No provision for income taxes was recorded for the years ended December 31, 2020, December 31, 2019 and December 31, 2018. The Company has incurred net operating losses only in the United States since its inception. The Company has not reflected any benefit of such net operating loss carryforwards in the financial statements.

The provision for income taxes differs from the amount expected by applying the federal statutory rate to the loss before taxes as follows:

	Year Ended December 31,
	2020			2019			2018
Federal statutory income tax rate		21.0	%		21.0	%		21.0	%
State income taxes		(1.9	%)		7.0	%		6.9	%
Change in valuation allowance		(21.5	%)		(31.4	%)		(29.4	%)
Research tax credits		2.7	%		3.6	%		2.7	%
Other permanent differences		(0.3	%)		(0.4	%)		(1.3	%)
Change in fair value of redeemable convertible preferred stock liability		0.0	%		0.2	%		0.1	%
Provision for income taxes		0.0	%		0.0	%		0.0	%

The tax effects of temporary differences and carryforwards of the deferred tax assets are presented below (in thousands):

	As of December 31,
	2020			2019
Deferred tax assets:
Net operating loss carryforwards	$	13,418		$	23,947
Research and development credit carryforwards		3,813			2,995
Lease liability		1,420			1,895
Intangible assets		1,274			1,809
Stock-based compensation		705			407
Accruals and reserves		505			436
Deferred revenue		17,332			—
Gross deferred tax assets		38,467			31,489
Less: Valuation allowance		(36,879	)		(29,462	)
Deferred tax assets, net of valuation allowance		1,588			2,027
Deferred tax liabilities:
Right-of-use assets		(1,100	)		(1,415	)
Property and equipment		(488	)		(612	)
Net deferred tax assets	$	—		$	—

The Company has established a full valuation allowance against its deferred tax assets due to the uncertainty surrounding the realization of such assets.

ASC 740 requires that the tax benefit of net operating losses, temporary differences and credit carryforwards be recorded as an asset to the extent that management assesses that realization is “more likely than not.” Realization of the future tax benefits is dependent on the Company’s ability to generate sufficient taxable income within the carryforward period. Because of the Company’s recent history of operating losses, management believes that recognition of the deferred tax assets arising from the above-mentioned future tax benefits is currently not likely to be realized and, accordingly, has provided a valuation allowance.

As of December 31, 2020, the Company had net operating loss carryforwards of $35.8 million available to reduce future taxable income, if any, for federal income tax purposes. As of December 31, 2020, the Company had net operating loss carryforwards of $83.8 million available to reduce future taxable income, if any, for California state income tax purposes. If not utilized, the federal carryforwards of $11.7 million and the state carryforwards of $83.8 million will begin to expire in 2037 and 2036, respectively. The federal net operating loss carryforwards of $24.1 million arising after December 31, 2017 do not expire.

The Company also had federal and state research and development credit carryforwards of $2.6 million and $2.5 million, respectively, as of December 31, 2020. The federal credits will expire starting in 2037 if not utilized and the state research credit can be carried forward indefinitely.

The Tax Reform Act of 1986 limits the use of net operating loss carryforwards in certain situations where changes occur in the stock ownership of a company. The annual limitation may result in the expiration of net operating losses and credits before utilization. The Company performed a Section 382 analysis through December 31, 2020. The Company has experienced ownership changes in the past and in the current year. As a result of the ownership changes, some of the tax attribute carryforwards may be permanently limited as they will expire unused. The Company is continuing to analyze the impact of the limitation. Subsequent ownership changes may affect the limitation in future years.

Related to unrecognized tax benefits noted below, the Company accrued no penalties or interest during the years ended December 31, 2020, December 31, 2019 and December 31, 2018. The Company does not expect its unrecognized tax benefit balance to change materially over the next 12 months.

The Company had $0.9 million and $0.7 million of unrecognized tax benefits as of December 31, 2020 and December 31, 2019, respectively.

The following table summarizes the activity related to the Company’s unrecognized tax benefits (in thousands).

Balance as of January 1, 2019	$	399
Increase related to prior year tax positions		59
Increase related to current year tax positions		288
Balance as of December 31, 2019	$	746
Increase related to prior year tax positions		2
Increase related to current year tax positions		153
Balance as of December 31, 2020	$	901

The Company files income tax returns in the U.S. federal jurisdiction and in California. For jurisdictions in which tax filings have been filed, all tax years remain open for examination by the federal and California state authorities for three and four years, respectively, from the date of utilization of any net operating losses or credits.