12. Income Taxes

        There was no provision for income taxes for the years ended December 31, 2017, 2016 and 2015, because the Company has incurred operating losses since inception. At December 31, 2017, the Company concluded that it is not more likely than not that the Company will realize the benefit of its deferred tax assets due to its history of losses. Accordingly, a full valuation allowance has been applied against the net deferred tax assets.

        On December 22, 2017, the Tax Cuts and Jobs Act of 2017 ("TCJA") was signed into law making significant changes to the Internal Revenue Code. Changes include, but are not limited to, a corporate tax decrease from 34% to 21% effective for tax years after December 31, 2017, the transition of U.S. Tax from a worldwide to a territorial system, and potential additional limitations on deductions related to interest expense and executive compensation. The Company has calculated the impact of TCJA in its year end income tax provision in accordance with its current understanding of the TCJA and guidance currently available as of this filing and recorded a provisional reduction to its gross deferred tax assets of $50.4 million in the fourth quarter of 2017, the period in which the legislation was enacted. The provisional reduction in the Company's gross deferred tax assets was fully offset by an equal reduction in the Company's valuation allowance, resulting in no additional net income tax expense from the tax law change.

        In addition, on January 1, 2017, upon the Company's adoption of ASU 2016-09, the Company recognized approximately $4.5 million of deferred tax assets that were not previously recognized on the Company's balance sheet under the prior accounting guidance. The increase in the deferred tax assets was fully offset by an increase in the Company's valuation allowance.

        As of December 31, 2017, 2016 and 2015, the Company had deferred tax assets, before valuation allowance, of approximately $99.8 million, $75.3 million and $50.6 million, respectively. Realization of the deferred assets is dependent upon future taxable income, if any, the amount and timing of which are uncertain. Accordingly, the net deferred tax assets have been fully offset by a valuation allowance. As of December 31, 2017, 2016 and 2015, the Company had federal net operating loss ("NOL") carryforwards of approximately $347.4 million, $196.4 million and $137.4 million, respectively. The federal NOL carryforwards will expire at various dates beginning in 2028, if not utilized. The Company filed certain amended state tax returns for tax years 2012-2015 during 2017 that resulted in increasing the Company's state NOL carryforward. As of December 31, 2017, 2016 and 2015, the Company had state NOL carryforwards of approximately $327.8 million, $18.1 million and $15.4 million, respectively. The state NOL carryforwards will expire at various dates beginning in 2022, if not utilized.

        A reconciliation of the U.S. statutory income tax rate to the Company's effective tax rate is as follows:

                                                                                                                                                                                    

        If the Company experiences a greater than 50 percentage point aggregate change in ownership of certain significant stockholders over a three-year period, a Section 382 ownership change could be deemed to have occurred. If a section 382 change occurs, the Company's future utilization of the net operating loss carryforwards and credits as of the ownership change will be subject to an annual limitation under Section 382 of the Internal Revenue Code of 1986, as amended, and similar state provisions. Such an annual limitation may result in the expiration of net operating losses before utilization.

        The Company's reserves related to taxes are based on a determination of whether and how much of a tax benefit taken by the Company in its tax filings or positions is more likely than not to be realized following resolution of any potential contingencies present related to the tax benefit. The Company recognized no material adjustment for unrecognized income tax benefits. Through December 31, 2017, the Company had no unrecognized tax benefits or related interest and penalties accrued.

        Significant components of the Company's deferred tax assets are summarized in the table below: