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Stock-based compensation
 3 Months Ended
Mar. 31, 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. As of March 31, 2024, the total number of shares of common stock available for issuance under the 2023 Plan was approximately 2.1 million.
Stock-based compensation expense
The Company recognized stock-based compensation expense totaling $3.8 million and $5.4 million during the three months ended March 31, 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 March 31,

20242023
Stock options$1,230 $1,723 
Restricted stock units2,451 3,668 
Employee stock purchase plan and other80 — 
$3,761 $5,391 
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 March 31,
20242023
Cost of product revenue491 103 
Selling, general and administrative2,335 2,361 
Research and development935 2,927 
$3,761 $5,391 
During the three months ended March 31, 2024 and 2023, the Company had $0.3 million and $0.5 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 
Granted2,572 $1.56 
Exercised— $— 
Canceled, forfeited, or expired(229)$16.55 
Outstanding at March 31, 20246,570 $9.16 
Exercisable at March 31, 20241,871 $22.75 
Vested and expected to vest at March 31, 20246,570 $9.16 
During the three months ended March 31, 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 
Granted2,177 $1.56 
Vested(785)$7.14 
Forfeited(205)$4.41 
Unvested at March 31, 20245,394 $4.16 
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 three months ended March 31, 2024 and 2023, no shares and less than 0.1 million shares, respectively, of common stock were issued under the 2013 ESPP.