League-wide Retirements
Jackie Robinson, the first African American player in the modern era of Major League Baseball, had his number 42 retired league-wide in 1997 (although those players who were wearing the number at the time were permitted to retain it for the duration of their careers — as of the 2013 season, Mariano Rivera is the only remaining active player wearing the number, and he has announced he will retire at the end of that season). Apart from Rivera, the only exception to this retirement is on April 15, the anniversary of Robinson's MLB debut, when all players wear 42. Wayne Gretzky's number 99 was likewise retired league-wide by the National Hockey League upon his retirement from the game.
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