12. Income Taxes

The Company accounts for income taxes under FASB Accounting Standards Codification 740 ("ASC 740"). Deferred income tax assets and liabilities are determined based upon differences between financial reporting and tax bases of assets and liabilities and are measured using the enacted tax rates and laws that will be in effect when the differences are expected to reverse. The components of the income tax provision for the years ended December 31, 2019, 2018 and 2017 are as follows:

A reconciliation of the U.S. statutory income tax rate to the Company’s effective tax rate is as follows for the years ended December 31, 2019, 2018 and 2017:

The principal components of the Company’s deferred tax assets and liabilities consist of the following at December 31, 2019 and 2018 (in thousands):

As of December 31, 2019, the Company had federal net operating loss (“NOL”) carryforwards of approximately $248.2 million and state net operating loss carryforwards of $251.7 million which are available to reduce future taxable income. The Company also had federal tax credits of approximately $11.9 million and state tax credits of $2.9 million which may be used to offset future tax liabilities. Net operating losses generated before 2018 of approximately $135.7 million will expire at various dates through 2037, and net operating losses of approximately $112.5 million, which were generated after 2017 have an indefinite carryforward period. The NOL and tax credit carryforwards are subject to review and possible adjustment by the Internal Revenue Service and state tax authorities. Net operating loss and tax credit carryforwards may become subject to an annual limitation in the event of certain cumulative changes in the ownership interest of significant shareholders over a three-year period in excess of 50%, as defined under Sections 382 and 383 of the Internal Revenue Code, respectively, as well as similar state provisions. This could limit the amount of tax attributes that can be utilized annually to offset future taxable income or tax liabilities. The amount of the annual limitation is determined based on the value of the Company immediately prior to the ownership change. Subsequent ownership changes may further affect the limitation in future years. The Company has not determined whether an ownership change has occurred and as such, the Company's NOLs may be limited.

ASC 740 requires a valuation allowance to reduce the deferred tax assets reported if, based on the weight of the evidence, it is more likely than not that some portion or all of the deferred tax assets will not be realized. After consideration of all the evidence, both positive and negative, the Company has recorded a valuation allowance against its deferred tax assets at December 31, 2018 and 2019, respectively because the Company's management has determined that it is more likely than not that these assets will not be fully realized. The increase in the valuation allowance of $23.2 million in 2019 and $19.5 million in 2018 primarily relates to the net loss incurred by the Company.

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. As of December 31, 2019 and 2018 the Company had no unrecognized tax benefits or accrued interest or penalties related to unrecognized tax benefits. The Company’s policy is to recognize both interest and penalties related to unrecognized tax benefits in income tax expense.

The Company completed a study to document its qualifying research credits for all years except the years ended December 31, 2018 and 2019. For the years ended December 31, 2018 and 2019, the Company generated research credits but has not conducted a study to document the qualified activities. This study may result in an adjustment to the Company’s research and development credit carryforwards; however, until a study is completed and any adjustment is known, no amounts are being presented as an unrecognized tax benefit for the years ended December 31, 2018 and 2019. A full valuation allowance has been provided against the Company’s research and development credits and, if an adjustment is required, this adjustment would be offset by an adjustment to the deferred tax asset established for the research and development credit carryforwards and the valuation allowance.

The federal and state income tax returns are generally subject to examinations for the tax years ended December 31, 2016 through December 31, 2019. To the extent the Company has tax attribute carryforwards, the tax years in which the attribute was generated may still be adjusted upon examination by the Internal Revenue Service or state tax authorities to the extent utilized in a future period. The Company files income tax returns in the U.S. federal and Massachusetts jurisdictions. There are currently no federal or state audits in process.