11.	Stock-Based Compensation

A summary of the Company’s stock option activity and related information is as follows:

				Options Outstanding
	Shares Available for Grant			Number of Stock Options Outstanding			Weighted- Average Exercise Price		Weighted- Average Remaining Contractual Life (Years)		Aggregate Intrinsic Value
											(In thousands)
Balance at December 31, 2017		969,783			8,592,747		$	3.69		7.4	$	47,864
Additional options authorized		2,706,158			—
Options granted		(2,003,229	)		2,003,229			7.15
Options exercised		—			(396,939	)		1.80
Options canceled		155,459			(155,459	)		5.41
Balance at June 30, 2018		1,828,171			10,043,578		$	4.43		7.6	$	25,933
Vested and exercisable at June 30, 2018					5,411,367		$	3.08		6.4	$	21,106
Vested and expected to vest at June 30, 2018					9,701,142		$	4.38		7.6	$	25,536

The weighted-average grant date fair value of options granted to employees was $4.18 and $3.13 per share during the six months ended June 30, 2018 and 2017, respectively. The grant date fair value of options vested was $2.5 million and $1.9 million during the six months ended June 30, 2018 and 2017, respectively.

Aggregate intrinsic value represents the difference between the estimated fair value of the underlying common stock and the exercise price of outstanding, in-the-money options. The aggregate intrinsic value of options exercised was $2.2 million and $1.5 million during the six months ended June 30, 2018 and 2017, respectively.

At June 30, 2018, total unrecognized compensation cost related to stock-based awards granted to employees, net of estimated forfeitures, was $14.8 million which is expected to be recognized over a weighted-average period of 3.0 years.

Determination of Fair Value

The determination of the fair value of stock options on the date of grant using an option-pricing model is affected by the estimated fair value of the Company’s common stock, as well as assumptions regarding a number of complex and subjective variables. The variables used to calculate the fair value of stock options using the Black-Scholes option-pricing model include actual and projected employee stock option exercise behaviors, expected price volatility of the Company’s common stock, the risk-free interest rate and expected dividends. Each of these inputs is subjective and generally requires significant judgment to determine.

Fair Value of Common Stock

Beginning March 31, 2016, the Company’s common stock shares were listed on The NASDAQ Global Market, or NASDAQ. Fair value of the common stock is the adjusted closing price of the Company’s common stock on the trading date on these stock exchanges.

Expected Term

The expected term represents the period that the Company’s option awards are expected to be outstanding. The Company considers several factors in estimating the expected term of options granted, including the expected lives used by a peer group of companies within the Company’s industry that the Company considers to be comparable to its business and the historical option exercise behavior of its employees, which the Company believes is representative of future behavior.

Expected Volatility

As the Company has a limited trading history for its common stock, the expected stock price volatility for the Company’s common stock was estimated by taking a combination of the average historic price volatility of the Company’s common stock and industry peers based on daily price observations over a period equivalent to the expected term of the stock option grants. Industry peers consist of several public companies in the Company’s industry which were the same as the comparable companies used in the common stock valuation analysis. The Company intends to continue to consistently apply this process using the same or similar public companies until a sufficient amount of historical information regarding the volatility of its own share price becomes available, or unless circumstances change such that the identified companies are no longer similar to the Company, in which case, more suitable companies whose share prices are publicly available would be used in the calculation.

Risk-Free Interest Rate

The risk-free interest rate is based on the zero-coupon U.S. Treasury notes, with maturities similar to the expected term of the options.

Expected Dividend Yield

The Company does not anticipate paying any dividends in the foreseeable future and, therefore, uses an expected dividend yield of zero in the Black-Scholes option-valuation model.

In addition to the Black-Scholes assumptions discussed immediately above, the estimated forfeiture rate also has a significant impact on the related stock-based compensation. The forfeiture rate of stock options is estimated at the time of grant and revised in subsequent periods if actual forfeitures differ from those estimates. The Company uses historical data to estimate pre-vesting option forfeitures and records stock-based compensation expense only for those awards that are expected to vest.

The fair value of employee stock option was estimated at the date of grant using a Black-Scholes option-pricing model with the following weighted-average assumptions:

	Six Months Ended June 30,
	2018		2017
Expected term (in years)		6.0		6.0
Expected volatility%	61.2%		67.1%
Risk-free interest rate%	2.8%		2.1%
Expected dividend yield%		0.0%		0.0%

Restricted Stock Units

From time to time, the Company grants Restricted Stock Units, or RSUs, to its board of directors for their services. These RSUs were fully vested upon issuance and will be released and settled upon termination of the board services or the occurrence of a change in control event. The fair value of RSUs is based on the closing market price of the Company’s common stock on the grant date.

In September 2016 and November 2017, the Company granted 112,578 shares and 43,554 shares of RSUs, to its board members and these RSUs had a grant-date fair value of $3.58 and $8.02 per share, respectively. 18,964 shares of these RSUs with a grant-date fair value of $3.58 per share were released in December 2017 in connection with the resignation of one board member. In connection with the departure of two board members in June 2018, 34,502 shares of RSUs with a grant date fair value of $3.58 per share and 12,467 shares of RSUs with a grant date fair value of $8.02 per share are expected to be released in the third quarter of fiscal 2018.

In November 2017, the Company granted 12,468 shares of RSUs with a grant-date fair value of $8.02 per share to one executive officer upon his termination. These RSUs were fully vested upon issuance and were released in the first quarter of fiscal 2018.

For the six months ended June 30, 2018 and 2017, no RSUs were issued and no stock-based compensation expense related to RSUs was recorded in the accompanying condensed consolidated statements of operations.

Stock-Based Compensation Expense

Total stock-based compensation expense recognized in the Company’s condensed consolidated statements of operations is classified as follows (in thousands):

	Three Months Ended June 30,				Six Months Ended June 30,
	2018		2017		2018		2017
Research and development	$	347	$	216	$	550	$	381
Selling and marketing		194		72		321		123
General and administrative		1,142		725		2,002		1,278
Total stock-based compensation expense	$	1,683	$	1,013	$	2,873	$	1,782

During the three and six months ended June 30, 2018 and 2017, there were no stock-based compensation expenses capitalized as a component of inventory or recognized in cost of revenue. Stock-based compensation relating to stock-based awards granted to consultants were insignificant during the three and six months ended June 30, 2018 and 2017.