Stock-Based Compensation (Tables)

12 Months Ended

Dec. 31, 2018

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Summary of Employee and Non-Employee Stock Option Activity

The following table summarizes employee and non-employee stock option activity for the year ended December 31, 2018:

	Shares Issuable Under Options			Weighted Average Exercise Price		Weighted Average Remaining Contractual Term		Aggregate Intrinsic Value
						(in years)		(inthousands)
Outstanding as of December 31, 2017		2,361,360		$	5.21		8.72	$	2,482
Granted		1,668,800			8.03
Exercised		(521,665	)		5.42
Forfeited		(365,808	)		5.20
Outstanding as of December 31, 2018		3,142,687		$	6.67		8.52	$	4,798
Options vested and expected to vest    as of December 31, 2018		2,939,785		$	6.49		8.45	$	4,786
Options exercisable as of December 31, 2018		1,160,819		$	5.85		7.68	$	2,470

Summary of Employee and Non-Employee Restricted Stock Activity

The following table summarizes employee and non-employee restricted stock activity for the year ended December 31, 2018:

	Shares			Weighted Average Grant Date Fair Value
Unvested restricted common stock as of December 31, 2017		33,307		$	1.84
Granted		—			—
Vested		(26,444	)		1.87
Forfeited		—			—
Unvested restricted common stock as of December 31, 2018		6,863		$	1.73

Summary of Stock-Based Compensation Expense

Total stock-based compensation expense recognized from the Company’s equity incentive plans, 2018 Plan, and the 2016 ESPP for the years ended December 31, 2018, 2017, and 2016 was as follows (in thousands):

	Year Ended December 31,
	2018				2017				2016
	Employees		Non-Employees		Employees		Non-Employees		Employees		Non-Employees
Research and development	$	1,440	$	234	$	961	$	—	$	389	$	—
General and administrative		2,606		—		1,531		—		832		—
Total stock-based     compensation expense	$	4,046	$	234	$	2,492	$	—	$	1,221	$	—

Summary of Weighted-Average Assumptions Used in Calculating Fair Value of Awards

The following table summarizes the weighted-average assumptions used in calculating the fair value of the awards:

	Year Ended December 31,
	2018			2017			2016
2018 Plan, 2016 Plan, and 2015 Plan
Expected term (in years)		5.20			5.88			5.99
Expected volatility		74	%		86	%		87	%
Risk-free interest		2.42	%		2.00	%		1.28	%
Dividend yield		0	%		0	%		0	%
2016 ESPP
Expected term (in years)		0.50			0.50			0.45
Expected volatility		66	%		78	%		82	%
Risk-free interest		2.20	%		1.06	%		0.50	%
Dividend yield		0	%		0	%		0	%