The Company incurred net operating losses for the years ended December 31, 2020, 2019, and 2018. The Company has not reflected a benefit of such net operating loss carryforwards in the accompanying financial statements. The Company has established a full valuation allowance against its net deferred tax assets due to the uncertainty surrounding realization of such assets.

A reconciliation of the federal statutory rate to the Company’s effective tax rate is as follows for the years ended December 31, 2020, 2019, and 2018 (in thousands):

	Year Ended December 31,
	2020			2019			2018
Tax benefit at federal statutory rate	$	(39,948	)	$	(22,081	)	$	(10,972	)
State income taxes		(16,570	)		(4,694	)	—
Tax credits		(8,010	)		(6,530	)		(2,387	)
Uncertain tax positions		2,003			2,044			708
Stock-based compensation		353			1,493			1,251
Nondeductible expense		51			124			58
Tax benefit recognized on utilization of attributes previous reserved	—				(13,582	)	—
Deferred adjustment from amended tax return	—				(1,073	)	—
Change in valuation allowance		62,280			44,748			11,346
Others		(159	)		(449	)		(4	)
Income tax provision	$	—		$	—		$	—

The tax effects of temporary differences that give rise to significant components of the Company’s deferred tax assets consist of (in thousands):

	December 31,
	2020			2019
Deferred tax assets:
Net operating loss	$	54,382		$	2,223
Accrued bonus		1,201			370
Tax credits		13,000			7,319
Stock-based compensation		6,493			2,015
Deferred revenue		32,129			37,621
Lease liability		12,457			11,781
Others		463			209
Gross deferred tax assets		120,125			61,538
Less valuation allowance		(105,226	)		(47,053	)
Total deferred tax assets	$	14,899		$	14,485
Deferred tax liabilities:
Depreciation and amortization	$	(6,815	)	$	(7,495	)
Right-of-use assets		(7,934	)		(6,990	)
Others		(150	)	—
Gross deferred tax liabilities		(14,899	)		(14,485	)
Deferred tax assets, net	$	—		$	—

In assessing the realization of deferred tax assets, management considers whether it is more likely than not that some portion or all of the deferred assets will be realized. The ultimate realization of the deferred tax assets is dependent upon the generation of future taxable income during the periods in which those temporary differences become deductible. Evaluating the need for a valuation allowance for deferred tax assets often requires judgment and analysis of all the positive and negative evidence available, including cumulative losses in recent years and projected future taxable income, to determine whether all or some portion of the deferred tax assets will not be realized. As of December 31, 2020, the Company has utilized a full valuation allowance to offset the net deferred tax assets as the Company believes it is not more likely than not that the net deferred tax assets will be fully realizable. The valuation allowance for deferred tax assets increased by $58.2 million during the year ended December 31, 2020.

As of December 31, 2020, the Company had federal and state net operating loss (NOL) carryforwards of approximately $195.3 million and $191.4 million, respectively. Federal NOL carryforwards have an indefinite life and deductions cannot exceed 80% of taxable income. State NOL carryforwards will begin to expire as early as 2030, if not utilized, or have an indefinite life. As of December 31, 2020, the Company also had federal and California tax credit carryforward of approximately $14.4 million and $3.8 million, respectively. The federal tax credits will begin to expire in 2036 while the California tax credits have no expiration date.

Generally, utilization of the NOL carryforwards and credits may be subject to an annual limitation due to the ownership change limitations provided by Section 382, which provides for limitations on NOL carryforwards and certain built-in losses following ownership changes, and Section 383, which provides for special limitations on certain excess credits, etc., of the Code, and similar state provisions. Accordingly, the Company’s ability to utilize NOL carryforwards may be limited as the result of such an “ownership change.” The carryforwards could be subject to an annual limitation, resulting in a reduction in the gross deferred tax assets before considering the valuation allowance. Further, a portion of the carryforwards may expire before being applied to reduce future earnings. The Company is not aware of any changes in ownership that would result in material limitations under Section 382 at this time.

The following table summarizes the activity related to the Company’s unrecognized tax benefits for the years ended December 31, 2020 and 2019 (in thousands):

Balance as of December 31, 2018	$	1,507
Decreases related to tax positions taken during the prior year		(80	)
Increases related to tax positions taken during the current year		1,608
Balance as of December 31, 2019		3,035
Decreases related to tax positions taken during the prior year		(446	)
Increases related to tax positions taken during the current year		2,003
Balance as of December 31, 2020	$	4,592

If the unrecognized tax benefits for uncertain tax positions as of December 31, 2020, is recognized, there will be no impact to the effective tax rate as the tax benefit would increase the net deferred tax assets, which is currently offset with a full valuation allowance. The Company’s policy is to include interest and penalties related to unrecognized tax benefits, if any, within the provision for taxes in the consolidated statements of operations. The Company did not accrue any interest or penalties and does not have any tax positions for interest or penalties for the year ended December 31, 2020. The Company does not have any tax positions for which it is reasonably possible that the total amount of gross unrecognized tax benefits will significantly change within 12 months of December 31, 2020.

The Company recognizes the tax benefit of an uncertain tax position only if it is more likely than not that the position is sustainable upon examination by the taxing authority, based on the technical merits. During the year ended December 31, 2020, the Company recorded a tax reserve of $2.0 million as a reduction of the tax credit carryover. The Company’s income tax returns generally remain subject to examination by federal and most state tax authorities. The Company is currently not subject to any income tax audits by federal or state taxing authorities. The statute of limitations for tax liabilities for all years remains open.