U.S and foreign components of consolidated loss before income taxes for the years ended December 31, 2020, 2019 and 2018, was as follows (in thousands):

	2020			2019			2018
Loss before income taxes:
U.S.	$	(61,246	)	$	(71,946	)	$	(58,048	)
Foreign		1,673			965			713
Loss before income taxes	$	(59,573	)	$	(70,981	)	$	(57,335	)

The provision for income taxes for the years ended December 31, 2020, 2019 and 2018, was as follows (in thousands):

	2020		2019		2018
Provision for income taxes:
Current:
Foreign	$	274	$	255	$	225
Federal		—		—		—
State		—		2		—
Total current		274		257		225
Deferred:
Foreign		—		—		—
Federal		6		4		3
State		4		2		1
Total deferred		10		6		4
Provision for income taxes	$	284	$	263	$	229

The difference between the provision for income taxes and the amount computed by applying the federal statutory income tax rate to loss before taxes for the years ended December 31, 2020, 2019 and 2018, was as follows (in thousands):


	2020			2019			2018
Federal statutory tax	$	(12,510	)	$	(14,906	)	$	(12,040	)
Federal research credits		(1,630	)		(1,857	)		(1,390	)
State research credits		(749	)		(821	)		(655	)
Expiration of federal carryovers		9,200			5,472			4,154
Expiration of state carryovers		—			—			1,344
Change in valuation allowance		6,738			13,059			9,913
Compensation related items		978			158			(361	)
State taxes		(1,921	)		(1,111	)		(1,141	)
Other		178			269			405
Provision for income taxes	$	284		$	263		$	229

Deferred income taxes reflect the net tax effects of temporary differences between the carrying amounts of assets and liabilities for financial reporting purposes and the amounts used for income tax purposes at the enacted rates. The significant components of the Company’s deferred tax assets and liabilities at December 31, 2020, 2019 and 2018, were as follows (in thousands):

	December 31,
	2020			2019
Deferred tax assets:
Net operating loss carryforwards	$	141,176		$	135,536
Research and development credit carryforwards		28,892			28,291
Capitalized research and development		10,756			12,832
Compensation related items		10,957			9,843
Operating leases		4,214			4,374
Other		6,051			4,547
Total deferred tax assets		202,046			195,423
Valuation allowance		(199,042	)		(192,304	)
Net deferred tax assets	$	3,004		$	3,119

Deferred tax liabilities:
Right-of-use assets	$	2,892		$	3,017
Amortization of goodwill		163			143
Total deferred tax liabilities	$	3,055		$	3,160

The valuation allowance increased by $6.7 million for the year ended December 31, 2020, compared to the increase of $13.1 million and $9.9 million for the years ended December 31, 2019 and 2018, respectively. The Company believes that, based on a number of factors, the available objective evidence creates sufficient uncertainty regarding the realizability of the deferred tax assets such that a valuation allowance has been recorded. These factors include the Company’s history of net losses since its inception, the need for regulatory approval of the Company’s products prior to commercialization and expected near-term future losses. The Company expects to maintain a valuation allowance until circumstances change.

For the year ended December 31, 2020, the Company reported pretax net losses on its consolidated statement of operations and calculated taxable losses for both federal and state taxes. The difference between reported net loss and taxable loss are due to differences between book accounting and the respective tax laws.

The Company's tax losses and credits are subject to varying carryforward periods. The gross amounts and dates of expiration of the significant carryforwards are as follows:

			Expires		Expires		Expires		No
	Total		2021-2023		2024-2030		2031-2040		Expiration
Federal losses carryovers	$	638,103	$	55,587	$	190,581	$	209,368	$	182,567
California loss carryovers		75,407		—		28,779		46,628		—
Federal research credits		19,249		7,786		2,248		9,215		—
California research credits		12,207		—		—		—		12,207
Federal foreign tax credits		610		—		610		—		—

The Company’s ability to utilize net operating loss and research and development credit carryforwards is limited by (a) its ability to generate future taxable income, (b) varying apportionment and allocation rules, and (c) limitations pursuant to the ownership change rules in accordance with Section 382 of the Internal Revenue Code of 1986 and with Section 383 of the Internal Revenue Code of 1986, as well as similar state provisions.

The Company’s unrecognized tax benefits relate to federal and California research tax credits. These tax credits have not been utilized on any tax return and currently have no impact on the Company’s tax expense due to the Company’s operating losses and the related valuation allowances.

The following is a tabular reconciliation of the total amounts of unrecognized tax benefits (in thousands):

	December 31,			December 31,
	2020			2019
Unrecognized tax benefits at beginning of period	$	10,842		$	11,063
Decreases related to expired carryforwards		(1,171	)		(729	)
Increases related to current year tax positions		439			508
Unrecognized tax benefits at end of period	$	10,110		$	10,842

The Company will recognize accrued interest and penalties related to unrecognized tax benefits in its income tax expense. To date, the Company has not recognized any interest and penalties in its consolidated statements of operations, nor has it accrued for or made payments for interest and penalties.