Stock-Based Compensation (Tables)

3 Months Ended

Mar. 31, 2022

Summary of Stock Option Activity

Stock option activity under the Plans is set forth below:

	Number of Shares Underlying Options (in thousands)			Weighted- Average Exercise Price		Weighted Average Remaining Contractual Term (in years)		Aggregate Intrinsic Value (in thousands)
Balance—December 31, 2021		5,842		$	12.73		7.0	$	34,928
Granted		1,624			13.03
Exercised		(245	)		8.37
Canceled or forfeited		(278	)		8.13
Balance—March 31, 2022		6,943		$	13.13	7.5		$	36,350
Options exercisable—March 31, 2022		2,651		$	14.83	5.1		$	17,638
Options vested and expected to vest—March 31, 2022		6,943		$	13.13	7.5		$	36,350

Summary of Restricted Stock Unit ("RSU") Activity

Restricted stock unit (“RSU”) activity under the Plans is set forth below:  

	RSUs Outstanding
	Number of RSUs (in thousands)			Weighted- Average Grant Date Fair Value Per Share
Balance—December 31, 2021		823		$	14.76
Granted		532			13.10
Vested		(112	)		15.20
Forfeited		(36	)		15.07
Balance—March 31, 2022		1,207		$	13.98

Schedule of Weighted-average Assumptions Used to Estimate Fair Value of Stock Options Using Black Scholes Option-Pricing Model

The weighted-average assumptions used to estimate the fair value of stock options using the Black-Scholes option-pricing model and the resulting weighted-average grant date fair value were as follows:

	Three Months Ended March 31,
	2022		2021
Assumptions:
Expected term (in years)	6.2		6.1
Volatility	77.8%		79.2%
Risk-free interest rate	1.7%		0.7%
Dividend yield		—		—
Fair Value:
Weighted-average grant date fair value per share	$	8.94	$	10.33

Summary of Stock-Based Compensation

Total stock-based compensation expense recognized was as follows (in thousands):

	Three Months Ended March 31,
	2022		2021
Research and development	$	2,185	$	1,028
General and administrative		2,084		1,450
Total stock-based compensation expense	$	4,269	$	2,478

Total Unrecognized Stock-Based Compensation Expense and Expected Period Over Which Such Expense Will Be Recognized

Total unrecognized stock-based compensation expense and the expected period over which such expense will be recognized are as follows ($ in millions):

	As of March 31, 2022
Stock options
Unrecognized stock compensation expense	$	34.5
Weighted-average remaining vesting period (years)	2.9
RSU
Unrecognized stock compensation expense	$	14.8
Weighted-average remaining vesting period (years)	2.3
ESPP
Unrecognized stock compensation expense	$	0.1
Weighted-average remaining vesting period (years)	0.1

2015 ESPP

Schedule of Black Scholes Option-Pricing Model

The weighted-average assumptions used to estimate the fair value of our common stock to be issued under the 2015 ESPP using a Black-Scholes option-pricing model and the resulting weighted-average grant date fair value were as follows:

	Three Months Ended March 31,
	2022		2021
Assumptions:
Expected term (in years)		0.5		0.5
Volatility	64.9%		81.5%
Risk-free interest rate	0.1%		0.1%
Dividend yield		—		—
Fair Value:
Weighted-average grant date fair value per share	$	5.43	$	5.42