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Stock-based compensation
 12 Months Ended
Dec. 31, 2023
Share-Based Payment Arrangement [Abstract]  
Stock-based compensation Stock-based compensation
2023 Incentive Award Plan
In June 2023, the Company’s stockholders approved the bluebird bio, Inc. 2023 Incentive Award Plan (“2023 Plan”), which became effective on June 16, 2023. The 2023 Plan replaced the 2013 Stock Option and Incentive Plan (“2013 Plan”).
The 2023 Plan allows for the granting of incentive stock options, non-qualified stock options, restricted stock units, and restricted stock awards to the Company’s employees, members of the board of directors, and consultants of the Company. The 2023 Plan initially authorized the issuance of up to 5.2 million shares of common stock.
Any awards outstanding under the Company’s 2013 Plan at the time of adoption of the 2023 Plan remained outstanding and effective. The shares of common stock underlying any awards that are forfeited, canceled, repurchased, expired or are otherwise terminated (other than by exercise) under the 2013 Plan are added to the shares of common stock available for
issuance under the 2023 Plan. The 2023 Plan will expire in 2033. As of December 31, 2023, the total number of shares of common stock that may be issued under the 2023 Plan is 5.1 million.
2021 Inducement Plan
On May 18, 2021, the Company's board of directors adopted the bluebird bio, Inc. 2021 Inducement Plan (the “Inducement Plan”) pursuant to the Nasdaq Stock Market LLC listing rules (“Rule 5635(c)(4)”). In accordance with Rule 5635(c)(4), equity-based incentive awards under the Inducement Plan may only be made to a newly hired employee who has not previously been a member of the Company's board of directors, or an employee who is being rehired following a bona fide period of non-employment by the Company as a material inducement to the employee’s entering into employment with the Company. The Inducement Plan initially reserved 600,000 shares, which was later increased to an aggregate of 1,250,000 shares in January 2022.
The exercise price of stock options granted under the Inducement Plan will not be less than the fair market value of a share of the Company's common stock on the grant date. Other terms of awards, including vesting requirements, are determined by the Company's board of directors or compensation committee and are subject to the provisions of the Inducement Plan. Stock options granted to employees under the Inducement Plan generally vest over a four-year period but may be granted with different vesting terms. Certain options may provide for accelerated vesting in the event of a change in control. Stock options granted under the Inducement Plan expire no more than 10 years from the date of grant. As of December 31, 2023, 1.1 million shares of common stock are available for future grant under the Inducement Plan.
2013 Stock Option and Incentive Plan
In June 2013, the Company’s board of directors adopted the 2013 Plan, which was subsequently approved by the Company's stockholders and became effective upon the closing of the Company’s IPO. The 2013 Plan expired in June 2023.
The 2013 Plan allowed for the granting of incentive stock options, non-qualified stock options, restricted stock units and restricted stock awards to the Company’s employees, members of the board of directors, and consultants of the Company. The Company initially reserved approximately 1.0 million shares of its common stock for the issuance of awards under the 2013 Plan. The 2013 Plan provided that the number of shares reserved and available for issuance would automatically increase each January 1, beginning on January 1, 2014, by four percent of the outstanding number of shares of common stock on the immediately preceding December 31 or such lesser number of shares as determined by the Company’s compensation committee. In January 2023 and January 2022, the number of common stock available for issuance under the 2013 Plan was increased by approximately 2.8 million and 3.3 million shares, respectively, as a result of this automatic increase provision.
Conversion and modification of equity awards outstanding at the Separation
In connection with the Separation on November 4, 2021, under the provisions of the existing plans, the Company adjusted its outstanding equity awards in accordance with the terms of the Employee Matters Agreement (an equitable adjustment) to preserve the intrinsic value of the awards immediately before and after the distribution of all of the outstanding shares of 2seventy bio common stock to our stockholders. Upon the distribution, employees holding stock options, restricted stock units (“RSUs”) and performance restricted stock units (“PRSUs”) denominated in pre-distribution bluebird stock received a number of otherwise-similar awards either in post-Distribution bluebird stock or in a combination of post-distribution bluebird stock and 2seventy bio stock based on conversion ratios outlined for each group of employees in the Employee Matters Agreement that the Company entered into in connection with the distribution. The equity awards that were granted prior to 2021 were converted under the shareholder method, wherein employees holding outstanding equity awards received equity awards in both bluebird and 2seventy bio. The conversion ratio for the shareholder method took into consideration a distribution ratio of one share of 2seventy bio common stock for every three shares of bluebird common stock. For equity awards granted in 2021, the number of awards that were outstanding at the Separation were proportionately adjusted to maintain the aggregate intrinsic value of the awards at the date of the Separation. The conversion ratio was determined based on the volume weighted-average trading price for bluebird common stock for the five trading days before and after the Separation. These modified awards otherwise retained substantially the same terms and conditions, including term and vesting provisions.
Additionally, bluebird will not incur any future compensation cost related to equity awards held by 2seventy bio employees and directors. The Company will incur future compensation cost related to 2seventy bio equity awards held by bluebird employees.
Stock-based compensation expense
The Company recognized stock-based compensation expense totaling $19.4 million and $35.1 million during the years ended December 31, 2023 and 2022, respectively. Stock-based compensation expense recognized by award type is as follows (in thousands):
Year ended December 31,
20232022
(As Restated)
Stock options$6,478 $14,168 
Restricted stock units12,619 20,138 
Employee stock purchase plan and other332 784 
$19,429 $35,090 
Stock-based compensation expense by classification included within the consolidated statements of operations and comprehensive loss was as follows (in thousands):
Year ended December 31,
20232022
(As Restated)
Cost of product revenue
$881 $— 
Selling, general and administrative9,548 15,831 
Research and development9,000 19,259 
$19,429 $35,090 
During the years ended December 31, 2023 and 2022, the Company had $1.7 million and $0.3 million of stock-based compensation expense that was capitalized into inventory, respectively.
As of December 31, 2023, the Company had $8.7 million and $18.8 million of unrecognized compensation expense related to unvested stock options and restricted stock units (exclusive of those with both service and performance conditions that have not yet been achieved), respectively, that is expected to be recognized over a weighted-average period of 1.96 years and 2.92 years, respectively.
Stock options
The fair value of each option issued to employees was estimated at the date of grant using the Black-Scholes option pricing model with the following weighted-average assumptions:
Year ended December 31,
20232022
Expected volatility70.9 %66.4 %
Expected term (in years)6.106.0
Risk-free interest rate4.1 %1.9 %
Expected dividend yield0.0 %0.0 %
The following table summarizes the stock option activity under the Company’s equity awards plans excluding awards held by employees of 2seventy bio:
Shares
(in thousands)		Weighted-
average
exercise price
per share		Weighted-
average
contractual
life
(in years)		Aggregate
intrinsic
value (a)
(in thousands)
Outstanding at December 31, 20222,668 $24.38 
Granted2,077 $4.89 
Exercised(3)$2.74 
Canceled or forfeited(515)$29.55 
Outstanding at December 31, 20234,227 $14.16 8.06$— 
Exercisable at December 31, 20231,469 $28.80 6.54$— 
Vested and expected to vest at December 31, 20234,227 $14.16 8.06$— 
(a)The aggregate intrinsic value is calculated as the difference between the exercise price of the underlying options and the fair value of the common stock for the options that were in the money at December 31, 2023.
The weighted-average fair values of options granted during the years ended December 31, 2023 and 2022 was $3.24 and $4.46, respectively. There was no intrinsic value of options exercised during the years ended December 31, 2023 and 2022, respectively.
Restricted stock units
The following table summarizes the restricted stock unit activity under the Company’s equity award plans excluding awards held by employees of 2seventy bio:
Shares
(in thousands)		Weighted-average
grant date
fair value
Unvested balance at December 31, 20222,415 $11.44 
Granted3,369 4.76 
Vested(1,057)14.35 
Forfeited(520)8.09 
Unvested balance at December 31, 20234,207 $6.08 
The intrinsic value of restricted stock units, including shares held by employees of 2seventy bio, vested during the years ended December 31, 2023 and 2022 was $5.4 million and $6.0 million, respectively. The total grant date fair value of restricted stock units vested during the years ended December 31, 2023 and December 31, 2022 was $15.2 million and $14.0 million, respectively.
Employee Stock Purchase Plan
In June 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. The 2013 ESPP authorizes the initial issuance of up to a total of 0.2 million shares of the Company’s common stock to participating employees. In June 2021, the Company amended the 2013 ESPP to authorize an additional approximately 1.4 million shares of the Company’s common stock available to participating employees. During each of the years ended December 31, 2023 and 2022, approximately 0.1 million shares of common stock were issued under the 2013 ESPP.