STOCK BASED COMPENSATION (Tables)

12 Months Ended

Dec. 31, 2017

STOCK BASED COMPENSATION

Summary of stock-based compensation included in statement of operations

	Year Ended December 31,
	2017		2016		2015
Research and development expenses	$	888	$	638	$	223
Selling, general and administrative expenses		7,057		5,149		1,986
Total stock-based compensation expense	$	7,945	$	5,787	$	2,209

Summary of restricted stock awards activity

		Weighted-Average
		Purchase Price
	Shares	per Share
Unvested at December 31, 2016	43,265	$	5.73
Granted	—		—
Vested	(32,449)		5.73
Unvested at December 31, 2017 (1)	10,816	$	5.73

(1) Excludes 21,127 shares of unvested restricted stock remaining from the early exercise of stock options as of December 31, 2017.

Summary of restricted stock units activity

		Weighted-Average
	Shares	Grant Date Fair Value
Outstanding at December 31, 2016	41,741	$	16.15
Granted	211,018		12.45
Settled	(14,757)		16.15
Forfeited	(19,130)		15.52
Outstanding at December 31, 2017	218,872	$	12.64

Summary of stock option activity

				Weighted-
			Weighted-	Average
			Average	Remaining		Aggregate
			Exercise Price	Contractual		Intrinsic
	Shares		per Share	Term (in years)		Value
Outstanding at December 31, 2016	2,326,801	$	13.07	8.7	$	7,927
Granted	1,380,123		12.30
Exercised	(158,801)		4.63
Cancelled	(510,433)		14.04
Outstanding at December 31, 2017	3,037,690	$	13.00	8.4	$	16,829
Exercisable at December 31, 2017	1,025,252	$	12.86	7.6	$	5,871
Vested and expected to vest at December 31, 2017	2,920,652	$	13.00	8.3	$	16,183

Schedule of weighted-average assumptions used in Black-Scholes option-pricing model

	Year ended December 31,
	2017		2016		2015
Risk-free interest rate	2.0	%	1.5	%	1.7	%
Volatility	71.0	%	76.3	%	77.0	%
Expected term (years)	6.01		6.02		6.20
Expected dividend yield	—	%	—	%	—	%