For the years ended December 31, 2015, 2014 and 2013, our total stock-based compensation expense was approximately $2.4 million, $1.6 million and $2.9 million, respectively.  We have not recorded any income tax benefit related to stock-based compensation for the years ended December 31, 2015, 2014 and 2013.

A summary of the status of our stock options as of December 31, 2015, and changes during the year then ended, is presented below: 

The weighted average grant-date fair value of options granted in 2015, 2014, and 2013 was $1.67, $1.56 and $1.81, respectively.  During 2015, 146,625 stock options with an aggregate intrinsic value of $144,000 were exercised in exchange for issuance of 124,238 shares of our common stock, resulting in gross proceeds of $66,000.  During 2014, 468,000 stock options with an aggregate intrinsic value of $582,000 were exercised in exchange for issuance of 299,360 shares of our common stock, resulting in gross proceeds of $70,000.  During 2013, 60,000 stock options with an aggregate intrinsic value of $126,000 were exercised in exchange for issuance of 39,649 shares of our common stock, resulting in gross proceeds of $39,000.  In 2015, 2014, and 2013, the aggregate fair value of stock options vested during the year was $277,000, $4,000 and $36,000, respectively.

A summary of the status of our unvested restricted stock as of December 31, 2015, and changes during the year then ended, is presented below:

During 2015, 2014 and 2013, 333,250, 216,250 and 745,000 shares, respectively, of restricted stock vested with aggregate vesting date fair values of $511,000, $387,000 and $2.3 million, respectively.

During 2015, 120,000 shares of restricted stock held by non-employee directors with an aggregate fair value of $193,000 vested as scheduled according to the terms of the restricted stock agreements.  Also during 2015, 193,250 shares of restricted stock held by employees with an aggregate fair value of $286,000 vested as scheduled according to the terms of the restricted stock agreements.  During 2015, 27,000 shares of restricted stock held by employees with an aggregate fair value of $50,000 were forfeited in connection with their separation from employment.  In April 2015, 20,000 shares of restricted stock held by an executive officer with an aggregate fair value of $32,000 vested upon reaching a milestone as defined by the terms of the restricted stock agreement.  In May 2015, 20,000 shares of restricted stock held by an executive officer with an aggregate fair value of $25,000 were forfeited in connection with his separation from employment.  In July 2015, 61,000 shares of restricted stock held by non-employee directors with an aggregate fair value of $107,000 were forfeited as a result of their departures from the Board.

During 2014, 61,250 shares of restricted stock held by non-employee directors with an aggregate fair value of $111,000 vested as scheduled according to the terms of the restricted stock agreements.  Also during 2014, 40,000 shares of restricted stock held by executive officers with an aggregate fair value of $52,000 vested upon reaching certain milestones as defined by the terms of the restricted stock agreements.  In March 2014, 100,000 shares of restricted stock with an aggregate fair value of $205,000 vested as scheduled according to the terms of the restricted stock agreement.  In May 2014, 175,000 shares of restricted stock held by our former CEO with an aggregate fair value of $278,000 were forfeited in connection with his separation from employment.  In September 2014, 125,000 shares of restricted stock held by our former CEO with an aggregate fair value of $166,000 were forfeited in connection with termination of his Consulting Agreement.  In December 2014, 15,000 shares of restricted stock held by our former CEO with an aggregate fair value of $19,000 vested as scheduled in accordance with the terms of the restricted stock agreement.

In February 2013, 100,000 shares of restricted stock with an aggregate fair value of $310,000 vested as scheduled according to the terms of a restricted stock agreement.  In March 2013, the Company received FDA approval to market Lymphoseek.  As a result of the Lymphoseek approval, 560,000 shares of restricted stock vested with an aggregate fair value of $1.8 million.  In April 2013, 85,000 shares of restricted stock held by non-employee directors with an aggregate fair value of $224,000 vested as scheduled according to the terms of the restricted stock agreements.  In July 2013, 29,250 shares of restricted stock with an aggregate fair value of $83,000 were forfeited as a result of a non-employee director departing from the Board.

During 2015, 2014 and 2013, we paid minimum tax withholdings related to stock options exercised and restricted stock vested of $24,000, $131,000, and $659,000, respectively.  As of December 31, 2015, there was approximately $1.1 million of total unrecognized compensation cost related to stock option and restricted stock awards, which we expect to recognize over remaining weighted average vesting terms of 1.6 years.  See Note 1(e).