Equity Incentive Plan (Tables)

9 Months Ended

Sep. 30, 2019

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Schedule of stock option activity

	Options Outstanding
				Weighted-
				Average	Aggregate
		Weighted-		Remaining	Intrinsic
	Number	Average		Contractual	Value
	of Shares	Exercise Price		Life (years)	(in thousands)
Balances, December 31, 2018	2,136,291	$	6.06	8.9	$	12,881
Granted	1,658,012		13.68
Exercised	(67,243)		3.83
Cancelled	(36,662)		11.03
Balances, September 30, 2019	3,690,398	$	9.46	8.8	$	12,830
Vested and expected to vest at September 30, 2019	3,690,398	$	9.46	8.8	$	12,830
Exercisable at September 30, 2019	1,736,235	$	5.56	8.1	$	11,606
Vested at September 30, 2019	993,078	$	5.91	7.8	$	6,296

Schedule of Share-based Payment Award, Stock Options, Valuation Assumptions

	Three Months Ended September 30,				Nine Months Ended September 30,
	2019		2018		2019		2018
Expected life (in years)	6.03		*		6.02		6.01
Volatility	80.5	%	*	%	80.8	%	78.3	%
Risk-free interest rate	1.77	%	*	%	2.19	%	2.88	%

Schedule of stock-based compensation expense

The compensation expense is allocated on a departmental basis, based on the classification of the option holder as follows (in thousands):

	Three Months Ended September 30,				Nine Months Ended September 30,
	2019		2018		2019		2018
Research and development	$	920	$	147	$	2,091	$	357
General and administrative		958		195		1,922		475
	$	1,878	$	342	$	4,013	$	832

2019 Employee Stock Purchase Plan

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Schedule of Share-based Payment Award, Stock Options, Valuation Assumptions

	Three Months Ended September 30,				Nine Months Ended September 30,
	2019		2018		2019		2018
Expected life (in years)	0.2-2.0		—		0.2-2.0		—
Volatility	74.5-101.6	%	—	%	74.5-101.6	%	—	%
Risk-free interest rate	1.50-  2.15	%	—	%	1.50-2.15	%	—	%