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Stock-based compensation
 6 Months Ended
Jun. 30, 2023
Share-Based Payment Arrangement [Abstract]  
Stock-based compensation Stock-based compensation
In January 2023 and 2022, the number of shares of common stock available for issuance under the 2013 Stock Option and Incentive Plan (“2013 Plan”) was increased by approximately 3.3 million and 2.8 million shares, respectively, as a result of the automatic increase provision of the 2013 Plan. On June 16, 2023, the Company’s stockholders approved the 2023 Incentive Award Plan (the “2023 Plan”), which replaced the 2013 Plan. Following approval of the 2023 Plan, no further awards will be granted under the 2013 Plan. As of June 30, 2023, the total number of shares of common stock available for issuance under the 2023 Plan was approximately 5.0 million.
Stock-based compensation expense
The Company recognized stock-based compensation expense totaling $5.8 million and $8.9 million during the three months ended June 30, 2023 and 2022, respectively. The Company recognized stock-based compensation expense totaling $11.1 million and $21.3 million for the six months ended June 30, 2023 and 2022, 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 June 30,For the six months ended June 30,

2023202220232022
Stock options
$1,710 $3,685 $3,433 $8,945 
Restricted stock units
3,969 4,850 7,637 11,884 
Employee stock purchase plan and other75 372 75 469 
$5,754 $8,907 $11,145 $21,298 
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 June 30,For the six months ended June 30,
2023202220232022
Cost of product revenue(10)— 93 — 
Selling, general and administrative2,681 3,652 5,042 9,488 
Research and development3,083 5,255 6,010 11,810 
$5,754 $8,907 $11,145 $21,298 
During the six months ended June 30, 2023, the Company had $1.1 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 and have been adjusted to reflect the effects of the November 2021 separation of our severe genetic disease and oncology programs into two separate, publicly traded companies:
Shares
(in thousands)
Weighted-
average
exercise price
per share
Outstanding at December 31, 20222,668 $24.38 
Granted
1,974 $5.04 
Exercised
(3)$2.74 
Canceled or forfeited(210)$22.51 
Outstanding at June 30, 20234,429 $15.91 
Exercisable at June 30, 20231,425 $34.27 
Vested and expected to vest at June 30, 20234,429 $15.91 
During the six months ended June 30, 2023, less than 0.1 million stock options were exercised, resulting in total proceeds to the Company of less than $0.1 million.
Restricted stock units
The following table summarizes the restricted stock unit activity under the Company’s equity award plans:
Shares
(in thousands)		Weighted-
average
grant date
fair value
Unvested at December 31, 20222,415 $11.44 
Granted
2,943 $4.97 
Vested
(420)$17.11 
Forfeited
(301)$9.10 
Unvested at June 30, 20234,637 $7.25 
Employee stock purchase plan
In June 2013, the Company 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 six months ended June 30, 2023 and 2022, less than 0.1 million shares and no shares, respectively, of common stock were issued under the 2013 ESPP.