
	2020		2019		2018
Domestic	$	(111,101)	$	(41,720)	$	(78,124)
Foreign	1,276		1,293		973
Loss before income taxes	$	(109,825)	$	(40,427)	$	(77,151)

A reconciliation of the federal statutory income tax rate to the effective income tax rate for the years ended December 31 are as follows (in thousands):


	2020		2019		2018
Income tax provision at federal statutory rate	$	(23,063)	$	(8,490)	$	(16,202)
Tax on repatriated foreign earnings and other nondeductible items	348		403		195
Section 162(m) limitations	5,044		1,438		—
Change in tax credits	3,123		(3,738)		(2,148)
Change in valuation allowance	21,707		17,842		19,935
Changes in federal and state tax rates	586		(4,058)		—
Stock option exercise (windfall) shortfall	(7,683)		(1,763)		257
Adjustments to deferred stock compensation	3,060		—		—
State and Foreign tax, and other	(2,869)		(1,365)		(1,788)
Total provision for income taxes	$	253	$	269	$	249

At December 31, 2020, for income tax return purposes the Company has gross federal and state NOL carryforwards totaling $553.7 million and tax credit carryforwards of $10.2 million. The gross federal NOL carryforwards generated during and after fiscal 2018 totaling $204.4 million are carried forward indefinitely, while all others, if not utilized, will expire beginning in 2025 through 2037. The research and development credit carryforwards generated prior to 2018 will expire beginning in 2028. The carryforwards may be subject to limitations under the Internal Revenue Code and applicable state tax law.

The Company does not expect to utilize any of its net operating loss and tax credit carryforwards in the near term. The Company may have already experienced one or more ownership changes. Depending on the timing of any future utilization of its carryforwards, the Company may be limited as to the amount that can be utilized each year as a result of such previous ownership changes. However, the Company does not believe such limitations will cause its carryforwards to expire unutilized.

Future changes in the Company’s stock ownership as well as other changes that may be outside the Company’s control could potentially result in further limitations on the Company’s ability to utilize its net operating loss and tax credit carryforwards.

The effect of temporary differences and carryforwards that give rise to deferred tax assets and liabilities for the years ended December 31 were as follows (in thousands):


	2020		2019
Deferred tax assets:
Net operating loss carryforwards	$	100,927	$	73,310
Research and development tax credit carryforwards	9,513		12,636
Operating lease liability	6,962		7,653
Stock-based compensation	5,161		9,680
Foreign tax credit carryforwards	648		633
Accruals and other	8,138		8,219
Total deferred tax assets before allowance	131,349		112,131
Less: Valuation allowance	(114,275)		(106,438)
Deferred tax assets, net	17,074		5,693
Deferred tax liabilities:
Debt discount (equity component)	12,045		—
Right of use asset	5,029		5,693
Deferred tax liability	17,074		5,693
Net deferred tax assets and liabilities	$	—	$	—

Certain of the amounts in the income tax rate table and deferred tax assets table above reflect reclassifications and corrections that were immaterial to the financial statements taken as a whole. The deferred tax assets and liabilities disclosure at December 31, 2019 has been adjusted to reflect the gross deferred tax right-of-use asset and related gross deferred lease liability recognized in accordance with ASC 842.

The Company has recorded a full valuation allowance related to its deferred tax assets due to the uncertainty of the ultimate realization of the future benefits from those assets. The table below summarizes changes in the deferred tax asset valuation allowance for the years ended December 31 (in thousands):


	2020		2019		2018
Balance at beginning of year	$	106,438	$	88,596	$	68,661
Charged to costs and expenses	8,423		13,784		19,935
Impact of change in tax rate	(586)		4,058		—
Balance at end of year	$	114,275	$	106,438	$	88,596

The total balance of unrecognized gross tax benefits for the years ended December 31, resulting from research and development tax credits claimed on the Company’s annual tax return was as follows (in thousands):


	2020		2019		2018
Unrecognized tax benefits at beginning of year	$	4,212	$	2,830	$	2,168
Additions based on current year tax positions	4,959		1,382		662
Unrecognized tax benefits at end of year	$	9,171	$	4,212	$	2,830

The Company classifies applicable interest and penalties on amounts due to tax authorities as a component of the provision for income taxes. The amount of accrued interest and penalties recorded in 2020, 2019 or 2018 was not significant. The Company does not anticipate that the amount of its existing unrecognized tax benefits will significantly increase or decrease within the next 12 months. Due to the presence of net operating loss carryforwards in most jurisdictions, the Company’s tax years remain open for examination by U.S. taxing authorities back to 2004.