Stock-based Compensation

3 Months Ended

Mar. 31, 2021

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Stock-based Compensation

9. Stock-based Compensation

During the three months ended March 31, 2021, the Company recognized the following stock-based compensation expense (in thousands):

	Three Months Ended March 31,
	2021		2020
Research and development	$	12,845	$	7,362
General and administrative		9,247		6,789
Total	$	22,092	$	14,151

Stock option activity

The following table summarizes stock option activity for the three months ended March 31, 2021:

	Shares			Weighted- average exercise price per share
Outstanding at December 31, 2020		8,101,980		$	42.44
Granted		882,558			139.05
Exercised		(342,471	)		33.12
Cancelled or forfeited		(69,404	)		43.31
Outstanding at March 31, 2021		8,572,663		$	52.76
Exercisable at March 31, 2021		3,962,897		$	31.48
Vested and expected to vest at March 31, 2021		8,572,663		$	52.76

As of March 31, 2021, total unrecognized compensation expense related to stock options was $191.3 million, which the Company expects to recognize over a remaining weighted-average period of 3.1 years.

Restricted stock activity

The following table summarizes restricted stock activity for the three months ended March 31, 2021:

	Restricted Stock			Weighted- Average Grant Date Fair Value
Unvested balance as of December 31, 2020		894,092		$	70.55
Granted		278,645			136.16
Vested		(109,355	)		40.64
Cancelled or forfeited		(11,003	)		57.94
Unvested balance as of March 31, 2021		1,052,379		$	91.16

As of March 31, 2021, total unrecognized compensation expense related to unvested restricted common shares was $76.5 million, which the Company expects to recognize over a remaining weighted-average vesting period of 2.8 years.