Income Taxes (Tables)

12 Months Ended

Dec. 31, 2019

Schedule of Reconciliation of U.S Federal Statutory Income Tax Rate to Company's Effective Income Tax Rate

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

	Year Ended December 31,
	2019			2018			2017
Federal statutory income tax rate		(21.0	)%		(21.0	)%		(34.0	)%
Research and development tax credits		(5.8	)		(7.2	)		(7.7	)
State taxes, net of federal benefit		(6.8	)		(7.2	)		(4.8	)
Stock-based compensation		(1.7	)		2.0			1.2
Revaluation of preferred stock warrant liability		—			—			33.1
Other		(0.7	)		0.8			(0.3	)
Change in deferred tax asset valuation allowance		36.0			32.6			12.5
Effective income tax rate		—	%		—	%		—	%

Schedule of Net Deferred Tax Assets

Net deferred tax assets as of December 31, 2019 and 2018 consisted of the following:

	December 31,
	2019			2018
Deferred tax assets:
Net operating loss carryforwards	$	72,752		$	50,723
Research and development tax credit carryforwards		36,602			32,522
Capitalized organization costs		244			275
Stock-based compensation expense		12,783			9,178
Lease Liability		5,500			—
Charitable Contributions		13			11
Deferred Revenue		32,713			37,499
Accrued expenses		1,251			4,279
Capitalized research and development expenses		58			66
Total deferred tax assets	$	161,916		$	134,553
Deferred tax liabilities:
Depreciation and amortization		(1,467	)		(2,544	)
Right of use assets		(3,103	)		—
Total deferred tax liabilities		(4,570	)		(2,544	)
Valuation allowance	$	(157,346	)	$	(132,009	)
Net deferred tax assets	$	—		$	—

Summary of Changes In Valuation Allowance for Deferred Tax Assets

Changes in the valuation allowance for deferred tax assets during the years ended December 31, 2019, 2018 and 2017 related primarily to the increases in NOLs, research and development tax credit carryforwards, stock-based compensation and decrease of the deferred rate due to tax reform were as follows:

	Year Ended December 31,
	2019			2018			2017
Valuation allowance at beginning of year	$	(132,009	)	$	(94,126	)	$	(82,994	)
Decreases recorded as benefit to income tax provision		—			—			29,546
Increases recorded to income tax provision		(25,337	)		(37,883	)		(40,678	)
Valuation allowance as of end of year	$	(157,346	)	$	(132,009	)	$	(94,126	)