In February 2019, Private Chinook adopted the 2019 Equity Incentive Plan (the “2019 Plan”). In connection with the Merger, we assumed Aduro’s two equity incentive plans, the 2015 Equity Incentive Plan (the “2015 Plan”) and the 2009 Stock Incentive Plan (the “2009 Plan,” and collectively the “Aduro Plans”). No additional grants may be made from the 2009 Plan; however, shares subject to awards granted under the 2009 Plan remain subject to the terms of the 2009 Plan. The number of shares subject to and the exercise prices applicable to these awards were adjusted to reflect the one-for-five reverse stock split.

As of September 30, 2021 and December 31, 2020, there were 1.4 million and 0.9 million shares available for future grant, respectively.


	Number of Shares Underlying Options (in thousands)			Weighted- Average Exercise Price		Aggregate Intrinsic Value
						(In thousands)
Balance—December 31, 2020		5,514		$	13.24	$	38,433
Granted		1,371			15.49
Exercised		(281	)		6.17	$	2,864
Canceled		(395	)		29.27
Balance—September 30, 2021		6,209		$	13.04	$	25,980
Options exercisable—September 30, 2021		2,374		$	16.20	$	13,193
Options vested and expected to vest—September 30, 2021		6,209		$	13.04	$	25,980

The aggregate intrinsic value represents the difference between the exercise price of the options and the closing price of our common stock for stock options that were in-the-money at September 30, 2021.

The weighted average grant-date fair value of options granted was $10.53 and $3.64 for the nine months ended September 30, 2021 and 2020, respectively. As of September 30, 2021, the total unrecognized compensation expense related to unvested options was $26.8 million, which is expected to be recognized over a weighted-average period of 2.8 years.

We estimate the fair value of stock options using the Black Scholes option-pricing model. The fair value of stock options is amortized on a straight-line basis over the requisite service period of the awards. The fair value of stock options is estimated at the date of grant using a Black-Scholes option-pricing model with the following weighted-average assumptions:

	Three Months Ended September 30,						Nine Months Ended September 30,
	2021			2020			2021			2020
Expected term (in years)	6.2			0			6.1			6.1
Volatility		77.6	%		0.0	%		79.0	%		79.5	%
Risk-free interest rate		1.0	%		0.0	%		0.8	%		0.8	%
Dividend yield		—			—			—			—

	RSUs Outstanding
	Number of RSUs (in thousands)			Weighted- Average Grant Date Fair Value Per Share
Balance—December 31, 2020		441		$	14.51
Granted		523			15.18
Vested		(25	)		14.75
Forfeited		(37	)		14.69
Balance—September 30, 2021		902		$	14.89

The total fair value of RSUs that vested in the nine months ended September 30, 2021 was $0.4 million. The fair value of RSUs is determined on the date of grant based on the market price of our common stock on that date. As of September 30, 2021, there was $10.5 million of unrecognized stock-based compensation expense related to RSUs, which is expected to be recognized over a weighted-average period of 2.3 years.

We had 0.7 million shares available for future issuance under the 2015 ESPP as of September 30, 2021.

The fair value of our common stock to be issued under the 2015 ESPP is estimated at the date of grant using a Black-Scholes option-pricing model with the following assumptions:


	Three Months Ended September 30,						Nine Months Ended September 30,
	2021			2020			2021			2020
Expected term (in years)		—			—			0.5			—
Volatility		0.0	%		0	%		60.9	%		0	%
Risk-free interest rate		0.0	%		0	%		0.0	%		0	%
Dividend yield		—			—			—			—

As of September 30, 2021, the unrecognized stock-based compensation expense related to the ESPP was not material.

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

	Three Months Ended September 30,				Nine Months Ended September 30,
	2021		2020		2021		2020
Research and development	$	1,522	$	151	$	4,282	$	334
General and administrative		1,765		164		5,078		380
Total stock-based compensation expense	$	3,287	$	315	$	9,360	$	714