Stock Awards (Tables)

3 Months Ended

Mar. 31, 2017

Table Text Blocks

Summary of unvested restricted stock

		Weighted-Average
		Grant Date
	Shares	Fair Value
Unvested at December 31, 2016	2,125	$	1.25
Vested	(1,871)		1.22
Repurchased	—		—
Unvested at March 31, 2017	254		1.49

Summary of stock option activity

		Weighted-		Remaining	Aggregate
		Average		Contractual	Intrinsic
		Exercise		Life	Value(1)
	Shares	Price		(in Years)	(in thousands)
Outstanding at December 31, 2016	2,622,741	$	11.67	8.30	$	44,025
Granted	750,300		35.94
Exercised	(124,135)		7.87
Canceled	(11,863)		6.80
Outstanding at March 31, 2017	3,237,043	$	17.45	8.58	$	72,949
Exercisable at March 31, 2017	958,200	$	7.76	7.78	$	30,882
Vested and expected to vest at March 31, 2017	3,237,043	$	17.45	8.58	$	72,949

(1) Intrinsic value represents the amount by which the fair market value as of March 31, 2017 of the underlying common stock exceeds the exercise price of the option.

Assumptions used in estimating fair value of stock options on grant date

	Three Months Ended
	March 31, 2017		March 31, 2016
Risk-free interest rate	2.11	%	1.59	%
Expected dividend yield	—	%	—	%
Expected term (years)	6.0		6.0
Expected stock price volatility	75.43	%	76.74	%

Summary of stock-based compensation expense recognized in statements of operations

Total stock‑based compensation expense recognized for all stock‑based compensation awards in the condensed consolidated statements of operations and comprehensive loss is as follows (in thousands):

	Three Months Ended
	March 31,
	2017		2016
Research and development	$	1,124	$	526
General and administrative		1,116		576
Total stock-based compensation expense	$	2,240	$	1,102