Chipotle Mexican Grill Inc. said four incumbent board members won't run for re-election in May.

The board of the Denver restaurant chain (NYSE: CMG), which has been criticized by at least one activist investor for exhibiting " a shocking dearth of racial and gender diversity," won't include John Charlesworth, Patrick Flynn, Darlene Friedman and Stephen Gillett after the company's annual shareholder meeting on May 25.

Last year, activist investor CtW Investment Group asked shareholders not to support the board re-election efforts of Flynn and Friedman, but that effort wasn't successful.

CtW argued that many of Chipotle's 12 board members had served too long on the board, as well as having other reservations, adding "i n our view, a board exhibiting low diversity, questionable independence and high tenure is most liable to function as an echo chamber and not as an effective means of differentiated thought leadership or oversight."

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