Stock-Based Awards (Tables)

9 Months Ended

Sep. 30, 2020

Summary of Stock Option Plans Activity

Activity under the Company’s stock option plans is set forth below:

	Number of Options			Weighted Average Exercise Price		Weighted Average Remaining Contractual Term		Aggregate Intrinsic Value
						(Years)		(in thousands)
Outstanding at December 31, 2019		7,186,298		$	5.25		8.5
Granted		4,175,235		$	32.53
Exercised		(1,387,629	)	$	2.55
Forfeited		(514,889	)	$	13.78
Outstanding at September 30, 2020		9,459,015		$	17.22		8.7	$	174,230
Vested and expected to vest at September 30, 2020		9,459,015		$	17.22		8.7	$	174,230
Vested and exercisable at September 30, 2020		2,194,929		$	4.52		7.8	$	65,422

Summary of Restricted Stock Activity

The following table summarizes restricted stock activity:

	Number of Shares			Weighted Average Fair Value at Date of Grant per Share
Unvested as of December 31, 2019		2,075,511		$	0.95
Vested		(1,791,880	)		0.91
Unvested as of September 30, 2020		283,631		$	1.15

Summary of Stock-based Compensation Expense

The following table sets forth the total stock-based compensation expense for all awards granted to employees and non-employees, including shares sold through the issuance of non-recourse promissory notes of which all the shares are considered to be options for accounting purposes in the Company’s consolidated statements of operations:

	Three Months Ended September 30,				Nine Months Ended September 30,
	2020		2019		2020		2019
	(in thousands)				(in thousands)
Research and development	$	4,181	$	926	$	8,369	$	1,925
General and administrative		4,401		1,262		8,930		4,115
Total stock-based compensation	$	8,582	$	2,188	$	17,299	$	6,040

Stock Option

Summary of Assumptions Used for Estimating the Fair Value of Stock Options Granted

The fair value of stock options granted to employees was estimated on the date of grant using the Black-Scholes option-pricing model using the following assumptions:

	Three Months Ended September 30,				Nine Months Ended September 30,
	2020		2019		2020		2019
Expected term of options (in years)	5.0 - 6.1			6.0	5.0 - 6.1		5.9 - 6.0
Expected stock price volatility	99.7% - 108.6%		88.5%		88.8% - 108.6%		88.5% - 89.4%
Risk-free interest rate	0.3% - 0.4%		1.9%		0.3% - 1.2%		1.9% - 2.5%
Expected dividend yield		—		—		—		—