2. Income Tax Expense

As part of the process of preparing our consolidated financial statements we are required to determine our income taxes in each of the jurisdictions in which we operate. This process involves estimating our actual current tax expense together with assessing temporary differences resulting from recognition of items for income tax and accounting purposes. These differences result in deferred tax assets and liabilities, which are included within our consolidated balance sheet. We must then assess the likelihood that our deferred tax assets will be recovered from taxable income during the carryback period or in the future; and to the extent we believe that recovery is not more likely than not, we must establish a valuation allowance. To the extent we establish a valuation allowance or increase this allowance in a period, we must reflect this increase as an expense within the tax provision in the statement of operations. We do not provide for income taxes on undistributed earnings of foreign subsidiaries, as our intention is to permanently reinvest these earnings.

The Tax Cut and Jobs Act of 2017 (the Tax Act) changed many aspects of U.S. corporate income taxation and included a reduction of the corporate income tax rate from 35% to 21%, implementation of a territorial tax system, and imposition of a tax on deemed repatriated earnings of foreign subsidiaries (the Transition Tax). We estimated the impact of the Tax Act in our financial statements as of December 31, 2017. We recorded $0.6 million in tax expense related to the Transition Tax and recognized $1.0 million in tax benefit related to the remeasurement of deferred taxes to the 21% tax rate. We timely filed our 2017 U.S. tax return with immaterial adjustments to the estimated tax provision recorded as of December 31, 2017.

We recognize, measure, present and disclose in our financial statements any uncertain tax positions that we have taken, or expect to take on a tax return. We operate in multiple taxing jurisdictions, both within and without the United States, and may be subject to audits from various tax authorities. Management’s judgment is required in determining our provision for income taxes, our deferred tax assets and liabilities, liabilities for uncertain tax positions, and any valuation allowance recorded against our net deferred tax assets. We will monitor the realizability of our deferred tax assets and adjust the valuation allowance accordingly.

Our policy is to classify interest and penalties related to unrecognized tax benefits as income tax expense.

Our 2018 income tax expense varies from the statutory rate mainly due to the generation of federal and state tax credits, permanent items, and different statutory rates from our foreign subsidiaries. Additionally, in the second quarter of 2018, we recognized certain discrete items primarily related to the exercise of stock options. Our 2017 income tax expense varied from the statutory rate mainly due to federal and state tax credits, permanent items, different statutory rates from our foreign entities, and discrete stock option exercises.

We have reviewed the tax positions taken, or to be taken, in our tax returns for all tax years currently open to examination by a taxing authority. As of September 30, 2018, the gross amount of unrecognized tax benefits exclusive of interest and penalties was $584,000. We remain subject to examination until the statute of limitations expires for each respective tax jurisdiction. The statute of limitations will be open with respect to these tax positions until 2025. A reconciliation of beginning and ending amount of our unrecognized tax benefits is as follows:


	Nine months ended September 30, 2018
	(in thousands)
Unrecognized tax benefits as of December 31, 2017	$	525
Additions for tax positions of current year		61
Additions for tax positions of prior years		(2	)
Reductions for settlements with taxing authorities		—
Reductions for lapses of the applicable statutes of limitations		—

Unrecognized tax benefits as of September 30,	$	584

As of September 30, 2018, a summary of the tax years that remain subject to examination in our taxing jurisdictions is as follows:


United States		2015 and forward
Foreign		2011 and forward