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Common Stock Warrants | 6 Months Ended
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Jun. 30, 2017
Warrants And Rights Note Disclosure [Abstract] |
Common Stock Warrants | 9. Common Stock Warrants As of June 30, 2017 and December 31, 2016, the following common stock warrants were outstanding:
Issuance Date |  | Expiration Date |  | Balance Sheet Classification |  | Exercise Price per Share |  |  | June 30, 2017 |  |  | December 31, 2016 |
March 2012 |  | March 2017 |  | Contingent warrant liability |  | $ | 35.20 |  |  |  | — |  |  |  | 479,277 |
September 2012 |  | September 2017 |  | Stockholders' deficit |  | $ | 70.80 |  |  |  | 1,967 |  |  |  | 1,967 |
February 2015 |  | February 2020 |  | Stockholders' deficit |  | $ | 66.20 |  |  |  | 9,063 |  |  |  | 9,063 |
February 2016 |  | February 2021 |  | Stockholders' deficit |  | $ | 15.40 |  |  |  | 8,249 |  |  |  | 8,249 |
|  |  |  |  |  |  |  |  |  |  | 19,279 |  |  |  | 498,556 |
In March 2012, in connection with an underwritten offering, the Company issued five-year warrants to purchase 741,729 shares of the Company’s common stock at an exercise price of $35.20 per share. These warrants contained provisions that were contingent on the occurrence of a change in control, which could conditionally obligate the Company to repurchase the warrants for cash in an amount equal to their estimated fair value using the Black-Scholes Model on the date of such change in control. Due to these provisions, the Company accounted for the warrants issued in March 2012 as a liability at estimated fair value. In addition, the estimated fair value of the liability related to the warrants was revalued at each reporting period until the earlier of the exercise of the warrants, at which time the liability would be reclassified to stockholders' equity at its then estimated fair value, or expiration of the warrants. In March 2017, all of these warrants expired unexercised.

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