an X-factor gene.
for example:
hank mcCoy was born a mutant because his father worked in a chemical lab and absorbed mad radiation.
his father's x-gene was dormant but it was expressed in his son, thus beast.
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but aliens can be mutants aswell, thanos is the only exception(devient gene)
alien mutants:
-warlock of the technarks
-a ton of skrulls
-terrax