Legion contains only humans.
Legion (TV series) . Dan Stevens stars as Haller , a mutant diagnosed with schizophrenia at a young age . Rachel Keller , Aubrey Plaza , Bill Irwin , Jeremie Harris , Amber Midthunder , Katie Aselton , and Jean Smart also star . In American comic books published by Marvel Comics , a mutant is a being ( usually otherwise human ) who possesses a genetic trait called an X-gene that allows the mutant to naturally develop superhuman powers and abilities .