In 1986 Bob Layton, writer of the X-Men spin-off "X-Factor", was running late on a deadline, and Simonson was called in to write a fill-in issue of "X-Factor". This story was never published, since Layton ultimately turned his story in on time, but while writing it Simonson found herself inspired by the characters, to the point where she brought a list of her ideas to editor Bob Harras in the hopes that Layton might use them for the series. Instead, Layton ended up dropping "X-Factor" shortly after, and at Chris Claremont and Ann Nocenti's suggestion, Harras chose Simonson as his replacement. In #6, her first issue, she and artist Jackson Guice introduced Apocalypse, a character who would go on to make repeated appearances in the X-Men franchise. From #10 of the title, she was joined by her husband, Walt Simonson, on pencils. In #25, the creators gave the character, Angel, blue skin and metal wings in a process which led to his being renamed as "Archangel". It was at Simonson's suggestion that X-Men writer Chris Claremont's "Mutant Massacre" story idea was turned into a crossover through all the "X-books", the first of its kind. Her run on "X-Factor" included the relevant installments of "Mutant Massacre", and the subsequent crossovers "Fall of the Mutants," "Inferno", and "X-Tinction Agenda". She ended her run on the title with #64 in 1991.