Income Taxes (Tables)

12 Months Ended

Dec. 31, 2016

Income Tax Disclosure [Abstract]

Summary of Reconciliation of U.S. Statutory Rate to Company's Effective Tax Rate

A reconciliation of U.S. statutory rate to the Company’s effective tax rate is as follows:

	Year Ended December 31,
	2016			2015			2014
Tax at Statutory Rate		34.0	%		34.0	%		34.0	%
State Taxes, net of federal benefit		4.2			4.0			4.5
Permanent Items		(1.0	)		(0.9	)		(1.0	)
Orphan Drug Credit Addback		(2.9	)		(4.2	)		—
Foreign Rate Differential		(3.1	)		(3.1	)		—
Federal and State Credits		10.3			13.2			8.5
Change in Valuation Allowance		(41.5	)		(43.1	)		(46.0	)
Other		—			0.1			—
		0.0	%		0.0	%		0.0	%

Summary of Significant Components of Company's Net Deferred Tax Asset

Significant components of the Company’s net deferred tax asset at December 31, 2016 and 2015 are as follows:

	December 31,
	2016			2015
	(in thousands)
Net operating losses	$	84,374		$	44,172
Capitalized start-up costs		2,155			2,335
Accounting method change		(673	)		(1,347	)
Tax credit carryforwards		33,477			17,013
Accrued expenses		2,287			1,062
Depreciation and amortization		999			716
Stock options		12,103			5,097
Others		350			21
Total net deferred tax asset before valuation allowance		135,072			69,069
Valuation allowance		(135,072	)		(69,069	)
Net deferred tax asset	$	-		$	-

Summary of Rollforward of Company's Unrecognized Tax Benefits

The following is a rollforward of the Company’s unrecognized tax benefits:

	Year Ended December 31,
	2016		2015		2014
	(in thousands)
Unrecognized tax benefits—as of the beginning of the year	$	—	$	—	$	2,880
Gross increases—current period tax positions		—		—		—
Gross decreases—tax positions of prior periods		—		—		(2,880	)
Unrecognized tax benefits—as of the end of the year	$	—	$	—	$	—