5. Stock-Based Compensation

The Company's 2000 Stock Option and Incentive Plan (2000 Plan) allowed for the grant of awards in respect of an aggregate of 150,297 shares of the Company's common stock in the form of incentive stock options, non-qualified stock options, stock appreciation rights, restricted stock and restricted stock units and other performance awards. The 2000 Plan has expired, and no further awards may be issued under the plan. Any shares of common stock subject to awards under the 2000 Plan that expire, terminate, or are otherwise surrendered, canceled, forfeited or repurchased without having been fully exercised, or resulting in any common stock being issued, will become available for issuance under the 2013 Stock Incentive Plan (2013 Plan) up to a specified number of shares.

Effective February 2003, the Company implemented the 2003 Equity Incentive Plan (2003 Plan), and it was amended and approved by the Company's stockholders in 2005. The 2003 Plan allowed for the grant of awards in respect of an aggregate of 4,336,731 shares of the Company's common stock. Stock options granted under the 2003 Plan may be either incentive stock options as defined by the Internal Revenue Code (IRC), or non-qualified stock options. In October 2013, the 2003 Plan was terminated, and no further awards may be issued under the plan. Any shares of common stock subject to awards under the 2003 Plan that expire, terminate, or are otherwise surrendered, canceled, forfeited or repurchased without having been fully exercised, or resulting in any common stock being issued, will become available for issuance under the 2013 Plan, up to a specified number of shares.

In October 2013, the Company implemented the 2013 Plan. The 2013 Plan provides for the grant of stock options and other stock-based awards, as well as cash-based performance awards. The aggregate number of shares of common stock initially available for issuance pursuant to awards under the 2013 Plan was 1,960,168 shares. The number of shares of common stock reserved for issuance will automatically increase on January 1 of each year from January 1, 2014 through and including January 1, 2023, by the lesser of (a) 1,960,168 shares, (b) 4.0% of the total number of shares of common stock outstanding on December 31 of the preceding calendar year, or (c) the number of shares of common stock determined by the Board of Directors. If an option expires or terminates for any reason without having been fully exercised, if any shares of restricted stock are forfeited, or if any award terminates, expires or is settled without all or a portion of the shares of common stock covered by the award being issued, such shares are available for the grant of additional awards. However, any shares that are withheld (or delivered) to pay withholding taxes or to pay the exercise price of an option are not available for the grant of additional awards.

The following stock-based compensation amounts were recognized for the periods indicated (in thousands):

	Three Months Ended June 30,						Six Months Ended June 30,
	2015			2014			2015			2014
Research and development		$	881		$	365		$	1,691		$	682
General and administrative			1,021			416			1,842			711
Total stock-based compensation expense		$	1,902		$	781		$	3,533		$	1,393

Employee Stock Options

The fair value of each option award is estimated on the date of grant using the Black-Scholes option-pricing model using the assumptions in the following table:

	Six Months Ended June 30,
	2015	2014
Expected dividend yield	0%	0%
Expected volatility	74%	67 %
Risk-free interest rate	1.6% - 2.1%	2.1% - 2.3%
Expected term	6.25 years	6.25 years

The following table summarizes stock option activity under the Plan during the six months ended June 30, 2015:

	Shares			Weighted- Average Exercise Price		Weighted-Average Remaining Contractual Term (Years)		Aggregate Intrinsic Value (in thousands)
Outstanding, December 31, 2014		3,572,116		$	11.40		7.3
Granted		137,388			32.17
Exercised		(205,617	)		2.26
Forfeited or expired		(19,425	)		18.25
Outstanding, June 30, 2015		3,484,462			12.72		7.1		$	87,994
June 30, 2015:
Exercisable		1,804,814			5.56		5.6			58,488
Vested and expected to vest		3,272,505			12.35		7.0			83,835

The weighted-average grant-date fair value of options granted for the six months ended June 30, 2015 was $21.47. The total intrinsic value of options exercised during the six months ended June 30, 2015 was approximately $6.5 million, and the total cash received for options exercised was approximately $0.4 million. The total fair value of shares vested in the six months ended June 30, 2015 was approximately $3.3 million. As of June 30, 2015, the total unrecognized compensation expense related to non-vested stock options, net of related forfeiture estimates, was approximately $18.7 million, which the Company expects to recognize over a weighted-average period of approximately three years.