Income Taxes (Tables)

12 Months Ended

Dec. 31, 2017

Schedule of Reconciliation of Effective Income Tax Expense (Benefit)

A reconciliation of income tax expense (benefit) computed at the statutory federal income tax rate of 34% to income tax expense (benefit) as reflected in the financial statements is as follows (in thousands):

	Years Ended December 31,
	2017			2016			2015
Federal income tax benefit at statutory rate	$	(24,876	)	$	(21,556	)	$	(7,756	)
State income tax benefit, net of federal benefit		(7,756	)		(1,617	)		(1,511	)
Step-up in assets upon conversion to C-corporation		—			—			(709	)
Research and development credits		(6,297	)		(7,275	)		—
Stock-based compensation expense for incentive stock options and employee stock purchase plan		(3,012	)		1,805			283
Other non-deductible expenses and reconciling items		(26	)		170			129
Change in corporate tax rates		16,598			42			111
Change in valuation allowance		25,369			27,996			9,453
Total income tax expense (benefit)	$	—		$	(435	)	$	—

Components of Net Deferred Tax Assets

The significant components of the Company’s net deferred tax assets are as follows (in thousands):

	December 31, 2017			December 31, 2016
Deferred tax assets:
Net operating loss carryforwards	$	37,658		$	25,625
Research and development tax credits		21,137			10,890
Step-up in assets upon conversion to C-corporation		721			953
Stock-based compensation expense for non-qualified stock options and restricted stock units		4,263			1,401
Unrealized losses on marketable securities		89			—
Deferred rent		458			258
Accruals and other		1,540			1,132
Total deferred tax assets before valuation allowance		65,866			40,259
Valuation allowance		(65,669	)		(40,084	)
Total deferred tax assets		197			175
Deferred tax liabilities:
Depreciation		(197	)		(19	)
Unrealized gains on marketable securities		—			(156	)
Total deferred tax liabilities		(197	)		(175	)
Net deferred tax assets	$	—		$	—