Stock Plans and Stock-Based Compensation (Tables)

12 Months Ended

Dec. 31, 2021

Share-based Payment Arrangement [Abstract]

Summary of Option Activity Includes Options Granted

The following table, which includes options granted under the Company’s Stock Plans, summarizes option activity:

	Shares Subject to Outstanding Options			Weighted Average Exercise Price Per Share		Weighted Average Remaining Contractual Term (in years)		Aggregate Intrinsic Value (in thousands)
Outstanding at December 31, 2020		9,892,697		$	13.88
Options granted		3,758,769		$	33.03
Options exercised		(680,755	)	$	13.68
Options forfeited or canceled		(950,799	)	$	19.08
Outstanding at December 31, 2021		12,019,912		$	19.46		8.02	$	253,620
Options vested and expected to vest as of December 31, 2021		12,019,912		$	19.46		8.02	$	253,620
Options exercisable as of December 31, 2021		5,158,246		$	14.55		7.15	$	133,742

T

Summary of Restricted Stock Units Activity

Restricted Stock Units

The Company grants restricted stock units (RSUs) to its employees and directors under the 2018 Plan. The shares subject to the RSUs vest annually or quarterly over four years for employees and annually for directors.

	Total Restricted Stock Units			Weighted Average Grant Date Fair Value
Nonvested at December 31, 2020		738,650		$	27.84
RSUs granted		932,362			34.05
RSUs vested		(382,183	)		31.60
RSUs forfeited or canceled		(212,741	)		26.06
Nonvested at December 31, 2021		1,076,088		$	32.23

Summary of Employee and Non-Employee Stock-based Compensation Expense

The following table summarizes employee and non-employee stock-based compensation expense for the years ended December 31, 2021, 2020 and 2019, and also the allocation within the consolidated statements of operations and comprehensive loss (in thousands):

	Year Ended December 31,
	2021		2020		2019
Research and development	$	28,302	$	11,195	$	4,152
General and administrative		26,226		10,630		4,829
Total stock-based compensation	$	54,528	$	21,825	$	8,981

Schedule of Assumptions Used to Calculate Fair Value of Stock-Based Compensation

The following assumptions were used to calculate the fair value of stock-based compensation for the years ended December 31, 2021, 2020, and 2019:

	Stock Options
	Year Ended December 31,
	2021	2020	2019
Risk-free interest rate	1.0% - 1.4%	0.4% - 0.5%	1.6% - 2.3%
Expected term (in years)	6.02	6.02	6.02
Volatility	75.3% - 77.6%	76.5% - 78.5%	71.8% - 74.6%
Dividend yield	0%	0%	0%

	ESPP
	Year Ended December 31,
	2021	2020	2019
Risk-free interest rate	0.0% - 0.6%	0.1% - 0.2%	1.6% - 2.3%
Expected term (in years)	0.5-2.0	0.5-2.0	0.5-2.0
Volatility	61.2% - 95.7%	66.6% - 136.0%	64.8% - 77.1%
Dividend yield	0%	0%	0%