For due to a full valuation allowance against our deferred tax assets

 

 

		December 31,
		2017				2016				2015
Tax at statutory federal rate		$	( 67,145	)		$	( 58,868	)		$	( 33,181	)
State tax, net of federal benefit			( 6,203	)			2,013				( 5,753	)
Research and development credits			( 5,962	)			( 4,206	)			( 2,757	)
Stock-based compensation expense			3,151				2,694				894
Non-deductible compensation			—				1,864				—
Change in valuation allowance			3,241				55,173				40,445
Impact of the 2017 Tax Act			74,361				—				—
Other			( 1,443	)			1,330				352
Provision for income taxes		$	—			$	—			$	—

 

Deferred income tax assets and liabilities arising from differences between accounting for fina

 

		December 31,
		2017				2016
Deferred tax assets:
Net operating loss carryforward		$	134,659			$	138,727
Research and development credits			25,919				16,687
Stock-based compensation			8,633				10,272
Other			2,920				3,204
Total gross deferred tax assets			172,131				168,890
Valuation allowance			(172,131	)			(168,890	)
Net deferred tax assets		$	—			$	—

 

We have established a valuation allowance to offset net deferred tax assets as of and due to the uncertainty of realiz

 

As of we had federal and state net operating loss (“NOL”) carryforwards of million and million, respectively, expiring beginning in As of we had federal and California state research and development credit carryforwards of million and million, respectively. The federal research and development credit carryforwards will begin to expire in The California state credits can be carried forward indefinitely.

 

Internal Revenue Code (“IRC”) Section and places a limitation on the amount of taxable income that can be offset by NOL and credit carryforwards after a change in control (generally greater than change in ownership within a -year period) of a loss corporation. California has similar rules. Generally, after a change in control, a loss corporation cannot deduct NOL and credit carryforwards in excess of the IRC Section and limitation. We have performed an IRC Section and analysis and determined there were ownership changes in and We are currently in the process of completing the IRC Section and analysis for 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

 

We file U.S. and state income tax returns with varying statutes of limitations. The tax years from to remain open to examination due to th

 

The Tax Cuts and Jobs Act ( Tax Act”) was enacted in The Tax Act, among other things, reduces the U.S. federal corporate tax rate from to effective requires companies to pay a -time transition tax on earnings of certain foreign subsidiaries that were previously tax deferred and creates new taxes on certain foreign earnings. We revalued our deferred tax assets as of based on a U.S. federal tax rate of which resulted in a reduction to our deferred tax assets of million fully offset by a reduction to the valuation allowance. We are required to pay a -time transition tax on earnings of our foreign subsidiary as the foreign subsidiary has an accumulated deficit.

 

 

As of both and we had unrecognized tax benefits of million