The Company has incurred net operating losses for all the periods presented. The Company has not reflected the benefit of any such net operating loss carryforwards in the accompanying financial statements.

The effective tax rate for the years ended December 31, 2020 and 2019 is different from the federal statutory rate primarily due to the valuation allowance against deferred tax assets as a result of insufficient sources of income. The reconciliation of the federal statutory income tax rate to the Company’s effective income tax rate is as follows:

	Year Ended December 31,
	2020			2019
Income tax benefit at the federal statutory rate		21.0	%		21.0	%
State income taxes, net of federal benefit	3.2				1.1
Research and development tax credits	1.3				2.8
Other		(0.6	)		(0.3	)
Mark to market gain/loss	1.7				—
Change in valuation allowance		(26.6	)		(24.6	)
Total		—	%		—	%

Deferred income taxes reflect the net effects of temporary differences between the carrying amounts of assets and liabilities for financial reporting purposes and the amounts used for income tax purposes. The principal components of the Company’s net deferred tax assets consisted of the following (in thousands):

	December 31,
	2020			2019
Deferred tax assets
Federal and state net operating loss carryforwards	$	27,567		$	21,262
Research and development tax credits		4,887			4,140
Other		748			488
Gross deferred tax assets		33,202			25,890
Less: valuation allowance		(33,141	)		(25,841	)
Total deferred tax assets		61			49
Deferred tax liabilities
Depreciation		(61	)		(49	)
Total deferred tax liabilities		(61	)		(49	)
Net deferred tax assets	$	—		$	—

The Company has incurred annual net operating losses in each year since inception. The Company has not reflected the benefit of any such net operating loss carryforwards in the financial statements. Due to the Company’s history of losses, and lack of other positive evidence, the Company has determined that it is more likely than not that its net deferred tax assets will not be realized and, therefore, the net deferred tax assets are fully offset by a valuation allowance at December 31, 2020 and 2019. The Company increased its valuation allowance by $7.3 million for the year ended December 31, 2020 in order to maintain a full valuation allowance against its deferred tax assets.

As of December 31, 2020, the Company had federal net operating loss carryforwards, or NOLs, of $125.4 million and federal tax credits of $5.4 million available to offset tax liabilities. The Company’s federal NOLs and federal tax credit carryforwards begin to expire in 2035 and 2036, respectively. Of the federal NOLs, $91.3 million have an indefinite life. The Company also had gross state NOLs of $20.2 million and state tax credits of $1.8 million which are available to offset state tax liabilities. The state NOLs expire in 2036 and the state tax credit carryforwards can be carried forward indefinitely. Federal and state NOLs and tax credit carryforwards are also subject to annual limitations in the event that cumulative changes in the ownership interests of significant stockholders exceed 50% over a three-year period, as defined under Sections 382 and 383 of the Internal Revenue Code of 1986. The Company has not completed an analysis to determine if the NOLs and tax credits are limited due to a change in ownership. Should there be ownership changes that occurred, the Company’s ability to utilize existing carryforwards could be substantially restricted.

The Company determines its uncertain tax positions based on whether and how much of a tax benefit taken by the Company in its tax filings is more likely than not to be sustained upon examination by the relevant income tax authorities.

	Year Ended December 31,
	2020		2019
Balance—beginning of year	$	1,406	$	873
Addition based on tax position related to current year		406		533
Addition based on tax position related to prior year		169		—
Balance—end of year	$	1,981	$	1,406

The unrecognized tax benefits, if recognized, would not have an impact on the Company’s effective tax rate assuming the Company continues to maintain a full valuation allowance position. Based on prior year’s operations and experience, the Company does not expect a significant change to its unrecognized tax benefits over the next twelve months. The unrecognized tax benefits may increase or change during the next year for unexpected or unusual items for items that arise in the ordinary course of business.

The Company files income tax returns in the U.S., California, Florida, Illinois, Indiana, Maryland, Massachusetts and Texas. Due to net operating loss carryforwards, all years remain open for income tax examination, however, the Company is not currently under examination by any taxing authority for any open tax year.