In the alternate future seen in Spider-Man: Reign, Mary Jane died of cancer due to prolonged exposure to Peter's radioactive body fluids, her last words being "Go..." as he leaped out of a window to respond to sirens. It is believed that Mary Jane was trying to say "Go get 'em, Tiger." After her death, Peter buried his red and blue costume with her, wearing a black suit until his retirement. However, with the coming of the Sinister Six (and thanks to some encouragement from the tentacles of the now-deceased Doctor Octopus), Peter reclaims his original suit from Mary Jane's coffin and returns to his life as Spider-Man. After the Six are defeated and the WEBB surrounding New York is destroyed, Peter visits Mary Jane's grave, assuring her that he'll be with her again some day, but, for now, he still has his responsibility.
Read more about this topic: Alternative Versions Of Mary Jane Watson