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Stock-based compensation
 6 Months Ended
Jun. 30, 2021
Share-based Payment Arrangement [Abstract]  
Stock-based compensation Stock-based compensationIn January 2021 and 2020, the number of shares of common stock available for issuance under the 2013 Stock Option and Incentive Plan (“2013 Plan”) was increased by approximately 2.7 million and 2.2 million shares, respectively, as a result of the automatic increase provision of the 2013 Plan. As of June 30, 2021, the total number of shares of common stock available for issuance under the 2013 Plan was approximately 3.9 million.
Stock-based compensation expense
The Company recognized stock-based compensation expense totaling $31.1 million and $73.5 million for the three and six months ended June 30, 2021, respectively. The Company recognized stock-based compensation expense totaling $48.5 million and $84.8 million for the three and six months ended June 30, 2020, respectively. Stock-based compensation expense by award type 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,

2021202020212020
Stock options
$17,324 $26,073 $37,983 $50,513 
Restricted stock units
8,534 14,143 23,267 25,996 
Employee stock purchase plan and other5,193 8,313 12,275 8,313 
$31,051 $48,529 $73,525 $84,822 
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,
2021202020212020
Research and development$15,768 $23,098 $35,636 $39,367 
Selling, general and administrative15,283 25,431 37,889 45,455 
$31,051 $48,529 $73,525 $84,822 
Stock-based compensation of $0.4 million and $0.7 million was capitalized into inventory for the three and six months ended June 30, 2021, respectively. Stock-based compensation of less than $0.1 million and $0.1 million was capitalized into inventory for the three and six months ended June 30, 2020, respectively. As of June 30, 2021, the Company had approximately $193.9 million of unrecognized stock-based compensation expense, which is expected to be recognized over a weighted-average period of approximately 2.1 years.
Unrestricted stock awards
During the first quarter of 2021, the Company granted 0.4 million unrestricted stock awards to employees as part of its 2020 annual incentive program. In addition, the Company implemented a retention program designed to incentivize and retain employees through the separation of its severe genetic disease and oncology programs, which is intended to occur by the end of 2021. Under the retention program, employees are entitled to a one-time bonus payment, consisting of both a cash payment and unrestricted stock awards, with the condition that the employee remains employed at the end of 2021. For the three and six months ended June 30, 2021, respectively, the Company recognized $10.7 million and $24.0 million in expense related to this program, which includes $5.2 million and $11.9 million in stock compensation expense related to the anticipated grants of stock.
Stock option activity
The following table summarizes the stock option activity under the Company’s equity award plans:
Shares
(in thousands)
Weighted-
average
exercise price
per share
Outstanding at December 31, 20206,262 $105.02 
Granted
811 $29.52 
Exercised
(209)$6.02 
Canceled, forfeited, or expired
(765)$108.51 
Outstanding at June 30, 20216,099 $97.93 
Exercisable at June 30, 20213,854 $110.83 
Vested and expected to vest at June 30, 20216,099 $97.93 
During the six months ended June 30, 2021, 0.2 million stock options were exercised, resulting in total proceeds to the Company of $1.3 million.
Restricted stock unit activity
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 balance at December 31, 20201,495 $102.34 
Granted
1,128 $21.38 
Vested
(421)$59.35 
Forfeited
(337)$77.19 
Unvested balance at June 30, 20211,865 $59.02 
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 include 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, 2021 and 2020, less than 0.1 million shares of common stock were issued under the 2013 ESPP.