Stockholders' Equity (Tables)

12 Months Ended

Dec. 31, 2019

Stockholders' Equity

Schedule of stock option activity

				Weighted
		Weighted		average	Aggregate
	Number	average		contractual	intrinsic
	of shares	exercise price		term	value
Outstanding at December 31, 2018	2,282,636		5.06
Granted	1,484,352		17.00
Exercised	(252,773)		4.53
Forfeited	(55,768)		7.26
Expired	—		—
Outstanding at December 31, 2019	3,458,447		10.19	8.0		322,229,820
Vested and expected to vest at December 31, 2019	3,449,778		10.21	8.0		321,347,251
Exercisable at December 31, 2019	1,861,890		7.23	6.5		178,992,676

Summary of Black-Scholes option valuation assumptions

Black-Scholes option valuation assumptions	2019				2018				2017
Risk-free interest rates	1.4	-	2.7	%	2.0	-	3.1	%	1.6	-	2.2	%
Dividend yield		—				—				—
Volatility	87	-	91	%	74	-	87	%	73	-	77	%
Weighted average expected term	5.00	-	6.13	years	3.66	-	6.13	years	3.50	-	6.12	years

Schedule of stock-based compensation expense

	2019		2018		2017
Research and development	$	1,584,165	$	391,946	$	613,629
General and administrative		4,530,995		1,363,028		1,458,581
Total	$	6,115,160	$	1,754,974	$	2,072,210