11. Income Taxes

Our tax provision for interim periods is determined using an estimate of our annual effective tax rate, adjusted for discrete items arising in that quarter. Our effective tax rate differs from the U.S. statutory tax rate primarily due to valuation allowances on our deferred tax assets as it is more likely than not that some or all of our deferred tax assets will not be realized. Income taxes were a benefit of $1.3 million and an expense of $0.1 million for the three and nine months ended September 30, 2019, respectively, compared to a tax expense of $0.2 million and $2.9 million for the three and nine months ended September 30, 2018, respectively.