Stock-based compensation | 6. Stock ‑based compensation In December 2011, the Company adopted the 2012 Incentive Plan (the 2012 Plan). The 2012 Plan became effective upon the closing of the Company’s IPO in February 2012. The 2012 Plan provides for the grant of incentive stock options, nonqualified stock options, stock appreciation rights, restricted stock awards, restricted stock units and other stock ‑based and cash awards. Upon effectiveness, the number of shares of common stock that are reserved under the 2012 Plan is the sum of 3,428,571 shares plus the number of shares available under the Company’s prior 2010 Plan. The number of shares reserved under the 2012 Plan is increased by the number of shares of common stock (up to a maximum of 571,242 shares) subject to outstanding awards under the 2010 Plan that expire, terminate or are otherwise surrendered, cancelled, forfeited or repurchased. The 2012 Plan includes an “evergreen provision” that allows for an annual increase in the number of shares of common stock available for issuance under the 2012 Plan. The annual increase will be added on the first day of each year beginning in 2013 and each subsequent anniversary until the expiration of the 2012 Plan, equal to the lowest of 1,285,714 shares of common stock, 4.0% of the number of shares of common stock outstanding and an amount determined by the board of directors. On January 1, 2016 and 2015, the shares available under the 2012 Plan increased by 1,285,714 and 1,081,045 shares of common stock, respectively. In December 2014, the Company established an inducement award program (in accordance with NASDAQ Listing Rule 5635(c)(4)) under which it may grant non-statutory stock options to purchase up to an aggregate of 750,000 shares of common stock to new employees as inducement for prospective employees to enter into employment with the Company. The program is governed by the terms of the 2012 Plan but the shares are not issued pursuant to the 2012 Plan. The Company has granted 210,000 options to purchase shares under this program as of March 31, 2016 and 2015. There have been 75,000 shares returned to this program due to cancellations as of March 31, 2016. No shares had been cancelled and returned to the program as of March 31, 2015. Restricted common stock No restricted common stock was granted during the three months ended March 31, 2016 and 2015. The total fair value of shares vested during the three months ended March 31, 2015 was approximately $59,000 . All issued awards were fully vested as of December 31, 2015. Restricted stock units A summary of the Company’s restricted stock units (RSUs) activity and related information is as follows: Weighted- average grant date fair value Shares per share Unvested at December 31, 2015 $ Vested $ Forfeited — $ — Unvested at March 31, 2016 — $ — No RSUs were granted during the three months ended March 31, 2016 and 2015. The approximate total fair value of RSUs vested during the three months ended March 31, 2016 and 2015 was $65,000 and $873,000 , respectively . As of March 31, 2016, there was no unrecognized stock-based compensation expense related to unvested RSUs granted under the 2012 Plan. During the first quarter of 2013, the Company amended the terms of certain RSUs related to a total of 697,060 shares of common stock to allow for tax withholdings greater than the minimum required statutory withholding amount, of which there are no remaining outstanding as of March 31, 2016. As a result of this change in the terms of the awards, the outstanding RSUs were considered to be liability instruments. As a result of this modification, the Company recorded a liability for the fair value of the awards as of each reporting date with the change in fair value recorded through the statement of operations. The Company recorded stock ‑based compensation expense equal to the greater of the original grant date fair value of the awards or the settlement date fair value. As of March 31, 2016, all RSUs were fully vested. During the three months ended March 31, 2016 and 2015, the Company made approximate deposits with the taxing authorities of $5,000 and $223,000 , respectively, in respect of the tax liability for awards that settled during such periods. Stock options A summary of the Company’s stock option activity and related information follows: Weighted-average Weighted-average remaining Aggregate exercise price per contractual term intrinsic value Shares share (years) (in thousands) Outstanding at December 31, 2015 $ $ Granted $ Exercised $ Forfeited $ Outstanding at March 31, 2016 $ $ Vested at March 31, 2016 $ $ Vested and expected to vest at March 31, 2016(1) $ $ (1) This represents the number of vested options as of December 31, 2015, plus the number of unvested options expected to vest as of December 31, 2015, based on the unvested options at December 31, 2015, adjusted for the estimated forfeiture rate. The fair value of each stock option is estimated on the grant date using the Black-Scholes option-pricing model using the following weighted average assumptions: Three Months Ended March 31, 2015 Risk-free interest rate % % Volatility % % Dividend yield — — Expected term (years) |