8. Income Taxes

We did not provide for income taxes in 2016 and 2015 because we had a net operating loss for tax purposes in those years and the tax benefit that would have resulted from the statutory rate was fully offset by the valuation allowance.

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. We offset our deferred tax assets by a valuation allowance because we are uncertain about the timing and amount of any future profits. Significant components of our deferred tax assets are as follows (in thousands):


	December 31,
	2016		2015
Deferred tax assets:
Net operating loss carryforwards	$	22,300	$	17,100
Stock-related compensation		9,100		10,600
Research & development credit carryforwards		6,200		6,000
Other		200		500
		37,800		34,200
Valuation allowance		(37,800)		(34,200)
	$	—	$	—

As of the beginning of 2016, we increased both our net operating loss carryforwards and our valuation allowance by $0.9 million when we adopted ASU 2016-09 for certain tax deductions associated with stock option transactions greater than the stock-related compensation expense in our financial statements. The valuation allowance increased by $3.6 million in 2016 and $3.4 million in 2015.

Our pre-tax net operating loss carryforwards of $65.6 million are federal and expire between 2029 and 2036. As of December 31, 2016, we had federal research and development tax credits of approximately $10.4 million, which expire in the years 2023 through 2036.

Unrecognized tax benefits

We have unrecognized tax benefits related to tax credits. We added to our unrecognized tax benefits in 2016 and 2015 as follows (in thousands):



	2016		2015
Beginning balance	$	4,000	$	3,900
Additions based on tax positions related to the current year		200		100
Ending balance	$	4,200	$	4,000