6.	Stock Based Compensation

The following table summarizes stock option activity under the Company’s equity incentive plans and related information:

	Number of Shares Underlying Outstanding Options			Weighted- Average Exercise Price		Weighted-Average Remaining Contractual Term (Years)
Outstanding — December 31, 2017		821,741		$	13.69		9.17
Options granted		227,900		$	14.83
Options exercised		(59	)	$	4.63
Options canceled/forfeited		(23,189	)	$	4.46
Outstanding — March 31, 2018		1,026,393		$	14.15		9.03
Exercisable — March 31, 2018		228,386		$	34.41		7.72
Vested and expected to vest — March 31, 2018		1,026,393		$	14.15		9.03

Valuation Assumptions

The Company estimated the fair value of stock options granted using the Black-Scholes option-pricing formula and a single option award approach. Due to its limited history as a public company and limited number of sales of its common stock, the Company estimated its volatility considering a number of factors including the use of the volatility of comparable public companies. The expected term of options granted under the Plan, all of which qualify as “plain vanilla” per SEC Staff Accounting Bulletin 107, is determined based on the simplified method due to the Company’s limited operating history and is 6.01 years based on the average between the vesting period and the contractual life of the option. The risk-free rate is based on the yield of a U.S. Treasury security with a term consistent with the option. This fair value is being amortized ratably over the requisite service periods of the awards, which is generally the vesting period.

The fair value of employee stock options was estimated using the following weighted-average assumptions for the three months ended March 31, 2018 and 2017:

	Three Months Ended March 31,
	2018			2017
Employee Stock Options:
Risk-free interest rate		2.40	%		1.57	%
Expected term (in years)		6.01			6.04
Dividend yield		—			—
Volatility		109.32	%		73.28	%
Weighted-average fair value of stock options granted	$	12.34		$	21.15

Total stock-based compensation recognized was as follows (in thousands):

	Three Months Ended March 31,
	2018		2017
Research and development	$	115	$	20
General and administrative⁽¹⁾		491		100
Total stock-based compensation	$	606	$	120

(1)

2018 includes $0.1 million in modification stock-based compensation expense related to a Board member’s departure.

As of March 31, 2018, 150,175 shares of common stock were available for future grant and 1,026,393 options to purchase shares of common stock were outstanding. As of March 31, 2018, the Company had unrecognized employee stock-based compensation expense of $5.6 million, related to unvested stock awards, which is expected to be recognized over an estimated weighted-average period of 3.15 years.