From Cassius Comics

A human born with the "X-Gene", mutants, or Homo Superior possess unique abilities not found in normal Homo Sapiens. Although some of these unique traits are evident from birth (usually when they involve a unique physical trait), most X-Genes trigger with the onset of puberty. Still others never trigger at all, as some require a catalyst such as radiation that may never come to pass in the person's life.

Although there were once hundreds of thousands, if not millions of mutants on Earth, their numbers have dwindled to a mere 200 or so, following the events of M-Day.