In 1975, editor Len Wein gave Claremont the writing duties for the relaunched Uncanny X-Men series.[5] During his 17 years as X-Men writer, Claremont wrote or co-wrote many classic X-Men stories, such as "The Dark Phoenix Saga" and "Days of Future Past." The second X-Men film was loosely based on his X-Men graphic novel God Loves, Man Kills.[8]
During his X-Men tenure, Claremont became especially known for his strong characterizations of the female members of the team,[9] particularly Phoenix and Storm. Under Claremont's direction, Jean Grey a.k.a. Marvel Girl, one of Marvel's first female heroes, underwent a huge transformation into the omnipotent Phoenix.[10] Similarly, Storm a.k.a. Ororo Munroe, became one of the first relevant African-American superheroines of the era. She was the first black female to play either a major or supporting role in the big two comic book houses, Marvel and DC Comics; one of her defining moments being successfully battling Cyclops for leadership of the X-Men.[11] In addition, Claremont co-created numerous other important female X-Men characters, including Rogue, Psylocke, Shadowcat, Phoenix, Mystique, Lady Mastermind, Emma Frost, Siryn, Jubilee, Rachel Summers, and Madelyne Pryor. (He also co-created such notable male characters as Sabretooth, Avalanche, Strong Guy, Multiple Man, Captain Britain, Mister Sinister, and Gambit.)
In addition, Claremont helped launch spin-offs such as X-Men, New Mutants, Excalibur and Wolverine. In 1986, Marvel launched an X-Men spinoff, X-Factor, altering the Phoenix/Jean Grey continuity Claremont had established. In 1991, Marvel launched a second X-Men title simply called X-Men with Claremont as writer. After writing three issues of the new X-Men title, Claremont left the book (and for the time being, Marvel Comics) after a series of clashes with editor Bob Harras.[