13.	Income Taxes

During the years ended December 31, 2015, 2014 and 2013, the Company recorded no income tax benefits for the net operating losses incurred in each year or interim period, due to its uncertainty of realizing a benefit from those items.

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,
	2015			2014			2013
Federal statutory income tax rate		(34.0	)%		(34.0	)%		(34.0	)%
Research and development tax credits		(5.7	)		(3.8	)		(6.1	)
State taxes, net of federal benefit		(5.3	)		(5.3	)		(5.3	)
Stock-based compensation		4.1			2.3			1.2
Revaluation of preferred stock warrant liability		—			3.3			—
Other		0.3			0.2			—
Change in deferred tax asset valuation allowance		40.6			37.3			44.2
Effective income tax rate		—	%		—	%		—	%

Net deferred tax assets as of December 31, 2015 and 2014 consisted of the following:

	December 31,
	2015			2014
Deferred tax assets:
Net operating loss carryforwards	$	25,293		$	7,946
Research and development tax credit carryforwards		4,180			1,055
Capitalized organization costs		527			571
Stock-based compensation expense		1,897			436
Charitable Contributions		5			—
Accrued expenses		977			447
Capitalized research and development expenses		126			136
Total deferred tax assets	$	33,005			10,591
Deferred tax liabilities:
Depreciation and amortization		(228	)		(69	)
Total deferred tax liabilities		(228	)		(69	)
Valuation allowance	$	(32,777	)		(10,522	)
Net deferred tax assets	$	—		$	—

As of December 31, 2015, the Company had net operating loss carryforwards for federal and state income tax purposes of $65,401 and $64,266, respectively, which both begin to expire in 2031. As of December 31, 2015, the Company also had available research and development tax credit carryforwards for federal and state income tax purposes of $3,045 and $1,331, respectively, which begin to expire in 2031 and 2027, respectively. Utilization of the net operating loss carryforwards and research and development tax credit carryforwards may be subject to a substantial annual limitation under Section 382 of the Internal Revenue Code of 1986 due to ownership changes that have occurred previously or that could occur in the future. These ownership changes may limit the amount of carryforwards that can be utilized annually to offset future taxable income. In general, an ownership change, as defined by Section 382, results from transactions increasing the ownership of certain shareholders or public groups in the stock of a corporation by more than 50% over a three-year period. Since our formation, we have raised capital through the issuance of capital stock on several occasions. These financings, combined with the purchasing shareholders' subsequent disposition of those shares, may have resulted in a change of control as defined by Section 382 or could result in a change of control in the future upon subsequent disposition. We conducted an analysis under Section 382 to determine if historical changes in ownership through August 31, 2015 would limit or otherwise restrict our ability to utilize these NOL and R&D credit carryforwards. As a result of this analysis, we do not believe there are any significant limitations on our ability to utilize these carryforwards. However, future changes in ownership after August 31, 2015 could affect the limitation in future years. Any limitation may result in expiration of a portion of the NOL or R&D credit carryforwards before utilization. At December 31, 2015, $856 of the federal net operating loss carryforwards, and $856 of the state net operating loss carryforwards relate to excess stock based compensation tax benefits for which the benefit will be recorded to additional paid-in capital when recognized.

The Company has evaluated the positive and negative evidence bearing upon its ability to realize the deferred tax assets. Management has considered the Company’s history of cumulative net losses incurred since inception and its lack of commercialization of any products or generation of any revenue from product sales since inception and has concluded that it is more likely than not that the Company will not realize the benefits of the deferred tax assets. Accordingly, a full valuation allowance has been established against the deferred tax assets as of December 31, 2015 and 2014. Management reevaluates the positive and negative evidence at each reporting period.

Changes in the valuation allowance for deferred tax assets during the years ended December 31, 2015, 2014 and 2013 related primarily to the increases in net operating loss carryforwards, research and development tax credit carryforwards and stock-based compensation were as follows:

	Year Ended December 31,
	2015			2014			2013
Valuation allowance at beginning of year	$	(10,522	)	$	(4,294	)	$	(1,599	)
Decreases recorded as benefit to income tax provision		—			—			—
Increases recorded to income tax provision		(22,255	)		(6,228	)		(2,695	)
Valuation allowance as of end of year	$	(32,777	)	$	(10,522	)	$	(4,294	)