6. Income Taxes

Our effective tax rate differs from the statutory federal income tax rate, primarily as a result of the changes in valuation allowance. There was no provision for income taxes for the year ended December 31, 2018. For the year ended December 31, 2017, the provision for income tax is a benefit of $0.5 million. Current tax expense of $1.0 million for the year ended December 31, 2016 represents federal and state alternative minimum tax.

A reconciliation of the federal statutory income tax to our effective income tax is as follows (in thousands):


	Year Ended
	December 31,
	2018		2017		2016
			(Revised)		(Revised)
Federal statutory income tax	$	(14,795)	$	(13,243)	$	15,680
State and local income taxes		(4,767)		(1,806)		2,818
Research and development credit		(6,170)		(5,554)		(2,544)
Stock based compensation		444		2,709		733
Effect of the 2017 Tax Cut and Jobs Act		—		19,596		—
Other		414		720		1,008
Net change in valuation allowance		24,874		(2,885)		(16,695)
Income tax provision (benefit)	$	—	$	(463)	$	1,000

The tax effect of temporary differences that give rise to a significant portion of the deferred tax assets and liabilities at December 31, 2018 and 2017 is presented below (in thousands):


	December 31,
	2018		2017
			(Revised)
Deferred income tax assets
Net operating loss carryforwards	$	49,889	$	25,565
Research credits		23,151		16,642
Depreciation		207		437
Unrealized loss on securities		269		504
Accrued compensation		1,097		748
Deferred revenue		11,222		16,820
State taxes		—		(2)
Gross deferred income tax assets		85,835		60,714
Valuation allowance		(82,537)		(57,663)
Net deferred income tax assets		3,298		3,051
Deferred income tax liabilities
Patent costs		(3,142)		(2,873)
Licensing costs		(125)		(142)
Capitalized legal costs		(31)		(36)
Gross deferred income tax liabilities		(3,298)		(3,051)
Net deferred income tax asset	$	—	$	—

The Tax Cuts and Jobs Act (TCJA) was enacted in December 2017 and made substantial changes in the U.S. tax system. One of the changes was elimination of the AMT tax system for corporations and allowance of an income tax refund for AMT tax credit carryforwards as of December 31, 2017. We have reported an income tax receivable of $1.6 million as of December 31, 2018 and 2017 to reflect the U.S. AMT credit carryforwards we have available. Due to the uncertainty surrounding the realization of the benefits of our deferred tax assets in future tax periods, we have placed a valuation allowance against our deferred tax assets at December 31, 2018 and 2017. The Company recognizes valuation allowances to reduce deferred tax assets to the amount that is more likely than not to be realized. The Company’s net deferred income tax asset is not more likely than not to be realized due to the lack of sufficient sources of future taxable income and cumulative losses that have resulted over the years. During the year ended December 31, 2018, the valuation allowance increased by $24.9 million. Upon analysis, there were changes in ownership under Section 382 of the Internal Revenue Code and related state provisions as a result of our sale of preferred stock and sale of common stock during 2013. Section 382 limits the amount of net operating losses and tax credit forwards that may be available after a change in ownership. The Company has adjusted its net operating loss and tax credit carryforwards to reflect the impact of the section 382 limitations. The Company’s tax returns remain open to potential inspection for the years 2013 and onwards for federal purposes and 2012 and onwards for state purposes.

As of December 31, 2018, we had cumulative net operating loss carryforwards for federal and state income tax purposes of $191.1 million and $138.9 million respectively, and available tax credit carryforwards of approximately $14.9 million for federal income tax purposes and $8.2 million for state income tax purposes, which can be carried forward to offset future taxable income, if any. The federal net operating loss carryforwards consists of $102.6 million of losses incurred prior to January 1, 2018 and which can be used to offset 100% of future taxable income and, $88.6 million of losses incurred after January 1, 2018 which can be used to offset up to 80% of taxable income in subsequent years.

Our federal net operating loss carryforwards expire starting in 2026, state net operating losses expire starting in 2032, and federal tax credit carryforwards expire starting in 2019. Utilization of the net operating losses and tax credits are subject to a substantial annual limitation due to ownership changes which occurred. As a result of these changes, provisions in the Internal Revenue Code of 1986 under Section 382 and similar state provisions may result in the expiration of certain of our net operating losses and tax credits before we can use them.