Stock-based Compensation (Tables)

12 Months Ended

Dec. 31, 2021

Stock-Based Compensation Expense by Award Type

The Company’s stock-based compensation expense by award type was as follows (in thousands):

	Years Ended December 31,
	2021		2020		2019
Stock options	$	35,320	$	31,178	$	25,964
Restricted stock units		2,835		—		257
Employee stock purchase plan		653		771		633
	$	38,808	$	31,949	$	26,854

Stock-Based Compensation Expense

The Company recorded aggregate stock-based compensation expense in the consolidated statements of operations and comprehensive income (loss) as follows (in thousands):

	Years Ended December 31,
	2021		2020		2019
Research and development	$	19,602	$	16,280	$	13,031
General and administrative		19,206		15,669		13,823
	$	38,808	$	31,949	$	26,854

Schedule of Fair Value of Options Granted on Weighted-Average Assumptions

The fair values of options granted were estimated at each grant date using the Black-Scholes valuation model with the following weighted-average assumptions:

	Years Ended December 31,
	2021			2020			2019
Expected volatility		68	%		71	%		74	%
Expected term (years)		6.0			6.0			6.1
Risk-free interest rate		0.6	%		1.4	%		2.3	%
Expected dividend yield		0.0	%		0.0	%		0.0	%

Summary of Unvested RSUs Activity Under 2015 Plan

The following table summarizes restricted stock unit activity under the 2015 Plan (in thousands, except per share data):

				Weighted-
				average
				Grant Date
	Shares			Fair Value
Unvested balance at December 31, 2020		—		$	—
Granted		291		$	43.67
Vested		—		$	—
Forfeited		(14	)	$	44.55
Unvested balance at December 31, 2021		277		$	43.62

2014 and 2015 Equity Incentive Plan [Member]

Summary of Stock Option Activity

The following table summarizes stock option activity under the 2014 Plan and 2015 Plan (in thousands, except per share data):

						Weighted-
						average
				Weighted-		Remaining
				average		Contractual		Aggregate
				Exercise		Life		Intrinsic
	Shares			Price		(Years)		Value (a)
Outstanding at December 31, 2020		6,361		$	31.21		7.2	$	101,356
Granted		1,437		$	43.04
Exercised		(404	)	$	11.46
Cancelled or forfeited		(268	)	$	45.98
Outstanding at December 31, 2021		7,126		$	34.16		6.8	$	41,128
Exercisable at December 31, 2021		4,566		$	29.62		5.9	$	40,607
Vested and expected to vest at December 31, 2021		7,126		$	34.16		6.8	$	41,128

(a) The aggregate intrinsic value is calculated as the difference between the exercise price of the underlying options and the fair value of the common stock for the options that were in the money at the dates reported.