Stockholders' Equity (Tables)

9 Months Ended

Sep. 30, 2021

Equity [Abstract]

Summary of Stock Option Activity

Stock option activity for employee and nonemployee awards and related information is as follows (in thousands, except per share data):

	Number of Options			Weighted- Average Exercise Price Per Share
Outstanding at December 31, 2020		3,788		$	9.06
Granted		2,455			23.26
Exercised		(114	)		1.37
Forfeited/expired		(480	)		23.02
Outstanding at September 30, 2021		5,649		$	14.20

Schedule of Assumptions Used in Black-Scholes Model Pricing Model to to Determine Fair Value of Stock Option and Shares Purchasable Under ESPP

The assumptions used in the Black-Scholes model to determine the fair value of stock option grants and shares purchasable under the ESPP were as follows:

	Nine Months Ended September 30,
Stock Option Grants	2021	2020
Risk-free interest rate	0.5% - 1.2%	0.3% - 1.5%
Expected volatility	87% - 88%	88% - 92%
Expected term (in years)	5.5 - 6.1	5.4 - 6.1
Expected dividend yield	—%	—%

	Nine Months Ended September 30,
ESPP	2021	2020
Risk-free interest rate	0.1%	0.2%
Expected volatility	75%	96%
Expected term (in years)	0.5	0.5
Expected dividend yield	—%	—%

Schedule of Allocated Stock-based Compensation Expense

The allocation of stock-based compensation expense was as follows (in thousands):

	Three Months Ended September 30,				Nine Months Ended September 30,
	2021		2020		2021		2020
Research and development expense	$	2,433	$	565	$	6,930	$	850
General and administrative expense		2,117		731		5,610		1,582
Total stock-based compensation expense	$	4,550	$	1,296	$	12,540	$	2,432