6. Income Taxes

The Company did not provide for income taxes in 2020 and 2019 because it had book and federal taxable losses in those years and the tax benefit that would have resulted from the pre-tax losses was fully offset by a change in the valuation allowance.

The reconciliation of the statutory federal income tax rate to the Company’s effective tax rate for the years ended December 31, 2020 and 2019 was as follows:



	Year ended December 31,
	2020		2019
Tax at federal statutory rate	21.0	%		21.0	%
State tax, net of federal benefit	—			—
Share-based compensation	(42.8)			(2.4)
Research and development credits	1.6			0.6
Change in valuation allowance	20.2			(19.2)
Effective income tax rate	—	%		—	%

Deferred tax assets and valuation allowance

Deferred tax assets reflect the tax effects of net operating loss and tax credit carryforwards and temporary differences between the carrying amounts of assets and liabilities for financial reporting purposes and the amounts used for income tax purposes. The Company’s deferred taxes assets at December 31, 2020 and 2019 were valued at the corporate tax rate of 21%. The Company offsets its deferred tax assets by a valuation allowance because it is uncertain about the timing and amount of any future profits. Significant components of its deferred tax assets are as follows (in thousands):


	December 31,
	2020		2019
Deferred tax assets:
Net operating loss carryforwards	$	18,333	$	17,110
Share-based compensation		3,230		5,817
Research and development credit carryforwards		6,687		6,587
Other		264		237
Total deferred tax assets:		28,514		29,751
Valuation allowance		(28,447)		(29,725)
Net deferred tax assets:		67		26
Deferred tax liabilities:
Property and equipment		(5)		(7)
Operating lease right-of-use assets		(62)		(19)
Total deferred tax liabilities:		(67)		(26)
Net deferred tax asset / (liability):	$	—	$	—

The valuation allowance decreased by $1.3 million in 2020. The valuation allowance decreased by $2.4 million due to a write down of the share-based compensation deferred tax asset and is offset in part by a $1.1 million increase due to additional book losses during the year. The valuation allowance increased by $0.9 million in 2019 due to additional book losses during the year.

The Company’s pre-tax net operating loss carryforwards of $87.3 million are federal, of which $74.1 million expires between 2029 and 2039 and $13.2 million carries forward indefinitely. As of December 31, 2020, the Company had federal research and development tax credits of approximately $11.1 million, which expire in the years 2023 through 2040.

Unrecognized tax benefits

As of December 31, 2020 and 2019, the Company has unrecognized tax benefits related to tax credits of $4.5 million and $4.4 million, respectively. None of the unrecognized tax benefits as of December 31, 2020, if recognized, would impact the effective tax rate due to the valuation allowance and no interest or penalties have been recognized. A reconciliation of the beginning and ending balance of unrecognized tax benefits is as follows (in thousands):



		Year ended December 31,
	2020		2019
Beginning balance	$	4,400	$	4,400
Additions based on tax positions related to the current year		100		—
Ending balance	$	4,500	$	4,400

As of December 31, 2020, there were no unrecognized tax benefits that we expect would change significantly over the next 12 months.

The Company files U.S. and Texas income tax returns. In the United States, the statute of limitations with respect to the federal income tax returns for tax years after 2016 are open to audit; however, since the Company has net operating losses, the taxing authority has the ability to review tax returns prior to the 2017 tax year and make adjustments to these net operating loss carryforwards. We are not under audit in any taxing jurisdiction at this time.