7.	Stockholders’ Equity Common Stock

Stock Options

The following table summarizes the Company’s stock option activity since December 31, 2017:

	Number of Shares			Weighted Average Exercise Price		Weighted Average Remaining Contractual Term		Aggregate Intrinsic Value
						(in years)
Outstanding as of December 31, 2017		6,125,692		$	13.00		7.70	$	15,808
Granted		1,820,776			10.08
Exercised		(196,508	)		0.69
Forfeited		(305,920	)		16.96
Outstanding as of September 30, 2018		7,444,040		$	12.45		7.55	$	9,360,614
Options vested and expected to vest as of September 30, 2018		7,444,040			12.45		7.55		9,360,614
Options exercisable as of September 30, 2018		4,176,275		$	12.23		6.61	$	9,357,014

The weighted average grant-date fair value of stock options granted during the three and nine months ended September 30, 2018 and 2017 was $5.67, $6.83, $8.43 and $7.06 per share, respectively.

Restricted Stock Units

The Company has granted restricted stock units with time-based vesting conditions. The table below summarizes the Company’s restricted stock unit activity since December 31, 2017:

	Number of Shares			Weighted Average Grant Date Fair Value
Unvested restricted stock units as of December 31, 2017		356,778		$	10.01
Granted		2,000		$	7.43
Forfeited		(7,370	)	$	9.78
Vested		(61,500	)	$	10.12
Unvested restricted stock units as of September 30, 2018		289,908		$	9.98

Stock-based Compensation Expense

The Company recorded stock-based compensation expense related to stock options, restricted stock units and the Company’s Employee Stock purchase Plan (“ESPP”) in the following expense categories of its condensed consolidated statements of operations and comprehensive loss:

	Three Months Ended September 30,				Nine Months Ended September 30,
	2018		2017		2018		2017
Research and development expenses	$	2,246	$	2,158	$	6,436	$	6,124
General and administrative expenses		2,071		2,211		6,451		7,026
	$	4,317	$	4,369	$	12,887	$	13,150

Employee Stock Purchase Plan

The ESPP provides that eligible employees may contribute up to 15% of their eligible earnings toward the semi-annual purchase of the Company' common stock. The ESPP is qualified under Section 423 of the Internal Revenue Code. The employee's purchase price is derived from a formula based on the closing price of the common stock on the first day of the offering period versus the closing price on the date of purchase (or, if not a trading day, on the immediately preceding trading day). The offering period under the ESPP has a duration of six months, and the purchase price with respect to each offering period beginning on or after such date is, until otherwise amended, equal to 85% of the lesser of (i) the fair market value of the Company's common stock at the commencement of the applicable six-month offering period or (ii) the fair market value of the Company's common stock on the purchase date. The Company estimates the fair value of common stock under the ESPP using a Black-Scholes valuation model. The fair value was estimated on the date of grant using the Black-Scholes option valuation model and the straight-line attribution approach with the following weighted-average assumptions: risk-free interest rate (2.30%); expected term (0.5 years); expected volatility (60.2%); and an expected dividend yield (0%). The Company recorded an immaterial amount of stock-based compensation expense under the ESPP for the three and nine months ended September 30, 2018.

The total number of available ESPP shares is increased annually, beginning in 2016 and ending in 2025. The ESPP allows for share replenishment equal to the lesser of (i) 400,000 shares and (ii) 1% of the number of shares of the Company’s common stock outstanding on the last day of the preceding calendar year, or an amount determined by the Company’s Board of Directors. As of September 30, 2018, a total of 1.6 million shares were reserved and available for issuance under the ESPP.