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Stock award plan (Tables)

3 Months Ended

Mar. 31, 2018

Disclosure of Compensation Related Costs, Share-based Payments [Abstract]

A summary of stock option activity is as follows:



	Number of options		Weighted- average exercise price		Weighted- average remaining contractual term	Aggregate intrinsic value
						(in thousands)
Outstanding at December 31, 2017	6,448,642		$	29.00
Granted	1,916,939		$	18.71
Exercised	(77,312	)	$	14.70
Forfeited/Cancelled	(111,492	)	$	34.42
Outstanding at March 31, 2018	8,176,777		$	26.65	7.67 years	$	56,205
Vested or Expected to vest at March 31, 2018	3,711,261		$	21.15	8.98 years	$	32,239
Exercisable at March 31, 2018	4,166,448		$	32.12	6.40 years	$	21,177

Schedule of assumptions used to estimate fair values of grants made on the date of grant

The fair value of grants made in the three months ended March 31, 2018 was contemporaneously estimated on the date of grant using the following assumptions:



		Three months ended March 31, 2018
Risk-free interest rate		2.25% — 2.73%
Expected volatility		64%—90%
Expected term		5.04 – 10.00 years

Summary of information on the Company's restricted stock

The following table summarizes information on the Company’s restricted stock awards and units:



	Restricted Stock Awards and Units
	Number of Shares		Weighted Average Grant Date Fair Value
January 1, 2018	393,011		$	15.64
Granted	338,991		$	18.01
Vested	(97,545	)	$	16.89
Forfeited	(19,082	)	$	17.06
Unvested at March 31, 2018	615,375		$	16.74

Schedule of share-based compensation expense recorded in the statement of operations

The Company recorded share-based compensation expense in the statement of operations related to incentive stock options, nonstatutory stock options, restricted stock awards, restricted stock units and the ESPP as follows:



	Three Months Ended March 31,
	2018		2017
Research and development	$	3,747	$	4,467
Selling, general and administrative	4,001		4,562
Total	$	7,748	$	9,029