8. Income taxes

        As of December 31, 2014, the Company had federal net operating loss carryforwards of approximately $108.0 million and state net operating loss carryforwards of $106.9 million, which are available to reduce future taxable income. The Company also had federal tax credits of $2.3 million and state tax credits of $842,000, which may be used to offset future tax liabilities. The net operating loss (NOL) and tax credit carryforwards will expire at various dates through 2034. Net operating loss and tax credit carryforwards are subject to review and possible adjustment by the Internal Revenue Service and state tax authorities and may become subject to an annual limitation in the event of certain cumulative changes in the ownership interest of significant stockholders over a three-year period in excess of 50%, as defined under Sections 382 and 383 of the Internal Revenue Code, 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.

        A reconciliation of income taxes computed using the U.S. federal statutory rate to that reflected in operations follows:

                                                                                                                                                                                    

        The principal components of the Company's deferred tax assets are as follows (in thousands):

                                                                                                                                                                                    

        The Company has recorded a valuation allowance against its deferred tax assets at December 31, 2014 and 2013 because the Company's management believes that it is more likely than not that these assets will not be fully realized. The increase in the valuation allowance of $10.3 million, $14.5 million and $18.3 million in the years ended December 31, 2012, 2013 and 2014, respectively, 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. From inception and through December 31, 2014, the Company had no unrecognized tax benefits or related interest and penalties accrued. The Company has not, as yet, conducted a study of research and development (R&D) credit carryforwards. This study may result in an adjustment to the Company's R&D credit carryforwards; however, until a study is completed and any adjustment is known, no amounts are being presented as an uncertain tax position. A full valuation allowance has been provided against the Company's R&D credits and, if an adjustment is required, this adjustment would be offset by an adjustment to the valuation allowance. Thus, there would be no impact to the consolidated balance sheet or statement of operations if an adjustment were required. The Company would recognize both accrued interest and penalties related to unrecognized benefits in income tax expense. The Company's uncertain tax positions are related to years that remain subject to examination by relevant tax authorities. Since the Company is in a loss carryforward position, the Company is generally subject to examination by the U.S. federal, state and local income tax authorities for all tax years in which a loss carryforward is available.