10. Stock-based compensation

In January 2017, the number of shares of common stock available for issuance under the 2013 Stock Option and Incentive Plan (“2013 Plan”) was increased by approximately 1.6 million shares as a result of the automatic increase provision of the 2013 Plan. As of September 30, 2017, the total number of shares of common stock available for issuance under the 2013 Plan was approximately 1.7 million.

Stock-based compensation expense

Stock-based compensation expense by award type included within the condensed consolidated statements of operations and comprehensive loss was as follows (in thousands):

	For the				For the
	Three Months Ended September 30,				Nine Months Ended September 30,
	2017		2016		2017		2016
Stock options	$	10,786	$	8,294	$	30,892	$	26,278
Restricted stock units		3,067		1,495		7,659		4,248
Employee stock purchase plan		130		116		402		305
	$	13,983	$	9,905	$	38,953	$	30,831

As of September 30, 2017, the Company had $122.5 million of unrecognized stock-based compensation expense related to unvested stock options, restricted stock units and the employee stock purchase plan, which is expected to be recognized over a weighted-average period of 2.7 years.

Stock-based compensation expense by classification included within the condensed consolidated statements of operations and comprehensive loss was as follows (in thousands):

	For the				For the
	Three Months Ended September 30,				Nine Months Ended September 30,
	2017		2016		2017		2016
Research and development	$	7,049	$	5,399	$	19,500	$	15,410
General and administrative		6,934		4,506		19,453		15,421
	$	13,983	$	9,905	$	38,953	$	30,831

Stock options

The following table summarizes the stock option activity under the Company’s equity award plans (shares in thousands):

	Shares			Weighted-average exercise price per share
Outstanding at December 31, 2016		3,735		$	52.17
Granted		1,066		$	83.88
Exercised		(613	)	$	23.59
Canceled or forfeited		(172	)	$	83.09
Outstanding at September 30, 2017		4,016		$	63.62
Exercisable at September 30, 2017		2,037		$	49.05
Vested and expected to vest at September 30, 2017		4,016		$	63.62

During the nine months ended September 30, 2017, 0.6 million shares of common stock were exercised, resulting in total proceeds to the Company of $14.5 million. In accordance with the Company’s equity award plans, the shares were issued from a pool of shares reserved for issuance under the equity award plans.

Restricted stock units

The following table summarizes the restricted stock unit activity under the Company’s equity award plans (shares in thousands):

	Shares			Weighted-average grant date fair value
Unvested balance at December 31, 2016		263		$	63.07
Granted		297		$	84.79
Vested		(78	)	$	73.58
Forfeited		(26	)	$	63.18
Unvested balance at September 30, 2017		456		$	75.41

Employee stock purchase plan

On June 3, 2013, the Company’s board of directors adopted its 2013 Employee Stock Purchase Plan (“2013 ESPP”), which was subsequently approved by its stockholders and became effective upon the closing of the Company’s initial public offering on June 24, 2013. The 2013 ESPP authorizes the initial issuance of up to a total of 238,000 shares of the Company’s common stock to participating employees. The first offering period under the 2013 ESPP opened on August 1, 2014. During the nine months ended September 30, 2017 and 2016, 20,773 shares and 18,338 shares of common stock were issued under the 2013 ESPP, respectively.