Stock-Based Compensation (Tables)

12 Months Ended

Dec. 31, 2018

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Summary of Option Activity Includes Options Granted

The following table, which includes options granted under the 2015 Plan, 2018 Plan and the Non-Plan Option, summarizes option activity:

	Shares Available for Grant			Shares Subject to Outstanding Options			Weighted Average Exercise Price Per Share		Weighted Average Remaining Contractual Term (in years)
Balance at December 31, 2015		871,226			20,200		$	0.40		8.79
Options authorized		1,133,987			—
Options granted		(746,825	)		746,825		$	0.59
Options exercised		—			(637,107	)	$	0.48
Options repurchased		24,987			—		$	0.40
Balance at December 31, 2016		1,283,375			129,918		$	1.20		9.84
`Options authorized		1,568,397			—
Options granted		(1,063,019	)		1,063,019		$	1.61
Options exercised		—			(643,024	)	$	1.45
Options forfeited or canceled		5,797			(5,797	)	$	1.28
Options repurchased		60,690			—		$	0.49
Balance at December 31, 2017		1,855,240			544,116		$	1.71		9.28
Options authorized		3,570,000
Options granted		(1,645,489	)		1,645,489		$	8.19
Options exercised		—			(720,756	)	$	4.64
Options forfeited or canceled		10,769			(10,769	)	$	5.25
Options repurchased		10,671
Balance at December 31, 2018		3,801,191			1,458,080		$	7.55		8.99
Options outstanding and exercisable as of    December 31, 2018					238,721		$	6.49		8.87
Options vested and expected to vest as of    December 31, 2018					1,458,080		$	7.55		8.99

Summary of Employee and Non-Employee Stock-based Compensation Expense

The following table summarizes employee and non-employee stock-based compensation expense for the years ended December 31, 2018, 2017 and 2016, and also the allocation within the consolidated statements of operations and comprehensive loss (in thousands):

	Year Ended December 31, 2018		Year Ended December 31, 2017		Year Ended December 31, 2016
Research and development	$	2,255	$	222	$	67
General and administrative		1,619		273		23
Total stock-based compensation	$	3,874	$	495	$	90

Schedule of Assumptions Used to Calculate Fair Value of Stock-Based Compensation

The following assumptions were used to calculate the fair value of stock-based compensation as of December 31, 2018, 2017 and 2016:

	Stock Options			ESPP
	Year Ended December 31, 2018	Year Ended December 31, 2017	Year Ended December 31, 2016	Year Ended December 31, 2018
Risk-free interest rate	1.2% - 3.1%	1.66% - 2.20%	1.2% - 2.45%	2.11% - 2.63%
Expected term (in years)	5.16-9.95	5.95-9.99	6.25-9.84	.5-2.0
Volatility	58.7%-75.5%	67.0%-71.7%	67.0 - 77.8%	54.3% - 65.5%
Dividend yield	0%	0%	0%	0%

Summary of Non-Vested Restricted Stock

A summary of the Company’s non-vested restricted stock for the periods presented is as follows:

	Number of Shares		Weighted Average Grant Date Fair Value		Remaining Contractual Term (Years)
Balance, December 31, 2015		2,372,684	$	0.000396		3.4
Vested during the year		694,444	$	0.000396
Balance, December 31, 2016		1,678,240	$	0.000396		2.4
Vested during the year		694,444	$	0.000396
Balance, December 31, 2017		983,796	$	0.000396		1.4
Vested during the year		694,444	$	0.000396
Balance, December 31, 2018		289,352	$	0.000396		0.4