8 .	Income Taxes

For the years ended December 31, 2015, 2014 and 2013, we did not record a provision for income taxes because we have incurred operating losses for all three years.

Deferred income tax assets and liabilities arising from differences between accounting for financial statement purposes and tax purposes, less valuation allowance at year-end are as follows (in thousands):

	December 31,
	201 5			201 4
Deferred tax assets:
Net operating loss carryforward	$	94,465		$	59,513
Research and development credits		11,181			7,667
Stock-based compensation		6,726			5,128
Other, net		1,345			964
Total deferred tax assets		113,717			73,272
Valuation allowance for net deferred tax assets		(113,717	)		(73,272	)
Net deferred taxes	$	—		$	—

Taxes on income vary from the statutory federal income tax rate applied to earnings before tax on income as follows (in thousands):

	December 31,
	2015			2014			2013
Statutory federal income tax rate of 34%	$	(33,181	)	$	(25,965	)	$	(18,796	)
Stock-based compensation expense		894			551			513
NOL not benefitted		31,935			25,085			17,983
Other, net		352			329			300
Provision for taxes	$	—		$	—		$	—

A valuation allowance is provided when it is more likely than not that the deferred tax assets will not be realized. We have established a valuation allowance to offset net deferred tax assets at December 31, 2015 and 2014 due to the uncertainty of realizing future tax benefits from our net operating loss carryforwards and other deferred tax assets. The net change in the total valuation allowance for the year ended December 31, 2015 and 2014 was an increase of approximately $40,445,000 and $32,035,000, respectively.

At December 31, 2015, we had federal and California state net operating loss (“NOL”) carryforwards of approximately $242,769,000 and $233,305,000, respectively, expiring beginning in 2018 for federal and 2017 for California state purposes. Approximately $4,235,000 of federal NOLs and $4,235,000 of state NOLs relate to stock-based compensation deductions in excess of book expense, the tax effect of which would be to credit additional paid-in capital, if realized. At December 31, 2015, we had federal and California state research credit carryforwards of approximately $7,434,000 and $5,639,000, respectively, expiring beginning in 2022 for federal. The California state credits can be carried forward indefinitely.

Internal Revenue Code section 382 places a limitation on the amount of taxable income that can be offset by NOL carryforwards after a change in control (generally greater than 50% change in ownership) of a loss corporation. California has similar rules. Generally, after a control change, a loss corporation cannot deduct NOL carryforwards in excess of the Section 382 limitation. We have performed an IRC Section 382 analysis and determined there were ownership changes in 2007, 2011, and 2013. We are currently in the process of completing the IRC Section 382 analysis for 2015 and we do not expect an ownership change for the year ended December 31, 2015. The limitation in the Federal and state carryforwards associated with the NOL and credit carryforwards reduce the deferred tax assets, which are further offset by a full valuation allowance. The limitation can result in the expiration of the NOLs and credit carryforwards available as of December 31, 2015 before utilization.

We file U.S. and state income tax returns with varying statutes of limitations. The tax years from 1998 to 2015 remain open to examination due to the carryover of unused NOL carryforwards and tax credits.

For benefits to be realized, a tax position must be more likely than not to be sustained upon examination. The amount recognized is measured as the largest amount of benefit that is greater than 50 percent likely of being realized upon settlement. It is our policy to recognize interest and penalties related to income tax matters in income tax expense. As of December 31, 2015 and 2014, we had no accrued interest and penalties related to uncertain tax positions.

As of December 31, 2015 and 2014, we had unrecognized tax benefits of $120,000, and the amount that would impact our effective tax rate, before the consideration of the valuation allowance, is $120,000. We do not expect any material changes to the estimated amount of liability associated with its uncertain tax positions within the next 12 months.