Roughly ten percent of the human population possesses the metagene - a gene that, when subjected to an outside catalyst, might rewrite a human's biology so that they have powers of some sort. Examples of outside catalysts include radiation, bioengineering, strange chemicals and alien tinkering. The metagene is unusual in that it seems to hide until it is activated. It is almost impossible to test for when it is dormant. Attempts to artificially activate the metagene are inconsistent at best, often killing the subject. Rarely are such attempts duplicatable.
One specific subset of metahumans are mutants: metahumans who are born with an activated metagene or whose metagene activates during puberty without an outside catalyst. See the MUTANT section.
Humans who gain their powers due to magic, technological enhancement, or alien heritage are not technically metahumans and rarely even possess the metagene.