Stockholders' Equity (Tables)

9 Months Ended

Sep. 30, 2021

Equity [Abstract]

Schedule of Shares of Common Stock Reserved for Issuance

As of September 30, 2021 and December 31, 2020, the Company had reserved shares of common stock for issuance as follows:

	September 30, 2021	December 31, 2020
Reserve balance for Sales Agreement	14,182,900	14,190,300
Common stock options outstanding	10,660,410	10,017,918
Common stock options available for grant	6,739,933	6,186,497
ESPP shares available for purchase	590,219	700,074
401(k) Matching Plan	17,813	21,930
Total	32,191,275	31,116,719

Summary of Activity under 2008 Plan and 2018 Plan

A summary of the activity under the 2008 Plan and the 2018 Plan is as follows:

	Outstanding Options			Weighted Average Remaining Contractual Life (In Years)	Aggregate Intrinsic Value (In Thousands)
	Number of Options	Weighted Average Exercise Price
Balances at December 31, 2020	10,017,918	$	10.52	6.45	$	198,097
Options granted	2,762,183	30.02
Options exercised	(1,627,867)	6.81
Options cancelled	(491,824)	21.55
Balances at September 30, 2021	10,660,410	$	15.63	6.83	$	80,898
Vested and expected to vest at September 30, 2021	10,467,350	$	15.46	6.79	$	80,604
Exercisable at September 30, 2021	8,647,960	$	12.31	6.23	$	80,387

Summary of Stock Based Compensation Expense

Employee and director stock-based compensation expense was allocated as follows (in thousands):

	Three Months Ended September 30,				Nine Months Ended September 30,
	2021		2020		2021		2020
Research and development	$	3,319	$	2,117	$	10,485	$	6,039
General and administrative	2,973		1,867		8,962		5,300
Total stock-based compensation expense	$	6,292	$	3,984	$	19,447	$	11,339