,977 1,427 Laboratory equipment 16,497 16,305 Leasehold improvements 13,697 13,697 Construction-in-progress 34,854 136,315 Total property and equipment, gross 196,004 169,399 Less accumulated depreciation and amortization (14,284 ) (12,444 ) Total Property and equipment, net $ 181,720 $ 156,955 As of March 31, 2017, total property and equipment, gross, includes $153.2 million related to the Company’s new headquarters at 60 Binney Street in Cambridge, Massachusetts, of which $147.9 million was incurred by the landlord. The majority of the 60 Binney Street building was placed into service during the first quarter of 2017, except for $30.8 million that remains in construction-in-progress as of March 31, 2017 related to two office floors that were subsequently completed and placed into service in April 2017. Construction-in-progress as of December 31, 2016 included

Accrued expenses and other curr

Accrued expenses and other current liabilities	3 Months Ended
	Mar. 31, 2017
Payables And Accruals [Abstract]
Accrued expenses and other current liabilities	6. Accrued expenses and other current liabilities Accrued expenses and other current liabilities consist of the following (in thousands): March 31, 2017 December 31, 2016 Employee compensation $ 6,251 $ 11,296 Accrued goods and services 29,538 34,275 Accrued license and milestone fees 285 2,464 Accrued professional fees 1,393 1,492 Deferred rent, current portion — 11 Contingent consideration, current portion 6,059 4,479 Other 781 643 Total accrued expenses and other current liabilities $ 44,307 $ 54,660

Commitments and contingencies

Commitments and contingencies	3 Months Ended
	Mar. 31, 2017
Commitments And Contingencies Disclosure [Abstract]
Commitments and contingencies	7. Commitments and contingencies The Company is party to various agreements, principally relating to licensed technology, that require future payments relating to milestones not met at March 31, 2017 and December 31, 2016 or royalties on future sales of specified products. The Company enters into standard indemnification agreements in the ordinary course of business. Pursuant to these agreements, the Company indemnifies, holds harmless, and agrees to reimburse the indemnified party for losses suffered or incurred by the indemnified party, generally the Company’s business partners or customers, in connection with claims by any third party with respect to the Company’s products or business activities. The term of these indemnification agreements is generally perpetual any time after execution of the agreement. The maximum potential amount of future payments the Company could be required to make under these indemnification agreements is unlimited. The Company has never incurred costs to defend lawsuits or settle claims related to these indemnification agreements. Operating Lease Commitments On June 3, 2013, the Company entered into a nine-year building lease for approximately 43,600 square feet of space located at 150 Second Street, Cambridge, Massachusetts, which commenced in December 2013. This lease was amended in June 2014 to add approximately 9,900 square feet. The lease originally had monthly lease payments of $0.2 million for the first 12 months, which increased to $0.3 million per month beginning in December 2014 due to the lease amendment, with annual rent escalations thereafter. Rent expense is recognized on a straight-line basis over the term of the lease. The lease provided a contribution from the landlord towards the initial build-out of the space of up to $7.8 million. The Company capitalizes the leasehold improvements as property and equipment and records the landlord incentive payments received as deferred rent and amortizes these amounts as reductions to rent expense over the lease term. In addition, i On September 30, 2016, the Company entered into an Assignment and Assumption of Lease (“Assignment”) relating to its lease at 150 Second Street. Under the Assignment, the Company will assign all of its rights, interests, obligations and responsibilities under the lease, to be effective as of the later of May 1, 2017 or the first day following the Company’s surrender of the leased premises in accordance with the lease. The Company vacated and surrendered the premises on On June 29, 2015, the Company entered into a lease agreement for additional office space located at 215 First Street, Cambridge, Massachusetts. Under the terms of the lease, the Company leased approximately 15,120 square feet starting on July 13, 2015 for $0.5 million per year in base rent, which is subject to a 3% annual rent increase plus certain operating expenses and taxes. The lease will continue until the end of the 60 th th April 12, 2017 On June 3, 2016, the Company entered into a strategic manufacturing agreement for the future commercial production of the Company’s Lenti-D and LentiGlobin product candidates with a contract manufacturing organization. Under this 12 year agreement, the contract manufacturing organization will complete the design, construction, validation and process validation of the leased suites prior to anticipated commercial launch of the product candidates. During construction, the Company is required to pay $12.5 million upon the achievement of certain contractual milestones, and may pay up to $8.0 million in additional contractual milestones if the Company elects its option to lease additional suites. The Company paid $5.0 million for the achievement of the first and second contractual milestones during 2016 and paid the third milestone of $3.0 million during the first quarter of 2017, which is reflected as a component of other non-current assets within the condensed consolidated balance sheets. under ASC 840-10, Leases - Overall On November 18, 2016, the Company entered into an agreement for future clinical and commercial production of the Company’s LentiGlobin gene therapy drug products with a contract manufacturing organization at an existing facility. The term of the agreement is five years with a three year renewal at the mutual option of each party. Under the agreement, the Company is required to pay an up-front fee of €3.0 million, €2.0 million of which was paid in the fourth quarter of 2016 and €1.0 million of which is expected to be paid in mid-2018, and annual maintenance and production fees of up to €9.8 million, depending on its production needs. The Company may terminate this agreement with six months’ notice and a one-time termination fee prior to July 1, 2018, or twelve months’ notice and a one-time termination fee thereafter. The Company concluded that this agreement contains an embedded lease as the clean rooms are designated for the Company’s exclusive use during the term of the agreement, and determined that it is not a capital lease under ASC 840-10, Leases – Overall non-cancellable 60 Binney Street Lease Commitments On September 21, 2015, the Company entered into a lease agreement for additional office and laboratory space located in a building (the “Building”) at 60 Binney Street, Cambridge, Massachusetts (the “60 Binney Street Lease”). Under the terms of the 60 Binney Street Lease, starting on October 1, 2016, the Company will lease approximately 253,108 square feet of office and laboratory space at $72.50 per square foot per year, or $18.4 million per year in base rent, which is subject to scheduled annual rent increases of 1.75% plus certain operating expenses and taxes. The Company also executed a $9.2 million letter of credit upon signing the 60 Binney Street Lease, which was required to be collateralized with a bank account at a financial institution in accordance with the 60 Binney Street Lease agreement. This letter of credit was increased to $13.8 million during the third quarter of 2016 as required under the terms of the lease. Subject to the terms of the lease and certain reduction requirements specified therein, including market capitalization requirements, this amount may decrease back to $9.2 million over time. The 60 Binney Street Lease will continue until the end of the 120 th Because the Company was involved in the construction project, including having responsibility to pay for a portion of the costs of finish work and mechanical, electrical, and plumbing elements of the Building, among other items, the Company was deemed for accounting purposes to be the owner of the Building during the construction period. Accordingly, costs that were incurred by the landlord directly or indirectly through reimbursement to the Company as part of its tenant improvement allowance have been recorded as an asset in “Property and equipment, net” with a related financing obligation in “Accrued expenses and other current liabilities” and “ Financing lease obligation, net of current portion The Company evaluated the 60 Binney Street Lease upon occupancy on March 27, 2017 and determined that the 60 Binney Street Lease did not meet the criteria for “sale-leaseback” treatment. This determination was based on, among other things, the Company's continuing involvement with the property in the form of non-recourse financing to the lessor. Accordingly, upon occupancy, the Company commenced depreciating the portion of the building in service over a useful life of 40 years and incurred interest expense related to the financing obligation, which was immaterial to the condensed consolidated statement of operations for the three months ended March, 31, 2017. The Company bifurcates its lease payments pursuant to the 60 Binney Street Lease into (i) a portion that is allocated to the Building and (ii) a portion that is allocated to the land on which the Building has been constructed, which is recorded as rental expense. The Company began making lease payments pursuant to the 60 Binney Street Lease in March 2017. The portion of the lease obligation allocated to the land is treated for accounting purposes as an operating lease that commenced upon execution of the 60 Binney Street Lease in September 2015. During the three months ended March 31, 2017, the Company recognized As of March 31, 2017, Property and equipment, net, includes $153.2 As of December 31, 2016, Property and equipment, net, includes $126.9 million related to the Building, of which $120.1 million has been incurred by the landlord. As of December 31, 2016, Prepaid expenses and other current assets includes $8.5 million of tenant improvement costs reimbursable by the landlord that had not yet been received, $7.8 million of which was received by the Company in January 2017.

Significant agreements

Significant agreements	3 Months Ended
	Mar. 31, 2017
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Significant agreements	8. Significant agreements Celgene Corporation Original Collaboration Agreement On March 19, 2013, the Company entered into a Master Collaboration Agreement (the “Collaboration Agreement”) with Celgene Corporation (“Celgene”) to discover, develop and commercialize potentially disease-altering gene therapies in oncology. The collaboration is focused on applying gene therapy technology to genetically modify a patient’s own T cells, known as chimeric antigen receptor, or CAR T cells, to target and destroy cancer cells. Additionally, on March 19, 2013, the Company entered into a Platform Technology Sublicense Agreement (the “Sublicense Agreement”) with Celgene pursuant to which the Company obtained a sublicense to certain intellectual property from Celgene, originating under Celgene’s license from Baylor College of Medicine. Under the terms of the Collaboration Agreement, the Company received a $75.0 million up-front, non-refundable cash payment. The Company was responsible for conducting discovery, research and development activities through completion of Phase I clinical studies, if any, during the initial term of the Collaboration Agreement, or three years. The collaboration is governed by a joint steering committee (“JSC”) formed by an equal number of representatives from the Company and Celgene. The JSC, among other activities, reviews the collaboration program, reviews and evaluates product candidates and approves regulatory plans. In addition to the JSC, the Collaboration Agreement provides that the Company and Celgene each appoint representatives to a patent committee, which is responsible for managing the intellectual property developed and used during the collaboration. Amended Collaboration Agreement On June 3, 2015, the Company and Celgene amended and restated the Collaboration Agreement (the “Amended Collaboration Agreement”). Under the Amended Collaboration Agreement, the parties will now focus the collaboration exclusively on anti- B-cell maturation antigen (“BCMA”) product candidates for a new three-year term. In connection with the Amended Collaboration Agreement, the Company received an upfront, one-time, non-refundable, non-creditable payment of $25.0 million to fund research and development under the collaboration. The collaboration will continue to be governed by the JSC. Under the terms of the Amended Collaboration Agreement, for up to two product candidates selected for development under the collaboration, the Company is responsible for conducting and funding all research and development activities performed up through completion of the initial Phase I clinical study of such product candidate. On a product candidate-by-product candidate basis, up through a specified period following enrollment of the first patient in an initial Phase I clinical study for such product candidate (the “Option Period”), the Company has granted Celgene an option to obtain an exclusive worldwide license to develop and commercialize such product candidate pursuant to a written agreement, the form of which the Company has already agreed upon. In the event that Celgene exercises its option with respect to any product candidate, the Company may elect to co-develop and co-promote the product candidate in the United States, provided that, if the Company does not exercise its option co-develop and co-promote the first product candidate in-licensed by Celgene under the Amended Collaboration Agreement, then the Company will not be permitted to exercise its option to co-develop and co-promote any future product candidates under the Amended Collaboration Agreement. If Celgene elects to exercise its option to exclusively in-license a product candidate, it must pay the Company an option fee in the amount of $10.0 million for the first product candidate and $15.0 million for any additional product candidates. On February 10, 2016, Celgene exercised its option to obtain an exclusive worldwide license to develop and commercialize bb2121, the first product candidate under the Amended Collaboration Agreement, pursuant to an executed license agreement entered into by the parties on February 16, 2016 and paid the associated $10.0 million option fee. The Company may now elect to co-develop and co-promote the product candidate within the United States which it currently expects to elect. The Company’s election to co-develop and co-promote must be made at the substantial completion of the Phase 1 trial of bb2121. The Company will share equally in all costs relating to developing, commercializing and manufacturing the product candidate within the United States if it elects to co-develop and co-promote bb2121 with Celgene. In the event the Company does not exercise its option to co-develop and co-promote bb2121, the Company will receive an additional fee in the amount of $10.0 million. Accounting Analysis The Company’s Amended Collaboration Agreement with Celgene contains the following deliverables: (i) research and development services, (ii) participation on the JSC, (iii) participation on the patent committee, (iv) a license to the first product candidate, (v) manufacture of vectors and associated payload for incorporation into the first optioned product candidate under the license, and (vi) participation on the JGC under the co-development and co-promotion agreement for the first optioned product candidate under the license. The license to the first product candidate was considered a deliverable at the inception of the arrangement and therefore the associated option fee was included in allocable arrangement consideration as the Company believed there was minimal risk with regard to whether Celgene will exercise the option based on the successful completion of preclinical activities and proximity of enrollment of the first patient in an initial Phase I clinical study for this product candidate. The Company determined that the obligation within the license to manufacture or have manufactured supplies of vectors and associated payloads for incorporation into the first optioned product candidate is a deliverable, consistent with the option to license the first product candidate. However, the Company determined that the options to license any additional product candidates are substantive options and therefore were not considered deliverables at execution of the Amended Collaboration Agreement. Celgene is not contractually obligated to exercise the options. Additionally, as a result of the uncertain outcome of the discovery, research and development activities, the Company is at risk with regard to whether Celgene will exercise the options to license additional product candidates. Moreover, the Company determined that the options are not priced at a significant and incremental discount. Accordingly, the options to other product candidates are not considered deliverables and the associated option fees are not included in allocable arrangement consideration. Upon execution of the Amended Collaboration Agreement in June 2015, the Company concluded that each of the three delivered elements at the inception of the agreement (research and development services, participation on the JSC and participation on the patent committee) had standalone value from the other undelivered elements. Additionally, the Amended Collaboration Agreement does not include return rights related to the collaboration term. Accordingly, each deliverable qualified as a separate unit of accounting. The Company determined that each of the delivered elements had the same period of performance (the three year term through projected initial Phase I clinical study substantial completion) and the same pattern of revenue recognition, ratably over the period of performance as there was no other discernible pattern of recognition. The Company identified the allocable arrangement consideration as the $25.0 million up-front research and development funding payment, $10.0 million option fee for the first product candidate, $20.0 million related to remaining deferred revenue from the original Collaboration Agreement, and $54.1 million of contingent revenue related to the estimated amounts that will be received from Celgene for manufacturing services. The $109.0 million total allocable arrangement consideration was allocated based on the relative estimated selling price of the separate units of accounting at the inception of the amended agreement, resulting in $17.3 million allocated to the three delivered elements at the inception of the agreement, which will be recognized over an initial three year term. The Company determined that each of the identified deliverables that qualify as a separate unit of accounting continue to have the same period of performance (the three year term through projected initial Phase I clinical study substantial completion) and the same pattern of revenue recognition, ratably over the period of performance as there is no other discernible pattern of recognition, and therefore there is no change in the recognition of $17.3 million allocated to these three elements. As of March 31, 2017, this will continue to be recognized over a three year term that began in June 2015. However, the Company concluded that the license to bb2121 does not have standalone value from one of the undelivered elements, the post-initial Phase I the manufacture of vectors and associated payload for bb2121 under the license, because the manufacturing is essential to the license agreement. The Company is required to reassess its conclusions on standalone value of deliverables upon delivery, and therefore, upon commencement of manufacturing services in the first quarter of 2017, the Company updated its assessment. The Company determined that there were no changes and that the bb2121 license agreement continues to not have standalone value from the manufacturing services. Accordingly, these two deliverables qualify as a single combined unit of accounting. Revenue recognition for the combined unit of accounting commenced during the first quarter of 2017 after the Company reached agreement with Celgene regarding the budget and timing for such manufacturing services. Accordingly, revenue of $4.9 million was recognized during the three months ended March 31, 2017 related to the combined unit of accounting. The Company recognizes revenue associated with the combined unit of accounting using the proportional performance method. In using this method, the Company estimated, through discussions with Celgene regarding their development plan for bb2121, the proportion of effort it incurred as a percentage of total effort it expects to incur and applied this ratio to the total estimated budget for the post-initial Phase I manufacture of vectors and associated payload for bb2121. In developing the total estimated budget, management assumed that the Company will exercise its option to co-develop and co-promote bb2121 and therefore is currently recognizing revenue related to 67.5% of worldwide development costs incurred, which represents the percentage the Company is contractually entitled to bill Celgene under the cost share provisions of the co-develop and co-promote agreement. While not material in the first quarter of 2017, costs incurred and billable by Celgene to the Company under the co-develop and co-promote agreement will be recorded in future quarters as contra-revenue. In the event that the Company does not exercise its option to co-develop and co-promote bb2121, the Company expects to recognize the remaining 32.5% of worldwide development costs, as Celgene would be responsible for 100% of costs incurred. Actual costs could materially differ from these estimates, and management has applied significant judgment in the process of developing its budget estimates. Any changes to these estimates will be recognized in the period in which they change as a cumulative catch up. Assuming the co-develop and co-promote agreement is executed and other revenue recognition criteria have been met, the Company expects to recognize approximately $77.6 million, before any offsetting expenses reimbursable to Celgene, between the commencement of post-Phase 1 development in the first quarter of 2017 and the end of development in 2021. The period of performance and recognition pattern will be revisited as the development plan changes or if other events impacting the deliverables occur. The Company also recognized $0.4 million of revenue related to other development costs incurred, which are not related to the combined unit of accounting but are billable under the collaboration agreement. The Company evaluated all of the milestones that may be received in connection with Celgene’s option to license a product candidate resulting from the collaboration. In evaluating if a milestone is substantive, the Company assesses whether: (i) the consideration is commensurate with either the Company’s performance to achieve the milestone or the enhancement of the value of the delivered item(s) as a result of a specific outcome resulting from the Company’s performance to achieve the milestone, (ii) the consideration relates solely to past performance and (iii) the consideration is reasonable relative to all of the deliverables and payment terms within the arrangement. All clinical and regulatory milestones that may be received under the option to the license agreement are considered substantive on the basis of the contingent nature of the milestone, specifically reviewing factors such as the scientific, clinical, regulatory, commercial and other risks that must be overcome to achieve the milestone as well as the level of effort and investment required. Accordingly, such amounts will be recognized as revenue in full in the period in which the associated milestone is achieved, assuming all other revenue recognition criteria are met. All commercial milestones will be accounted for in the same manner as royalties and recorded as revenue upon achievement of the milestone, assuming all other revenue recognition criteria are met. During the three months ended March 31, 2017 and 2016, the Company recognized $6.8 million and $1.5 million, respectively, of revenue associated with its collaboration with Celgene related to research and development services, delivery of the license to bb2121 and manufacturing services for bb2121 vector and plasmids. As of March 31, 2017 and December 31, 2016, there was $42.4 million and $46.4 million, respectively, of total deferred revenue related to the Company’s collaboration with Celgene, which is classified as current or non-current in the condensed consolidated balance sheets. As of March 31, 2017, Restricted cash and other non-current assets includes a $2.9 million receivable related to cost reimbursement from Celgene for bb2121 development costs incurred to date.

Stock-based compensation

Stock-based compensation	3 Months Ended
	Mar. 31, 2017
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Stock-based compensation	9. Stock-based compensation In January 2017, the number of shares of common stock available for issuance under the 2013 Stock Option and Incentive Plan (“2013 Plan”) was increased by approximately 1.6 million shares as a result of the automatic increase provision of the 2013 Plan. As of March 31, 2017, the total number of shares of common stock available for issuance under the 2013 Plan was approximately 2.0 million. Stock-based compensation expense Stock-based compensation expense by award type included within the condensed consolidated statements of operations and comprehensive loss was as follows (in thousands): Three Months Ended March 31, 2017 2016 Stock options $ 9,342 $ 8,926 Restricted stock units 1,956 1,127 Employee stock purchase plan 183 90 $ 11,481 $ 10,143 As of March 31, 2017, the Company had $121.7 million of unrecognized stock-based compensation expense related to unvested stock options, restricted stock units and the employee stock purchase plan, which is expected to be recognized over a weighted-average period of 2.8 Stock-based compensation expense by classification included within the condensed consolidated statements of operations and comprehensive loss was as follows (in thousands): Three Months Ended March 31, 2017 2016 Research and development $ 5,654 $ 4,681 General and administrative 5,827 5,462 $ 11,481 $ 10,143 Stock options The following table summarizes the stock option activity under the Company’s equity award plans (shares in thousands): Shares Weighted-average exercise price per share Outstanding at December 31, 2016 3,735 $ 52.17 Granted 696 $ 76.37 Exercised (181 ) $ 15.18 Canceled or forfeited (47 ) $ 88.88 Outstanding at March 31, 2017 4,203 $ 57.35 Exercisable at March 31, 2017 2,070 $ 42.63 Vested and expected to vest at March 31, 2017 4,203 $ 57.35 During the three months ended March 31, 2017, 0.2 million shares of common stock were exercised, resulting in total proceeds to the Company of $2.7 million. In accordance with the Company’s equity award plans, the shares were issued from a pool of shares reserved for issuance under the equity award plans. Restricted stock units The following table summarizes the restricted stock unit activity under the Company’s equity award plans (shares in thousands): Shares Weighted-average grant date fair value Unvested balance at December 31, 2016 263 $ 63.07 Granted 205 $ 78.88 Vested (39 ) $ 50.08 Forfeited (3 ) $ 57.10 Unvested balance at March 31, 2017 426 $ 71.92 Employee stock purchase plan On June 3, 2013, the Company’s board of directors adopted its 2013 Employee Stock Purchase Plan (“2013 ESPP”), which was subsequently approved by its stockholders and became effective upon the closing of the Company’s IPO on June 24, 2013. The 2013 ESPP authorizes the initial issuance of up to a total of 238,000 shares 2016, 11,079 and shares

Income taxes

Income taxes	3 Months Ended
	Mar. 31, 2017
Income Tax Disclosure [Abstract]
Income taxes	10. Income taxes Deferred tax assets and deferred tax liabilities are recognized based on temporary differences between the financial reporting and tax basis of assets and liabilities using statutory rates. A valuation allowance is recorded against deferred tax assets if it is more likely than not that some or all of the deferred tax assets will not be realized. Due to the uncertainty surrounding the realization of the favorable tax attributes in future tax returns, the Company has recorded a full valuation allowance against the Company’s otherwise recognizable net deferred tax assets.

Net loss per share

Net loss per share	3 Months Ended
	Mar. 31, 2017
Earnings Per Share [Abstract]
Net loss per share	11. Net loss per share The following common stock equivalents were excluded from the calculation of diluted net loss per share for the periods indicated because including them would have had an anti-dilutive effect (in thousands): Three Months Ended March 31, 2017 2016 Outstanding stock options 4,203 4,104 Restricted stock units 426 320 ESPP shares 11 10 4,640 4,434

Subsequent events

Subsequent events	3 Months Ended
	Mar. 31, 2017
Subsequent Events [Abstract]
Subsequent Events	12. Subsequent Events On April 26, 2017, the Company entered into a worldwide license agreement around its proprietary lentiviral vector platform with Novartis Pharma AG (“Novartis”). Under the terms of the agreement with Novartis, Novartis will non-exclusively license certain patent rights related to lentiviral vector technology to develop and commercialize chimeric antigen receptor T cell (CAR T) therapies for oncology, including CTL019, Novartis’s anti-CD19 CAR T investigational therapy. Financial terms of the agreement include a $7.5 million payment upon execution, $7.5 million of potential future milestone payments, and $1.1 million of payments for subsequently licensed products, as well as low single digit royalty payments on net sales of covered products. On April 28, 2017, the Company entered into a worldwide license agreement around its proprietary lentiviral vector platform with GlaxoSmithKline Intellectual Property Development Limited (“GSK”). Under the terms of the agreement, GSK will non-exclusively license certain patent rights related to lentiviral vector technology to develop and commercialize gene therapies for Wiscott-Aldrich syndrome and metachromatic leukodystrophy, two rare genetic diseases. Financial terms of the agreement include an upfront payment of $3.0 million as well as $1.3 million of potential milestone payments for each marketing authorization for each indication in any country as well as low single digit royalties on net sales of covered products.

Summary of significant accoun19

Summary of significant accounting policies and basis of presentation (Policies)	3 Months Ended
	Mar. 31, 2017
Accounting Policies [Abstract]
Basis of presentation and principles of consolidation	Basis of presentation and principles of consolidation The accompanying condensed consolidated financial statements are unaudited and have been prepared by the Company in accordance with accounting principles generally accepted in the United States (“GAAP”) as found in the Accounting Standards Codification (“ASC”) and Accounting Standards Update (“ASU”) of the Financial Accounting Standards Board (“FASB”). Certain information and footnote disclosures normally included in the Company’s annual financial statements have been condensed or omitted. These interim condensed consolidated financial statements, in the opinion of management, reflect all normal recurring adjustments necessary for a fair presentation of the Company’s financial position and results of operations for the interim periods ended March 31, 2017 and 2016. The results of operations for the interim periods are not necessarily indicative of the results of operations to be expected for the full year. These interim financial statements should be read in conjunction with the audited financial statements as of and for the year ended December 31, 2016, and the notes thereto, which are included in the Company’s Annual Report on Form 10-K filed with the Securities and Exchange Commission (the “SEC”) on February 22, 2017. The accompanying condensed consolidated financial statements include the accounts of the Company and its wholly-owned subsidiaries: Precision Genome Engineering, Inc. (“Pregenen”), bluebird bio France – SARL, bluebird bio Australia Pty Ltd., bluebird bio (UK) Ltd., bluebird bio (Bermuda) Ltd. Certain aggregations of prior year amounts have been made to conform to current year presentation. In the prior year balance sheet, tenant improvements receivable, prepaid expenses and other current assets are included within prepaid expenses and other current assets.
Summary of accounting policies	Summary of accounting policies The significant accounting policies and estimates used in the preparation of the condensed consolidated financial statements are described in the Company’s audited financial statements as of and for the year ended December 31, 2016, and the notes thereto, which are included in the Company’s Annual Report on Form 10-K. There have been no material changes in the Company’s significant accounting policies during the three months ended March 31, 2017.
Net loss per share	Net loss per share Basic net loss per share is calculated by dividing net loss attributable to common stockholders by the weighted average number of common shares outstanding during the period. Diluted net income per share is calculated by dividing the net income attributable to common stockholders by the weighted-average number of common stock equivalent shares outstanding for the period, including any dilutive effect from outstanding stock options, unvested restricted stock, restricted stock units, employee stock purchase plan, and warrants using the treasury stock method.
Property and equipment	Property and equipment Property and equipment is stated at cost. Maintenance and repairs that do not improve or extend the lives of the respective assets are expensed to operations as incurred. Upon disposal, the related cost and accumulated depreciation is removed from the accounts and any resulting gain or loss is included in the results of operations. Depreciation and amortization is calculated using the straight-line method over the estimated useful lives of the assets, which are as follows: Asset Estimated useful life Building 40 years Computer equipment and software 3 years Office and laboratory equipment 2 -5 years Leasehold improvements Shorter of the useful life or remaining lease term
Leases	Leases In September 2015, the Company entered into a lease agreement for additional office and laboratory space located at 60 Binney Street, Cambridge, Massachusetts, which was built between 2015 and March 2017, at which time the building became the Company’s corporate headquarters. This lease expires in 2027, subject to the Company’s right to extend the lease for an additional 10 years. Because the Company was involved in the construction project, it was deemed for accounting purposes to be the owner of the building during the construction period. Accordingly, the Company recorded project construction costs incurred by the landlord as an asset in “Property and equipment, net” and a related financing obligation in “Accrued expenses and other current liabilities” and “Financing lease obligation, net of current portion” on its condensed consolidated balance sheets. Upon completion of the construction of the building, the Company evaluated the lease and determined that it did not meet the criteria for “sale-leaseback” treatment. Accordingly, the Company depreciates the building and incurs interest expense related to the financing obligation recorded on its condensed consolidated balance sheet. The Company bifurcates its lease payments pursuant to the lease into (i) a portion that is allocated to the financing obligation related to the building and (ii) a portion that is allocated to the land on which the building was constructed. The portion of the lease obligation allocated to the land is treated for accounting purposes as an operating lease that commenced on September 2015 and is recorded on a straight-line basis over the initial lease term. See Note 7, “Commitments and contingencies,” for additional information.
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 and modifications to existing stock options, to be recognized in the consolidated statements of operations and comprehensive income (loss) based on their fair values. The Company uses the Black-Scholes option pricing model to determine the fair value of options granted. The Company’s stock-based awards are subject to either service or performance-based vesting conditions. Compensation expense related to awards to employees and directors with service-based vesting conditions is recognized on a straight-line basis based on the grant date fair value over the associated service period of the award, which is generally the vesting term. Compensation expense related to awards to non-employees with 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 accelerated attribution method. Compensation expense related to awards to employees with performance-based vesting conditions is recognized based on the grant date fair value over the requisite service period using the accelerated attribution method to the extent achievement of the performance condition is probable. Compensation expense related to awards to non-employees 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 accelerated attribution method to the extent achievement of the performance condition is probable. The Company expenses restricted stock unit awards to employees based on the fair value of the award on a straight-line basis over the associated service period of the award. Awards of restricted stock units to non-employees are adjusted through stock-based compensation expense at each reporting period end to reflect the current fair value of such awards and expensed using an accelerated attribution model. 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 subjective assumptions, including (i) the expected stock price volatility, (ii) the calculation of expected term of the award, (iii) the risk-free interest rate, and (iv) expected dividends. Due to the lack of company specific historical and implied volatility data of its common stock, the Company uses a weighted-average of expected volatility based on the estimated expected volatilities of a representative group of publicly traded companies; this representative group includes the Company’s data effective January 2017. The other public companies on which the Company has based its expected stock price volatility are 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 has estimated the expected term of its employee stock options using the “simplified” method, whereby, the expected term equals the arithmetic average of the vesting term and the original contractual term of the option due to its lack of sufficient historical data. 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, and does not expect to pay dividends in the foreseeable future. As a result of the adoption of ASU 2016-09, Improvements to Employee Share-Based Payment Accounting, effective January 1, 2017, the Company accounts for forfeitures as they occur instead of estimating forfeitures at the time of grant and revising those estimates in subsequent periods if actual forfeitures differ from its estimates. Stock-based compensation expense recognized in the financial statements is based on awards for which performance or service conditions are expected to be satisfied. Consistent with the guidance in FASB ASC Topic 505-50, Equity-Based Payments to Non-Employees, the fair value of each non-employee stock option and warrant award is estimated at the date of grant using the Black-Scholes option pricing model with assumptions generally consistent with those used for employee stock options, with the exception of expected term, which is over the contractual life.
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 in the financial statements and accompanying notes. Actual results could materially differ from those estimates. Management considers many factors in selecting appropriate financial accounting policies and controls, and in developing the estimates and assumptions that are used in the preparation of these financial statements. Management must apply significant judgment in this process. In addition, other factors may affect estimates, including: expected business and operational changes, sensitivity and volatility associated with the assumptions used in developing estimates, and whether historical trends are expected to be representative of future trends. The estimation process often may yield a range of potentially reasonable estimates of the ultimate future outcomes and management must select an amount that falls within that range of reasonable estimates. This process may result in actual results differing materially
Recent accounting pronouncements	Recent accounting pronouncements In May 2014, the FASB issued ASU No. 2014-09, Revenue from Contracts with Customers (“Topic 606”), which supersedes all existing revenue recognition requirements, including most industry-specific guidance. The new standard requires a company to recognize revenue when it transfers goods or services to customers in an amount that reflects the consideration that the company expects to receive for those goods or services. The new standard will be effective on January 1, 2018 and earlier application is permitted only for annual reporting periods beginning after December 15, 2016, including interim reporting periods within that reporting period. Topic 606 allows for either a full retrospective adoption, in which the standard is applied to all of the periods presented, or a modified retrospective application, in which the standard is applied to the most current period presented in the financial statements. As of March 31, 2017, revenue is generated exclusively from the Company’s collaboration arrangement with Celgene. The Company is currently evaluating the potential impact that Topic 606 may have on its financial position and results of operations as it relates to this single arrangement, and expects to elect the modified retrospective application as its transition method. The Company is continuing to assess the impact of ASC 606 to its collaboration with Celgene, it is expected that the evaluation of variable consideration, and in particular, milestone payments due from Celgene will require further judgement to assess whether to include them in the transaction price, which could accelerate revenue recognition compared to ASC 605. In February 2016, the FASB issued ASU 2016-02, Leases , (“ASU 2016-02”) , which requires a lessee to recognize assets and liabilities on the balance sheet for operating leases and changes many key definitions, including the definition of a lease. The new standard includes a short-term lease exception for leases with a term of 12 months or less, as part of which a lessee can make an accounting policy election not to recognize lease assets and lease liabilities. Lessees will continue to differentiate between finance leases (previously referred to as capital leases) and operating leases using classification criteria that are substantially similar to the previous guidance. The new standard will be effective beginning January 1, 2019, and early adoption is permitted for public entities. The Company is currently evaluating the potential impact ASU 2016-02 may have on its financial position and results of operations. In March 2016, the FASB issued ASU 2016-09, Improvements to Employee Share-Based Payment Accounting, which simplifies share-based payment accounting through a variety of amendments. The amendments are effective for annual periods beginning after December 15, 2016, and interim periods within those annual periods. The Company adopted this standard effective January 1, 2017. The adoption of this standard impacted the income tax footnote disclosure and did not have a material impact on the Company’s condensed consolidated financial statements as of March 31, 2017. Upon adoption of the new standard, all excess tax benefits and tax deficiencies (including tax benefits of dividends on share-based payment awards) are recognized as income tax expense or benefit in the income statement. The tax effects of exercised or vested awards are treated as discrete items in the reporting period in which they occur. The Company also recognizes excess tax benefits regardless of whether the benefit reduces taxes payable in the current period. The Company has applied the modified retrospective adoption approach beginning in 2017 and prior periods have not been adjusted. As a result, the Company established a net operating loss deferred tax asset of $76.7 million to account for prior period excess tax benefits through retained earnings, however an offsetting valuation allowance of $76.7 million will also be established through retained earnings because it is not more likely than not that the deferred tax asset will be realized due to historical and expected future losses, such that there is no impact on the Company’s condensed consolidated financial statements. The Company also elected to account for forfeitures as they occur, and recorded a cumulative catch up of $0.5 million within additional paid-in capital and retained earnings upon adoption. In August 2016, the FASB issued ASU 2016-15, Statement of Cash Flows: Classification of Certain Cash Receipts and Cash Payments (“Topic 230”). The new standard clarifies certain aspects of the statement of cash flows, including the classification of contingent consideration payments made after a business combination and several other clarifications not currently applicable to the Company. The new standard also clarifies that an entity should determine each separately identifiable source or use within the cash receipts and cash payments on the basis of the nature of the underlying cash flows. In situations in which cash receipts and payments have aspects of more than one class of cash flows and cannot be separated by source or use, the appropriate classification should depend on the activity that is likely to be the predominant source or use of cash flows for the item. The new standard will be effective for the Company on January 1, 2018. The adoption of this standard is not expected to have a material impact on the Company’s condensed consolidated statements of cash flows upon adoption. In November 2016, the FASB issued ASU 2016-18, Statement of Cash Flows: Restricted Cash (“ASU 2016-18”). The amendments in this update require that amounts generally described as restricted cash and restricted cash equivalents be included within cash and cash equivalents when reconciling the beginning-of-period and end-of-period total amounts shown on the statement of cash flows. ASU 2016-18 will be effective January 1, 2018, with early adoption permitted. As of March 31, 2017, the Company has not elected to early adopt this guidance, but expects the adoption to have an impact on its consolidated statement of cash flows as, upon adoption, it will include the Company’s restricted cash balance in the cash and cash equivalents reconciliation of operating, investing and financing activities. In April 2017, the FASB issued ASU 2017-08, Receivables – Nonrefundable Fees and Other Costs (“Subtopic 310-20”). The new standard amends the amortization period for certain purchased callable debt securities held at a premium by shortening the amortization period for the premium to the earliest call date. Subtopic 310-20 calls for a modified retrospective application under which . The new standard will be effective beginning January 1, 2019, and early adoption is permitted for public entities. financial position or results of operations upon adoption.

Summary of significant accoun20

Summary of significant accounting policies and basis of presentation (Tables)	3 Months Ended
	Mar. 31, 2017
Accounting Policies [Abstract]
Estimated Useful Lives of Assets	Depreciation and amortization is calculated using the straight-line method over the estimated useful lives of the assets, which are as follows: Asset Estimated useful life Building 40 years Computer equipment and software 3 years Office and laboratory equipment 2 -5 years Leasehold improvements Shorter of the useful life or remaining lease term

Marketable Securities (Tables)

Marketable Securities (Tables)	3 Months Ended
	Mar. 31, 2017
Investments Debt And Equity Securities [Abstract]
Summary of Available for Sale Securities Held	The following table summarizes the available-for-sale securities held at March 31, 2017 and December 31, 2016 (in thousands): Description Amortized Cost Unrealized Gains Unrealized Losses Fair Value March 31, 2017 U.S. government agency securities and treasuries $ 580,667 $ 11 $ (656 ) $ 580,022 Certificates of deposit 3,840 6 — 3,846 Total $ 584,507 $ 17 $ (656 ) $ 583,868 December 31, 2016 U.S. government agency securities and treasuries $ 600,001 $ 34 $ (575 ) $ 599,460 Certificates of deposit 6,480 6 (3 ) 6,483 Total $ 606,481 $ 40 $ (578 ) $ 605,943

Fair value measurements (Tables

Fair value measurements (Tables)	3 Months Ended
	Mar. 31, 2017
Fair Value Disclosures [Abstract]
Recorded Amount of Assets and Liabilities Measured at Fair Value on Recurring Basis	The following table sets forth the Company’s assets and liabilities that are measured at fair value on a recurring basis as of March 31, 2017 and December 31, 2016 (in thousands): Description Total Quoted prices in active markets (Level 1) Significant other observable inputs (Level 2) Significant unobservable inputs (Level 3) March 31, 2017 Assets: Cash and cash equivalents $ 216,001 $ 209,309 $ 6,692 $ — Marketable securities: U.S. government agency securities and treasuries 580,022 — 580,022 — Certificates of deposit 3,846 — 3,846 — Total assets $ 799,869 $ 209,309 $ 590,560 $ — Liabilities: Contingent consideration $ 9,189 $ — $ — $ 9,189 Total liabilities $ 9,189 $ — $ — $ 9,189 December 31, 2016 Assets: Cash and cash equivalents $ 278,887 $ 278,887 $ — $ — Marketable securities: U.S. government agency securities and treasuries 599,460 — 599,460 — Certificates of deposit 6,483 — 6,483 — Total assets $ 884,830 $ 278,887 $ 605,943 $ — Liabilities: Contingent consideration $ 7,756 $ — $ — $ 7,756 Total liabilities $ 7,756 $ — $ — $ 7,756
Roll-Forward of Fair Value of the Company's Contingent Consideration Obligations	The table below provides a roll-forward of fair value of the Company’s contingent consideration obligations, which include Level 3 inputs (in thousands): Three Months Ended March 31, 2017 Beginning balance $ 7,756 Additions — Changes in fair value 1,433 Payments — Ending balance $ 9,189

Property and equipment, net (Ta

Property and equipment, net (Tables)	3 Months Ended
	Mar. 31, 2017
Property Plant And Equipment [Abstract]
Summary of Property and Equipment Net	Property and equipment, net, consists of the following (in thousands): March 31, December 31, Building 122,451 — Computer equipment and software 5,528 1,655 Office equipment 2,977 1,427 Laboratory equipment 16,497 16,305 Leasehold improvements 13,697 13,697 Construction-in-progress 34,854 136,315 Total property and equipment, gross 196,004 169,399 Less accumulated depreciation and amortization (14,284 ) (12,444 ) Total Property and equipment, net $ 181,720 $ 156,955

Accrued expenses and other cu24

Accrued expenses and other current liabilities (Tables)	3 Months Ended
	Mar. 31, 2017
Payables And Accruals [Abstract]
Summary of Accrued Expenses and Other Current Liabilities	Accrued expenses and other current liabilities consist of the following (in thousands): March 31, 2017 December 31, 2016 Employee compensation $ 6,251 $ 11,296 Accrued goods and services 29,538 34,275 Accrued license and milestone fees 285 2,464 Accrued professional fees 1,393 1,492 Deferred rent, current portion — 11 Contingent consideration, current portion 6,059 4,479 Other 781 643 Total accrued expenses and other current liabilities $ 44,307 $ 54,660

Stock-based compensation (Table

Stock-based compensation (Tables)	3 Months Ended
	Mar. 31, 2017
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Summary of Stock-Based Compensation Expense by Award Type	Stock-based compensation expense by award type included within the condensed consolidated statements of operations and comprehensive loss was as follows (in thousands): Three Months Ended March 31, 2017 2016 Stock options $ 9,342 $ 8,926 Restricted stock units 1,956 1,127 Employee stock purchase plan 183 90 $ 11,481 $ 10,143
Schedule of Stock-Based Compensation Expense by Classification	Stock-based compensation expense by classification included within the condensed consolidated statements of operations and comprehensive loss was as follows (in thousands): Three Months Ended March 31, 2017 2016 Research and development $ 5,654 $ 4,681 General and administrative 5,827 5,462 $ 11,481 $ 10,143
Summary of Stock Option Activity Under Plan	The following table summarizes the stock option activity under the Company’s equity award plans (shares in thousands): Shares Weighted-average exercise price per share Outstanding at December 31, 2016 3,735 $ 52.17 Granted 696 $ 76.37 Exercised (181 ) $ 15.18 Canceled or forfeited (47 ) $ 88.88 Outstanding at March 31, 2017 4,203 $ 57.35 Exercisable at March 31, 2017 2,070 $ 42.63 Vested and expected to vest at March 31, 2017 4,203 $ 57.35
Restricted Stock Units [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Summary of Restricted Common Stock Awards	The following table summarizes the restricted stock unit activity under the Company’s equity award plans (shares in thousands): Shares Weighted-average grant date fair value Unvested balance at December 31, 2016 263 $ 63.07 Granted 205 $ 78.88 Vested (39 ) $ 50.08 Forfeited (3 ) $ 57.10 Unvested balance at March 31, 2017 426 $ 71.92

Net loss per share (Tables)

Net loss per share (Tables)	3 Months Ended
	Mar. 31, 2017
Earnings Per Share [Abstract]
Common Stock Equivalents Excluded from Calculation of Diluted Net Loss Per Share	The following common stock equivalents were excluded from the calculation of diluted net loss per share for the periods indicated because including them would have had an anti-dilutive effect (in thousands): Three Months Ended March 31, 2017 2016 Outstanding stock options 4,203 4,104 Restricted stock units 426 320 ESPP shares 11 10 4,640 4,434

Description of the business - A

Description of the business - Additional Information (Detail)	3 Months Ended
	Mar. 31, 2017
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Date of incorporation	Apr. 16, 1992

Summary of significant accoun28

Summary of significant accounting policies and basis of presentation - Additional Information (Detail) $ in Millions	Sep. 21, 2015	Mar. 31, 2017USD ($)Segment
Basis Of Presentation And Significant Accounting Policies [Line Items]
Number of operating segment | Segment		1
ASU 2016-09 [Member]
Basis Of Presentation And Significant Accounting Policies [Line Items]
Deferred tax assets, net operating loss		$ 76.7
Deferred tax assets, valuation allowance		76.7
ASU 2016-09 [Member] | Additional Paid-in Capital and Retained Earnings [Member]
Basis Of Presentation And Significant Accounting Policies [Line Items]
Cumulative catch up to account for forfeitures		$ 0.5
Lease starting on October 1, 2016 [Member] | 60 Binney Street Lease [Member]
Basis Of Presentation And Significant Accounting Policies [Line Items]
Lease expiration year	2,027
Additional lease period extendable	10 years

Summary of significant accoun29

Summary of significant accounting policies and basis of presentation - Estimated Useful Lives of Assets (Detail)	3 Months Ended
	Mar. 31, 2017
Building [Member]
Property Plant And Equipment [Line Items]
Estimated useful lives of assets	40 years
Computer Equipment and Software [Member]
Property Plant And Equipment [Line Items]
Estimated useful lives of assets	3 years
Office and Laboratory Equipment [Member] | Minimum [Member]
Property Plant And Equipment [Line Items]
Estimated useful lives of assets	2 years
Office and Laboratory Equipment [Member] | Maximum [Member]
Property Plant And Equipment [Line Items]
Estimated useful lives of assets	5 years
Leasehold Improvements [Member]
Property Plant And Equipment [Line Items]
Leasehold improvements	Shorter of the useful life or remaining lease term

Marketable securities - Summary

Marketable securities - Summary of Available for Sale Securities Held (Detail) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	$ 584,507	$ 606,481
Unrealized Gains	17	40
Unrealized Losses	(656)	(578)
Fair Value	583,868	605,943
U.S. Government Agency Securities and Treasuries [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	580,667	600,001
Unrealized Gains	11	34
Unrealized Losses	(656)	(575)
Fair Value	580,022	599,460
Certificates of Deposit [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	3,840	6,480
Unrealized Gains	6	6
Unrealized Losses		(3)
Fair Value	$ 3,846	$ 6,483

Fair value measurements - Recor

Fair value measurements - Recorded Amount of Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - Fair Value, Measurements, Recurring [Member] - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Assets:
Cash and cash equivalents	$ 216,001	$ 278,887
Total assets	799,869	884,830
Liabilities:
Contingent consideration	9,189	7,756
Total liabilities	9,189	7,756
U.S. Government Agency Securities and Treasuries [Member]
Assets:
Marketable securities	580,022	599,460
Certificates of Deposit [Member]
Assets:
Marketable securities	3,846	6,483
Quoted prices in active markets (Level 1) [Member]
Assets:
Cash and cash equivalents	209,309	278,887
Total assets	209,309	278,887
Significant other observable inputs (Level 2) [Member]
Assets:
Cash and cash equivalents	6,692
Total assets	590,560	605,943
Significant other observable inputs (Level 2) [Member] | U.S. Government Agency Securities and Treasuries [Member]
Assets:
Marketable securities	580,022	599,460
Significant other observable inputs (Level 2) [Member] | Certificates of Deposit [Member]
Assets:
Marketable securities	3,846	6,483
Significant unobservable inputs (Level 3) [Member]
Liabilities:
Contingent consideration	9,189	7,756
Total liabilities	$ 9,189	$ 7,756

Fair value measurements - Addit

Fair value measurements - Additional Information (Detail) - USD ($)	3 Months Ended
	Mar. 31, 2017	Dec. 31, 2016
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents maturities	Three months or less
Realized gain on available-for-sale securities	$ 0
Reclassification out of accumulated other comprehensive loss	0
Unrealized Loss on Securities	494,200,000	$ 376,100,000
Unrealized loss on securities, more than twelve months	53,300,000	95,500,000
Investments with other-than-temporary impairment	0	0
Contingent consideration, current	6,059,000	4,479,000
Contingent consideration, non current	3,130,000	$ 3,277,000
Pregenen [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Contingent cash payments	129,000,000
Pregenen [Member] | Preclinical Milestones [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Contingent cash payments	9,000,000
Pregenen [Member] | Clinical Milestone Payments [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Contingent cash payments	$ 20,100,000
Pregenen [Member] | Remaining Preclinical Milestones [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Milestone achievement end date	Jun. 30, 2017
Pregenen [Member] | Commercial Milestones Payments [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Contingent cash payments	$ 99,900,000
Minimum [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Milestone achievement period	2,017
Milestone discount rates	13.20%
Minimum [Member] | Pregenen [Member] | Other Milestone [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Milestone achievement period	2,021
Maximum [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Milestone achievement period	2,026
Milestone discount rates	15.40%
Maximum [Member] | Pregenen [Member] | Other Milestone [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Milestone achievement period	2,028

Fair value measurements - Roll-

Fair value measurements - Roll-Forward of Fair Value of the Company's Contingent Consideration Obligations (Detail) - Significant unobservable inputs (Level 3) [Member] - Contingent consideration obligations [Member] $ in Thousands	3 Months Ended
	Mar. 31, 2017USD ($)
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Beginning balance	$ 7,756
Changes in fair value	1,433
Ending balance	$ 9,189

Property and equipment, net - S

Property and equipment, net - Summary of Property and Equipment Net (Detail) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Property Plant And Equipment [Line Items]
Total property and equipment, gross	$ 196,004	$ 169,399
Less accumulated depreciation and amortization	(14,284)	(12,444)
Total Property and equipment, net	181,720	156,955
Building [Member]
Property Plant And Equipment [Line Items]
Total property and equipment, gross	122,451
Computer Equipment and Software [Member]
Property Plant And Equipment [Line Items]
Total property and equipment, gross	5,528	1,655
Office Equipment [Member]
Property Plant And Equipment [Line Items]
Total property and equipment, gross	2,977	1,427
Laboratory Equipment [Member]
Property Plant And Equipment [Line Items]
Total property and equipment, gross	16,497	16,305
Leasehold Improvements [Member]
Property Plant And Equipment [Line Items]
Total property and equipment, gross	13,697	13,697
Construction in Progress [Member]
Property Plant And Equipment [Line Items]
Total property and equipment, gross	$ 34,854	$ 136,315

Property and equipment, net - A

Property and equipment, net - Additional Information (Detail) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 196,004	$ 169,399
60 Binney Street [Member]
Property Plant And Equipment [Line Items]
Property and equipment, gross	153,200
Construction cost incurred by landlord	147,900
Construction In Progress 60 Binney Street [Member]
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 30,800	126,900
Construction cost incurred by landlord		$ 120,100

Accrued expenses and other cu36

Accrued expenses and other current liabilities - Summary of Accrued Expenses and Other Current Liabilities (Detail) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Payables And Accruals [Abstract]
Employee compensation	$ 6,251	$ 11,296
Accrued goods and services	29,538	34,275
Accrued license and milestone fees	285	2,464
Accrued professional fees	1,393	1,492
Deferred rent, current portion		11
Contingent consideration, current portion	6,059	4,479
Other	781	643
Total accrued expenses and other current liabilities	$ 44,307	$ 54,660

Commitments and contingencies -

Commitments and contingencies - Additional Information (Detail) € in Millions

Mar. 31, 2017USD ($)

Nov. 18, 2016EUR (€)

Jun. 03, 2016USD ($)

Sep. 21, 2015USD ($)ft²$ / ft²

Jun. 29, 2015USD ($)ft²

Dec. 31, 2014USD ($)

Jun. 03, 2013USD ($)ft²

Jan. 31, 2017USD ($)

Mar. 31, 2017USD ($)

Dec. 31, 2016EUR (€)

Sep. 30, 2016USD ($)

Dec. 31, 2016USD ($)

Commitments And Contingencies Disclosure [Line Items]

Lease period

9 years

Lease building space | ft²

43,600

Amendment effective date

2014-06

Additional lease building space | ft²

9,900

Lease payments

$ 200,000

Increase in monthly lease payments

$ 300,000

Contribution from the landlord towards the initial build-out of the space

7,800,000

Cash-collateralized irrevocable standby letter of credit

$ 600,000

$ 1,300,000

Lease starting date

Jun. 3, 2013

Lease termination date

Apr. 12, 2017

Term of agreement

5 years

Renewal period of agreement

3 years

Up-front fee | €

€ 3

Up-front fee paid | €

€ 2

Up-front fee payable in mid-2018 | €

1

Annual maintenance and production fees | €

€ 9.8

Termination description

The Company may terminate this agreement with six months’ notice and a one-time termination fee prior to July 1, 2018, or twelve months’ notice and a one-time termination fee thereafter.

Property and equipment, gross

196,004,000

$ 196,004,000

$ 169,399,000

Rental payment to land lord

1,500,000

Tenant improvement costs received

$ 7,800,000

Prepaid expenses and other current assets [Member]

Commitments And Contingencies Disclosure [Line Items]

Tenant improvement costs reimbursable not yet received

21,200,000

21,200,000

8,500,000

60 Binney Street Lease [Member]

Commitments And Contingencies Disclosure [Line Items]

Property and equipment, gross

153,200,000

153,200,000

126,900,000

Construction financing lease obligation

147,900,000

$ 147,900,000

$ 120,100,000

Building [Member]

Commitments And Contingencies Disclosure [Line Items]

Useful life of asset

40 years

Property and equipment, gross

$ 122,451,000

$ 122,451,000

Land [Member]

Commitments And Contingencies Disclosure [Line Items]

Non-cash rent expense

$ 500,000

Lease starting on July 13, 2015 [Member]

Commitments And Contingencies Disclosure [Line Items]

Lease period

60 months

Lease building space | ft²

15,120

Lease payments

$ 500,000

Lease starting date

Jul. 13, 2015

Early termination lease term

20 months

Operating lease description

Under the terms of the lease, the Company leased approximately 15,120 square feet starting on July 13, 2015 for $0.5 million per year in base rent, which is subject to a 3% annual rent increase plus certain operating expenses and taxes.

Operating lease, rent increase percentage

3.00%

Lease starting on January 1, 2016 [Member]

Commitments And Contingencies Disclosure [Line Items]

Lease building space | ft²

8,075

Lease payments

$ 300,000

Lease starting date

Jan. 1, 2016

Operating lease description

Under the terms of the lease, the Company has also leased an additional 8,075 square feet of office space in the same premises starting on January 1, 2016 for an additional $0.3 million per year in base rent, which is subject to a 3% annual rent increase plus certain operating expenses and taxes. The Company terminated this lease effective April 12, 2017.

Operating lease, rent increase percentage

3.00%

Strategic manufacturing agreement [Member]

Commitments And Contingencies Disclosure [Line Items]

Lease period

12 years

Lease payments

$ 5,100,000

Early termination lease term

24 months

Amount payable upon achievement of certain contractual milestones

$ 12,500,000

Milestone payments made

5,000,000

$ 3,000,000

Agreement termination description

Company may terminate this agreement any time after July 1, 2016 upon payment of a one-time termination fee and up to 24 months of fixed suite and labor fees.

Strategic manufacturing agreement [Member] | Maximum [Member]

Commitments And Contingencies Disclosure [Line Items]

Additional contractual payables to milestones

$ 8,000,000

Lease starting on October 1, 2016 [Member]

Commitments And Contingencies Disclosure [Line Items]

Lease period

120 months

Lease building space | ft²

253,108

Contribution from the landlord towards the initial build-out of the space

$ 42,400,000

Cash-collateralized irrevocable standby letter of credit

$ 9,200,000

Lease starting date

Oct. 1, 2016

Lease payments base annual rent

$ 18,400,000

Capital lease description

Under the terms of the 60 Binney Street Lease, starting on October 1, 2016, the Company will lease approximately 253,108 square feet of office and laboratory space at $72.50 per square foot per year, or $18.4 million per year in base rent, which is subject to scheduled annual rent increases of 1.75% plus certain operating expenses and taxes.

Increase in letter of credit under the terms of the lease

$ 13,800,000

Lease starting on October 1, 2016 [Member] | 60 Binney Street Lease [Member]

Commitments And Contingencies Disclosure [Line Items]

Annual lease rent per square foot | $ / ft²

72.50

Lease rent increase percentage

1.75%

Option to extend capital lease

10 years

Significant agreements - Additi

Significant agreements - Additional Information (Detail) - Celgene Corporation [Member] $ in Millions	Jun. 03, 2015USD ($)Deliverables	Mar. 19, 2013USD ($)	Mar. 31, 2017USD ($)	Mar. 31, 2016USD ($)	Dec. 31, 2016USD ($)	Feb. 17, 2016USD ($)	Feb. 16, 2016USD ($)
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Term of collaboration agreement		3 years
First Product Candidates [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Amount per product eligible to be received upon achievement of specified event	$ 10
Amount paid per product upon achievement of specified event							$ 10
Additional Product Candidates [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Amount per product eligible to be received upon achievement of specified event	$ 15
Co-Develop and Co-Promote Options not Exercise [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Amount per product eligible to be received upon achievement of specified event						$ 10
Collaborative Arrangement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Amount per product eligible to be received upon achievement of specified event			$ 77.6
Number of deliverable | Deliverables	3
Consideration allocated to agreement			$ 109
Deferred revenue recognition period			3 years
Deferred revenue recognized			$ 6.8	$ 1.5
Percentage of revenue related to worldwide development cost			67.50%
Percentage of worldwide development costs for responsible collaborator			100.00%
Revenue recognized related to other development costs incurred			$ 0.4
Deferred revenue			42.4		$ 46.4
Cost reimbursement receivable for product development costs incurred			$ 2.9
Collaborative Arrangement [Member] | Co-Develop and Co-Promote Options not Exercise [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Percentage of revenue related to worldwide development cost			32.50%
Collaborative Arrangement [Member] | Option Fee [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Amount per product eligible to be received upon achievement of specified event			$ 10
Collaborative Arrangement [Member] | Delivered Elements [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration agreement, cash payment received	$ 20
Consideration allocated to agreement			17.3
Up-front Payment Arrangement [Member] | Collaborative Arrangement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration agreement, cash payment received		$ 75	25
Up-front Payment Arrangement [Member] | Amended Collaborative Arrangement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration agreement, cash payment received	$ 25
Manufacturing Services [Member] | Collaborative Arrangement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Consideration allocated to agreement			54.1
Deferred revenue recognized			$ 4.9

Stock-based compensation - Addi

Stock-based compensation - Additional Information (Detail) - USD ($) $ in Millions	1 Months Ended	3 Months Ended
	Jan. 31, 2017	Mar. 31, 2017	Mar. 31, 2016
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Increased number of issuance of awards under the 2013 Plan	1,600,000
Number of shares available for issuance		2,000,000
Stock option share exercised		181,000
Proceed from option share exercised		$ 2.7
Stock Options [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Unrecognized stock- based compensation expense related to unvested stock options, restricted stock awards and employee stock purchase plan		$ 121.7
Expected weighted-average period related to unvested stock options, restricted stock awards and employee stock purchase plan		2 years 9 months 18 days
Restricted Stock Units [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Unrecognized stock- based compensation expense related to unvested stock options, restricted stock awards and employee stock purchase plan		$ 121.7
Expected weighted-average period related to unvested stock options, restricted stock awards and employee stock purchase plan		2 years 9 months 18 days
Employee Stock Purchase Plan [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Unrecognized stock- based compensation expense related to unvested stock options, restricted stock awards and employee stock purchase plan		$ 121.7
Expected weighted-average period related to unvested stock options, restricted stock awards and employee stock purchase plan		2 years 9 months 18 days
Common shares reserved for future issuance		238,000
Shares of common stock issued under plan		11,079	9,758

Stock-based compensation - Summ

Stock-based compensation - Summary of Stock-Based Compensation Expense by Award Type (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	$ 11,481	$ 10,143
Stock Options [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	9,342	8,926
Restricted Stock Units [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	1,956	1,127
Employee Stock Purchase Plan [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	$ 183	$ 90

Stock-based compensation - Sche

Stock-based compensation - Schedule of Stock-Based Compensation Expense by Classification (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	$ 11,481	$ 10,143
Research And Development [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	5,654	4,681
General And Administrative [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	$ 5,827	$ 5,462

Stock-based compensation - Su42

Stock-based compensation - Summary of Stock Option Activity Under Plan (Detail) shares in Thousands	3 Months Ended
	Mar. 31, 2017$ / sharesshares
Shares
Outstanding at beginning of period | shares	3,735
Granted | shares	696
Exercised | shares	(181)
Canceled or forfeited | shares	(47)
Outstanding at end of period | shares	4,203
Exercisable at end of period | shares	2,070
Vested and expected to vest at end of period | shares	4,203
Weighted-average exercise price per share
Outstanding at beginning of period | $ / shares	$ 52.17
Granted | $ / shares	76.37
Exercised | $ / shares	15.18
Canceled or forfeited | $ / shares	88.88
Outstanding at end of period | $ / shares	57.35
Exercisable at end of period | $ / shares	42.63
Vested and expected to vest at end of period | $ / shares	$ 57.35

Stock-based compensation - Su43

Stock-based compensation - Summary of Restricted Stock Units (Detail) - Restricted Stock Units [Member] shares in Thousands	3 Months Ended
	Mar. 31, 2017$ / sharesshares
Shares
Unvested balance at beginning of period | shares	263
Granted | shares	205
Vested | shares	(39)
Forfeited | shares	(3)
Unvested balance at end of period | shares	426
Weighted-average grant date fair value
Unvested balance at beginning of period | $ / shares	$ 63.07
Granted | $ / shares	78.88
Vested | $ / shares	50.08
Forfeited | $ / shares	57.10
Unvested balance at end of period | $ / shares	$ 71.92

Net loss per share - Common Sto

Net loss per share - Common Stock Equivalents Excluded from Calculation of Diluted Net Loss Per Share (Detail) - shares shares in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Common stock equivalents excluded from the calculation of diluted net loss per share	4,640	4,434
Outstanding Stock Options [Member]
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Common stock equivalents excluded from the calculation of diluted net loss per share	4,203	4,104
Restricted Stock Units [Member]
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Common stock equivalents excluded from the calculation of diluted net loss per share	426	320
ESPP Shares [Member]
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Common stock equivalents excluded from the calculation of diluted net loss per share	11	10

Subsequent Events - Additional

Subsequent Events - Additional Information (Detail) - Subsequent Event [Member] - USD ($) $ in Millions	Apr. 28, 2017	Apr. 27, 2017
Novartis Pharma AG [Member]
Subsequent Event [Line Items]
License agreement upfront payment		$ 7.5
License agreement, potential future milestone payments		7.5
Payments for subsequently licensed products and low single digit royalty payments on net sales of covered products		$ 1.1
GlaxoSmithKline Intellectual Property Development Limited [Member]
Subsequent Event [Line Items]
License agreement upfront payment	$ 3
License agreement, potential future milestone payments	$ 1.3