Stock-Based Awards

9 Months Ended

Sep. 30, 2021

Share-based Payment Arrangement [Abstract]

The Company has issued stock-based awards from various equity incentive and stock purchase plans, as more fully described in Note 10, "Stock-Based Awards" to the consolidated financial statements in the Company's 2020 Annual Report on Form 10-K.

Stock Option Activity

The following table summarizes stock option activity for the nine months ended September 30, 2021:


	Number of Options	Weighted-Average Exercise Price
Balance at December 31, 2020	12,959,926	$	16.31
Granted	1,944,717	73.31
Exercised	(831,207)	14.27
Forfeited	(259,060)	34.89
Balance at September 30, 2021	13,814,376	$	24.11
Vested and expected to vest at September 30, 2021	12,078,689	$	27.48
Exercisable at September 30, 2021	7,856,046	$	15.22

The estimated fair value of stock options granted to employees were calculated using the Black-Scholes option-pricing model using the following assumptions:


	Nine Months Ended September 30,
	2021	2020
Expected term (in years)	5.50 - 6.08	5.50 - 6.08
Volatility	62.2% - 63.7%	65.2% - 67.1%
Risk-free interest rate	0.5% - 1.2%	0.3% - 1.7%
Dividend yield	—	—

Restricted Stock Activity

The following table summarizes restricted stock unit ("RSU") activity for the nine months ended September 30, 2021:


	Number of RSU shares	Weighted-Average Fair Value at Date of Grant per Share
Unvested at December 31, 2020	2,301,679	$	27.08
Granted	1,046,259	69.50
Vested and released	(599,258)	25.63
Forfeited	(126,827)	60.92
Unvested at September 30, 2021	2,621,853	$	43.83
Expected to vest at September 30, 2021	2,621,853	$	43.83

Stock-Based Compensation Expense

The Company’s results of operations include expenses relating to stock-based compensation as follows (in thousands):


	Three Months Ended September 30,				Nine Months Ended September 30,
	2021		2020		2021		2020
Research and development	$	12,174	$	7,781	$	36,997	$	20,981
General and administrative	8,824		5,471		26,165		15,540
Total	$	20,998	$	13,252	$	63,162	$	36,521