A reconciliation of income tax (benefit) expense computed at the statutory corporate income tax rate to the effective income tax rate for the years ended December 31, 2021, 2020 and 2019 is as follows:


	Year Ended December 31,
	2021			2020			2019
Federal income tax at statutory rate		21.0	%		21.0	%		21.0	%
State income tax		4.5	%		4.5	%		4.2	%
Change in valuation allowance		(23.9	)%		(31.3	)%		(25.2	)%
Executive compensation		(2.0	)%		—			—
Equity investments		(0.7	)%		(0.6	)%		(5.7	)%
Tax credits		0.9	%		4.8	%		4.4	%
Non-deductible expenses and change in fair value of warrant liability		0.9	%		(0.2	)%		(0.1	)%
Other expenses		(0.6	)%		0.3	%		1.4	%

Effective tax rate		0.1	%		(1.5	)%		—

The Company’s deferred tax assets and liabilities consist of the following (in thousands):


	Year Ended December 31,
	2021			2020
Deferred tax assets:
Net operating loss carryforwards	$	174,127		$	91,467
Tax credit carryforwards		37,455			20,338
Accrued expenses		2,690			1,265
Deferred revenue		45,928			28,590
Stock-based compensation		318,049			—
Amortizable intangibles		3,834			4,198
Tenant allowance		2,927			2,206

Deferred tax assets before valuation allowance		585,010			148,064
Valuation allowance		(583,107	)		(143,827	)

Deferred tax assets, net of valuation allowance		1,903			4,237
Deferred tax liabilities:
Amortizable intangibles		(4,722	)		—
Property and equipment		(830	)		(830	)
Basis differences		(1,522	)		(5,270	)

Deferred tax liabilities		(7,074	)		(6,100	)

Net deferred taxes	$	(5,171	)	$	(1,863	)

Activity in the deferred tax assets valuation allowance is summarized as follows (in

thousands):


	Beginning of Period		Additions		End of Period
Deferred tax assets valuation allowance:
Year ended December 31, 2021	$	143,827	$	439,280	$	583,107
Year ended December 31, 2020	$	104,745	$	39,082	$	143,827

The Company has evaluated the positive and negative evidence bearing upon its ability to realize the deferred tax assets. The Company considered its history of cumulative net losses incurred since inception and has concluded that it is more likely than not that it will not realize the benefits of the deferred tax assets. Accordingly, a valuation allowance has been established against the deferred tax assets as of December 31, 2021 and 2020 that are not expected to be realized. The Company reevaluates the positive and negative evidence at each reporting period. The valuation allowance increased on a net basis by approximately $439.3 million during the year ended December 31, 2021 primarily due to an increase in the deferred tax asset related to stock-based compensation and the increase in the net operating losses and tax credits carryforwards.

As of December 31, 2021, the Company had federal net operating loss carryforwards of approximately $665.2 million, of which $139.2 million begin to expire in 2029. The Company has approximately $526.0 million of federal net operating losses as of December 31, 2021 that can be carried forward indefinitely. As of December 31, 2021, the Company had state net operating loss carryforwards of approximately $529.3 million, of which $485.9 million begin to expire in 2029. The Company has approximately $43.4 million of state net operating losses as of December 31, 2021 that can be carried forward indefinitely.

As of December 31, 2021, the Company had federal research and development tax credit carryforwards of approximately $23.3 million which begin to expire in 2029. As of December 31, 2021,

the Company also had

state research and development and investment tax credit carryforwards of approximately

$18.0 million which begin to expire in 2030.

Under Section

s 382 and 383 of the U.S. Internal Revenue Code, if a corporation undergoes an ownership change, the corporation’s ability to use its

pre-change

net operating loss carryforwards and other

pre-change

tax attributes, such as research tax credits, to offset its post-change income and taxes may be limited. In general, an ownership change generally occurs if there is a cumulative change in its ownership by 5% stockholders that exceeds 50 percentage points over a rolling three-year period. Similar rules may apply under U.S. state tax laws. The Company may have experienced an ownership change in the past and may experience ownership changes in the future as a result of future transactions in its share capital, some of which may be outside of the Company’s control. As a result, if the Company earns net taxable income, the Company’s ability to use its

pre-change

net operating loss carryforwards, or other

pre-change

tax attributes, to offset U.S. federal and state taxable income and taxes may be subject to significant limitations.

We assess the impact of various tax reform proposals and modifications to existing tax treaties in all jurisdictions where we have operations to determine the potential effect on our business and any assumptions we have made about our future taxable income. We cannot predict whether any specific proposals will be enacted, the terms of any such proposals or what effect, if any, such proposals would have on our business if they were to be enacted. Beginning in 2022, the Tax Cuts and Jobs Act of 2017 eliminates the currently available option to deduct research and development expenditures and requires taxpayers to amortize them over five years. The U.S. Congress is considering legislation that would defer the amortization requirement to future periods, however, we have no assurance that the provision will be repealed or otherwise modified.

The Company files tax returns as prescribed by the tax laws of the jurisdictions in which the Company operates. In the normal course of business, the Company is subject to examination by federal and state jurisdictions, where applicable. There are currently no pending tax examinations. As of December 31, 2021, the Company’s tax years are still open under statute from 2018 to the present in the United States and 2016 to the present in the Netherlands.

To the extent the Company has tax attribute carryforwards, the tax years in which the attribute was generated may still be adjusted upon examination by the Internal Revenue Service or state taxing authorities to the extent utilized in a future period.

The Company accounts for uncertain tax positions using a more likely than not threshold for recognizing and resolving uncertain tax positions. The evaluation of uncertain tax positions is based on factors that include, but are not limited to, changes in tax law, the measurement of tax positions taken or expected to be taken in tax returns, the effective settlement of matters subject to audit, new audit activity and changes in facts or circumstances related to a tax position. The Company evaluates uncertain tax positions on an annual basis and adjusts the level of the liability to reflect any subsequent changes in the relevant facts surrounding the uncertain positions. The Company accounts for interest and penalties related to uncertain tax positions as part of its provision for income taxes. As of December 31, 2021 and 2020, the Company had no recorded liabilities for uncertain tax positions and had no accrued interest or penalties related to uncertain tax positions. The Company does not expect a material change in unrecognized tax benefits in the next twelve months.