5. STOCK-BASED COMPENSATION

We use broad-based stock plans to attract and retain highly qualified officers and employees and to help ensure that management’s interests are aligned with those of our shareholders. We have also granted equity-based awards to directors, nonemployees, and certain employees of Cellectis.

In December 2014, we adopted the Calyxt, Inc. Equity Incentive Plan (2014 Plan), which allowed for the grant of stock options, and in June 2017, we adopted the 2017 Omnibus Plan (2017 Plan), which allowed for the grant of stock options, performance shares and other types of equity awards.

As of March 31, 2020, 1,976,394 shares were registered and available for grant under effective registration statements, while 2,770,295 shares were available for grant in the form of stock options, restricted stock, restricted stock units, and performance stock units under the 2017 Plan. Stock-based awards currently outstanding also include awards granted under the 2014 Plan, under which no further awards can be granted.

Stock Options

The estimated fair values of stock options granted, and the assumptions used for the Black-Scholes option pricing model were as follows:

We estimate the fair value of each option on the grant date or other measurement dates if applicable using a Black-Scholes option-pricing model, which requires us to make predictive assumptions regarding future stock price volatility, employee exercise behavior and dividend yield. The risk-free interest rate for periods during the expected term of the options is based on the United States Treasury zero-coupon yield curve in effect at the date of grant. We estimate our future stock price volatility using the historical volatility of comparable public companies over the expected term of the option. Our expected term represents the period that options granted are expected to be outstanding determined using the simplified method. We have not paid dividends on our common stock and we do not currently plan to pay any cash dividends in the foreseeable future.

Option strike prices are set at 100 percent or more of the closing share price on the date of grant, and generally vest over six years following the grant date. Options generally expire 10 years after the date of grant.

Information on stock option activity is as follows:

Stock-based compensation expense related to stock option awards is as follows:

At March 31, 2020, options outstanding and exercisable had no aggregate intrinsic value and the weighted average remaining contractual term was 6.6 years.

Net cash proceeds from the exercise of stock options less shares used for minimum withholding taxes and the intrinsic value of options exercised were as follows:

As of March 31, 2020, unrecognized compensation expense related to non-vested stock options was $10.6 million. This expense will be recognized over 56 months on average.

Restricted Stock Units

Units settled in stock subject to a restricted period may be granted under the 2017 Plan. Restricted stock units generally vest and become unrestricted over five years after the date of grant.

Information on restricted stock unit activity is as follows:

The total grant-date fair value of restricted stock unit awards that vested is as follows:

Stock-based compensation expense related to restricted stock units is as follows:

We treat stock-based compensation awards granted to employees of Cellectis as deemed dividends. We recorded deemed dividends as follows:

As of March 31, 2020, unrecognized compensation expense related to restricted stock units was $2.5 million. This expense will be recognized over 43 months on average.

Performance Stock Units

In June 2019, we granted 311,667 performance stock units under the 2017 Plan to three executive officers. The performance stock units will vest at 50%, 100% or 120% of the shares under the award at the end of a three-year performance period based upon increases in the value of our common stock from the grant price of $12.48. The performance stock units will be settled in restricted stock upon vesting, with restrictions on transfer lapsing on the second anniversary of the restricted stock issuance date.

Stock-based compensation expense related to performance stock units is as follows:

As of March 31, 2020, unrecognized compensation expense related to performance stock units was $1.9 million. This expense will be recognized over 51 months on average.