Stock-based Compensation (Tables)

12 Months Ended

Dec. 31, 2016

Stock-based Compensation

Summary of stock option activity

						Weighted Average
				Weighted Average Exercise Price
	Number of Shares					Remaining Contractual Life (Years)		Aggregate Intrinsic Value
Outstanding at December 31, 2015		1,748,875		$	15.04		9.38	$	8,224,947
Granted		973,638		$	33.10
Exercised		(84,127	)	$	5.11
Cancelled or forfeited		(61,275	)	$	22.19
Outstanding at December 31, 2016		2,577,111		$	22.01		8.79	$	66,466,245
Exercisable at December 31, 2015		168,360		$	2.47		8.41	$	2,907,260
Vested at December 31, 2015 and expected to vest		1,668,837		$	15.64		9.42	$	6,842,807
Exercisable at December 31, 2016		704,111		$	14.86		8.42	$	23,146,606
Vested and expected to vest at December 31, 2016		2,577,111		$	22.01		8.79	$	66,466,245

(a)           The aggregate intrinsic value is calculated as the difference between the exercise price of the underlying options and the estimated fair value of the common stock for the options that were in-the-money at December 31, 2015 and 2016.

Summary of aggregate stock-based compensation recorded in consolidated statements of operations and comprehensive loss

	Year ended December 31, 2016		Year ended December 31, 2015		Year ended December 31, 2014
Research and development	$	5,988,231	$	1,293,841	$	179,641
General and administrative		9,668,141		3,956,376		236,983
	$	15,656,372	$	5,250,217	$	416,624

Summary of grant date fair value and assumptions used in determining fair value of stock options

	Year ended December 31, 2016			Year ended December 31, 2015			Year ended December 31, 2014
Expected volatility		91.84	%		90.00	%		86.94	%
Risk-free interest rate		1.45	%		1.57	%		1.16	%
Expected term (in years)		6.08			6.08			4.19
Expected dividend yield		0.00	%		0.00	%		0.00	%

Summary of restricted stock units activity

	Number of Shares		Weighted Average Grant Date Fair Value Per Share
Outstanding at December 31, 2015		—	$	—
Granted		57,500		34.90
Vested		—		—
Forfeited and cancelled		—		—
Outstanding at December 31, 2016		57,500	$	34.90

Summary of restricted stock activity

	Number of common shares
Nonvested shares at December 31, 2015		1,750,794
Shares granted		—
Shares vested		1,750,794
Nonvested shares at December 31, 2016		—