9. Stock‑based Compensation

Total compensation cost recognized for all stock‑based compensation awards in the Company’s condensed consolidated statements of operations was as follows (in thousands):


	Three Months Ended				Nine Months Ended
	September 30,				September 30,
	2018		2017		2018		2017
Research and development	$	3,506	$	2,431	$	11,412	$	9,130
General and administrative		3,193		2,037		8,839		6,157
Total stock-compensation expense	$	6,699	$	4,468	$	20,251	$	15,287

Restricted Stock

From time to time, upon approval by the Company’s board of directors, certain employees and advisors have been granted restricted shares of common stock. These shares of restricted stock are subject to repurchase rights. Accordingly, the Company has recorded the proceeds from the issuance of restricted stock as a liability in the condensed consolidated balance sheets included as a liability. The restricted stock liability is reclassified into stockholders’ equity as the restricted stock vests. A summary of the status of and changes in unvested restricted stock as of December 31, 2017 and September 30, 2018 is as follows:


		Weighted
		Average
		Grant Date
		Fair Value
	Shares	Per Share
Unvested Restricted Common Stock as of December 31, 2017	513,225	$	18.70
Issued	—		—
Vested	(225,225)	$	6.75
Forfeited	—		—
Unvested Restricted Common Stock as of September 30, 2018	288,000	$	28.05

For the nine months ended September 30, 2018, the expense for restricted stock awards related to non-employees and employees was $1.9 million and $0, respectively.

As of September 30, 2018, the Company had no unrecognized stock‑based compensation expense related to its employee unvested restricted stock awards and $9.0 million in unrecognized stock-based compensation expense related to its non-employee unvested restricted stock awards.

Stock Options

Certain of the Company’s stock option agreements allowed for the exercise of unvested awards. During 2014, options to purchase 75,304 shares of common stock for $0.03 per share were exercised prior to their vesting. The unvested shares were subject to repurchase by the Company if the employees ceased to provide service to the Company, with or without cause. As such, the Company did not treat the exercise of unvested options as a substantive exercise. The Company recorded the proceeds from the exercise of unvested stock options as a liability in the condensed consolidated balance sheets. The liability for unvested common stock subject to repurchase was reclassified into stockholders’ equity as the shares vested. As of June 30, 2018, the early exercise stock options were fully vested.

The following is a summary of stock option activity for the nine months ended September 30, 2018:


		Weighted Average		Remaining	Aggregate Intrinsic
	Shares	Exercise Price		Contractual Life (years)	Value (in thousands)
Outstanding at December 31, 2017	4,372,136	$	17.28	8.5	$	60,591
Granted	1,643,185	$	36.29	—		—
Exercised	(634,331)	$	13.73	—		—
Cancelled	(558,504)	$	24.88	—		—
Outstanding at September 30, 2018	4,822,486	$	23.38	8.2	$	48,750
Exercisable at September 30, 2018	1,881,620	$	17.65	7.5	$	28,201

The table above reflects restricted stock issued upon exercise of unvested stock options as exercised on the dates that the shares are no longer subject to repurchase. The Company had no unvested restricted common stock outstanding at September 30, 2018 and had 4,572 shares of unvested restricted common stock outstanding at December 31, 2017, resulting from the exercise of unvested stock options.

Using the Black‑Scholes option pricing model, the weighted average fair value of options granted to employees and directors during the nine months ended September 30, 2018 and 2017 was $25.65 and $16.71, respectively. The expense related to options granted to employees and directors was $14.9 million and $9.1 million for the nine months ended September 30, 2018 and 2017, respectively.

The fair value of each option issued to employees and directors was estimated at the date of grant using the Black‑Scholes option pricing model with the following weighted‑average assumptions:


	Three Months Ended				Nine Months Ended
	September 30,				September 30,
	2018		2017		2018		2017
Risk free interest rate	2.9	%	1.9	%	2.6	%	2.1	%
Expected dividend yield	—		—		—		—
Expected term (in years)	6.25		6.25		6.25		6.25
Expected volatility	77.3	%	78.0	%	79.8	%	77.9	%

There were no options issued to persons other than employees and directors during the nine months ended September 30, 2018. As of September 30, 2018, the Company had unrecognized stock‑based compensation expense related to its employee and director stock options of $51.8 million which the Company expects to recognize over the remaining weighted average vesting period of 2.53 years.