Stock-Based Compensation (Tables)

12 Months Ended

Dec. 31, 2021

Summary of Stock Option Activity

Stock option activity under the Plans is set forth below:

	Outstanding Awards
		Number of Shares Underlying Outstanding Options (in thousands)			Weighted Average Exercise Price		Weighted Average Remaining Contractual Term (in years)		Aggregate Intrinsic Value (in thousands)
Balance—December 31, 2020			5,514		$	13.24		6.84	$	38,433
Granted			1,499			15.22
Exercised			(525	)		4.61
Canceled or forfeited			(646	)		29.55
Balance—December 31, 2021			5,842		$	12.73
Options exercisable December 31, 2021			2,459		$	14.83	4.74		$	17,255
Vested and expected to vest, December 31, 2021			5,842		$	12.73		7.00	$	34,928

Summary of RSU Activity

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, 2020		441		$	14.51
Granted		588			14.87
Vested		(141	)		14.52
Forfeited		(65	)		14.69
Balance, December 31, 2021		823		$	14.76

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:

	Years Ended December 31,
	2021		2020
Assumptions:
Expected term (in years)	6.1		6.1
Volatility	78.8%		77.6% – 90.0%
Risk-free interest rate	0.9%		0.1% – 0.9%
Dividend yield	0%		0%
Fair Value:
Weighted-average grant date fair value per share	$	10.34	$	9.99

Summary of Stock-Based Compensation

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

	Years Ended December 31,
	2021		2020
Research and development	$	6,007	$	1,759
General and administrative		6,778		1,852
Total stock-based compensation expense	$	12,785	$	3,611

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 December 31, 2021
Stock-options
Unrecognized stock compensation expense	$	24.7
Weighted-average remaining vesting period (years)	2.7
RSU
Unrecognized stock compensation expense	$	9.7
Weighted-average remaining vesting period (years)	2.1
ESPP
Unrecognized stock compensation expense	$	0.2
Weighted-average remaining vesting period (years)	0.4

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:

	Years Ended December 31,
	2021		2020
Assumptions:
Expected term (in years)		0.5		0.5
Volatility	63.3%		81.5% – 127.5%
Risk-free interest rate	0.1%		0.1% – 0.2%
Dividend yield	—		—
Fair Value:
Weighted-average grant date fair value per share	$	5.16	$	5.42