The Company adopted the 2008 Stock Option and Incentive Plan, or the 2008 Plan, and the 2015 Equity Incentive Award Plan, or the 2015 Plan, providing for the issuance of stock-based compensation awards to its employees, officers, directors, advisors and consultants. With the establishment of the 2015 Plan, the Company no longer grants stock options under the 2008 Plan, and the shares available for future grants under the 2008 Plan were transferred to the 2015 Plan. In July 2018, the Company adopted the 2018 Equity Inducement Award Program, or the 2018 Plan. The operative terms of the 2018 Plan adhere to the terms and conditions of the 2015 Plan.

Only stock options were granted under the 2008 Plan. The 2015 Plan and the 2018 Plan provide for the grant of stock and stock-based awards including stock options, restricted stock awards, restricted stock units and stock appreciation rights.

Options granted pursuant to the 2008 Plan and the 2015 Plan may be either incentive stock options or non-statutory stock options. Options granted pursuant to the 2018 Plan are non-statutory stock options. Under the 2008 Plan, incentive stock options could only be granted to employees at an exercise price of no less than the fair value of the common stock on the grant date and non-statutory options may be granted to employees or consultants at an exercise price of no less than 85% of the fair value of the common stock on the grant date, as determined by the board of directors. Under the 2015 Plan and the 2018 Plan, for both incentive stock options and non-statutory options, the exercise price should not be less than the fair value of the common stock on the date of grant. Under the 2008 Plan, the 2015 Plan and the 2018 Plan, if, at the time of grant, the optionee is a 10% shareholder, owning stock representing more than 10% of the voting power of all classes of stock of the Company, the exercise price must be at least 110% of the fair value of the common stock on the grant date as determined by the board of directors. Options generally vest ratably over four years, and expire in 10 years from the date of grant, or five years from the date of grant for 10% shareholders.

In July 2015, the Company adopted the 2015 Employee Stock Purchase Plan, or the 2015 ESPP. At December 31, 2018 and 2017, 1,780,020 shares and 1,103,481 shares were reserved for issuance and no shares have been issued under the 2015 ESPP.

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 authorized		8,326,158			—
Granted		(6,997,114	)		6,997,114			8.44
Exercised		—			(2,608,812	)		2.03
Cancelled		1,377,341			(1,377,341	)		6.12
RSUs granted		(1,767,542	)		—
Balance at December 31, 2018		1,908,626			11,603,708		$	6.64		7.6	$	10,151
Vested and exercisable at December 31, 2018					4,299,323		$	4.28		5.7	$	8,581
Vested and expected to vest at December 31, 2018					11,124,469		$	6.58		7.6	$	10,092

The weighted-average grant date fair value of options granted to employees was $4.89, $3.38 and $2.72 per share for the years ended December 31, 2018, 2017 and 2016. The grant date fair value of options vested was $6.3 million, $4.8 million and $2.4 million, respectively, for the years ended December 31, 2018, 2017 and 2016, 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 $17.7 million, $2.6 million and $2.3 million for the years ended December 31, 2018, 2017 and 2016, respectively.

At December 31, 2018, total unrecognized compensation cost related to stock-based awards granted to employees, net of estimated forfeitures, was $29.0 million which is expected to be recognized over a weighted-average period of 3.2 years.

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.

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

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.

As the Company does not have a sufficient trading history for its common stock, the expected stock price volatility for the Company’s common stock was estimated by taking the average historic price volatility of industry peers based on daily price observations over a period equivalent to the expected term of the stock option grants. 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 stock price becomes available.

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.

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-pricing model.

In addition to the Black-Scholes assumptions discussed above, the estimated forfeiture rate also has a significant impact on 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 options was estimated at the date of grant using a Black-Scholes option-pricing model with the following weighted-average assumptions:

	Year Ended December 31,
	2018		2017		2016
Expected term (in years)		6.0		5.9		6.0
Expected volatility (%)	60.4%		66.0%		67.1%
Risk-free interest rate (%)	2.8%		2.1%		1.3%
Expected dividend yield (%)	0%		0%		0%

From time to time, the Company grants RSUs to its board of directors for their services. These RSUs are either fully vested upon issuance or vest over a period of time from the grant date and will be released and settled upon termination of the board member’s services or the occurrence of a change in control event. In December 2016, the Company granted RSUs to certain executive officers and one consultant for services rendered, and these RSUs were fully vested upon issuance. In November 2017, the Company granted RSUs to one executive officer upon his termination, and these RSUs were fully vested upon issuance. In July 2018, the Company granted RSUs to two new executive officers, and these RSUs vest in equal annual installments over three years from the grant date. The fair value of RSUs is based on the closing market price of the Company’s common stock on the grant date. The weighted -average grant date fair value of RSUs granted in fiscal year 2017 and 2016 was $8.02 per share and $3.52 per share, respectively.

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

	RSUs
	Number of Shares			Weighted Average Grant Date Fair Value
Unvested at December 31, 2017		—		$	—
RSUs granted		1,767,542		$	9.65
RSUs vested		(31,308	)	$	9.17
Unvested at December 31, 2018		1,736,234		$	9.65
Vested and unreleased		121,507
Outstanding at December 31 2018		1,857,741

The total grant date fair value of RSUs awarded was $17.0 million, $0.4 million and $0.5 million for the years ended December 31, 2018, 2017 and 2016, respectively. The total fair value of RSUs vested was $0.3 million, $0.4 million and $0.5 million for the years ended December 31, 2018, 2017 and 2016, respectively.

As of December 31, 2018, total unrecognized stock-based compensation cost related to RSUs was $13.3 million, which is expected to be recognized over a weighted-average period of 2.6 years. As of December 31, 2018, 1,616,819 shares of RSUs are expected to vest.

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

	Year Ended December 31,
	2018		2017		2016
Research and development	$	1,411	$	952	$	593
Selling and marketing		700		303		120
General and administrative		12,058		4,064		2,194
Total stock-based compensation expense	$	14,169	$	5,319	$	2,907

During the years ended December 31, 2018, 2017 and 2016, 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 was insignificant for the years ended December 31, 2018, 2017 and 2016.