In 1963 Lee/Kirby came up with a reusable explanation for characters having unique powers. Mutation. They collaborated on the first twenty issues of X-MEN from 1963 to 1966 and created the first 17 Marvel Universe Homo Superiors: Professor X, Cyclops, Iceman, Angel, Beast, Marvel Girl, Magneto, Vanisher, Blob, Quicksilver, Scarlet Witch, Toad, Mastermind, Unus, Lucifer, Juggernaut and Mimic.
A decade later Kirby created 5 more mutants for Marvel without Lee's input: Burner, Lifter, Peepers, Shocker and Slither.