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Stock-based compensation
 9 Months Ended
Sep. 30, 2024
Share-Based Payment Arrangement [Abstract]  
Stock-based compensation Stock-based compensation
In June 2023, the Company’s stockholders approved the bluebird bio, Inc. 2023 Incentive Award Plan (the “2023 Plan”), which replaced the 2013 Stock Option and Incentive Plan (“2013 Plan”). Following approval of the 2023 Plan, no further awards will be granted under the 2013 Plan. The Company also maintains the 2021 Inducement Plan (the “2021 Plan”), pursuant to which equity-based awards may be granted to new hires. As of September 30, 2024, the total number of shares of common stock available for issuance under the 2023 Plan was approximately 1.5 million, and the total number of shares of common stock available for issuance under the 2021 Plan was approximately 0.6 million.
Stock-based compensation expense
The Company recognized stock-based compensation expense totaling $2.7 million and $4.9 million during the three months ended September 30, 2024 and 2023, respectively. The Company recognized stock-based compensation expense totaling $9.7 million and $16.0 million during the nine months ended September 30, 2024 and 2023, respectively. Stock-based compensation expense recognized by award type is included within the condensed consolidated statements of operations and comprehensive loss was as follows (in thousands):
For the three months ended September 30,For the nine months ended September 30,

2024202320242023
Stock options$920 $1,758 $3,343 $5,192 
Restricted stock units1,755 2,916 6,217 10,552 
Employee stock purchase plan and other26 194 109 269 
$2,701 $4,868 $9,669 $16,013 
Stock-based compensation expense by classification included within the condensed consolidated statements of operations and comprehensive loss was as follows (in thousands): 
For the three months ended September 30,For the nine months ended September 30,
2024202320242023
Cost of product revenue102 342 882 435 
Selling, general and administrative1,650 2,461 5,783 7,503 
Research and development681 2,065 2,736 8,075 
Restructuring Expenses268 — 268 — 
$2,701 $4,868 $9,669 $16,013 
During the nine months ended September 30, 2024 and 2023, the Company had $1.0 million and $1.4 million of stock compensation expense that was capitalized into inventory.
Stock options
The following table summarizes the stock option activity under the Company’s equity award plans excluding awards held by employees of 2seventy bio:
Shares
(in thousands)
Weighted-
average
exercise price
per share
Outstanding at December 31, 20234,227 $14.16 
Granted3,010 $1.47 
Exercised— $— 
Canceled, forfeited, or expired(905)$10.52 
Outstanding at September 30, 20246,332 $8.66 
Exercisable at September 30, 20242,290 $18.54 
Vested and expected to vest at September 30, 20246,332 $8.66 
During the nine months ended September 30, 2024, no stock options were exercised.
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 at December 31, 20234,207 $6.08 
Granted3,357 $1.23 
Vested(1,083)$7.08 
Forfeited(907)$4.05 
Unvested at September 30, 20245,574 $3.29 
Employee stock purchase plan
In June 2013, the Company's board of directors adopted its 2013 Employee Stock Purchase Plan (“2013 ESPP”), which authorized 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 1.4 million shares of the Company’s common stock available to participating employees. During each of the nine months ended September 30, 2024 and 2023, 0.1 million shares and 0.1 million shares, respectively, of common stock were issued under the 2013 ESPP.