The X-gene, the genetic propensity for Homo Superior to develop mutant abilities, does not have a mutagen (a catalyst for mutant abilities caused by mutation). The propensity for homo superior mutation is part of the DNA sequence of the 23 chromosomes of all Humanity living on Marvel Earth-616 and is activated by PUBERTY.
This means the Celestials, the source of genetic manipulation on Marvel Earth-616 encoded the capacity for mutant abilities within the general genome of all humans on Earth. However, these genetic transformations are random and do not activate with every interaction of human propagation.
Mutants in the Marvel Universe, baring unforeseen circumstances need nothing more than to reach puberty before their powers begin activating and increasing. Depending on the mutation, some will reach fantastic degrees of power with only a minimum of training. Others will require support before their powers will improve significantly.
A number of mutants will undergo Secondary Mutation, a drastic degree of mutation under stress which will increase the mutant's already considerable ability. See: Iceman or The White Queen.
Mutagen: In genetics, a mutagen is a physical or chemical agent that changes the genetic material, usually DNA, of an organism and thus increases the frequency of mutations above the natural background level. As many mutations cause cancer, mutagens are therefore also likely to be carcinogens.
- Before M-day, it was estimated that 1 in 25,000 humans could have the propensity for some degree of mutant ability and be considered be a member of Homo Superior. After M-day, and the probability-altering magic of the Scarlet Witch, that number has dropped considerable, such that there may only be few as a thousand mutants in total across the entirety of Marvel Earth-616 where there were once as many as 30 million.
According to Marvel Editor-In-Chief Joe Quesada, the "Decimation" (M-Day) event was designed to reduce the number of mutant characters in the Marvel Universe, which he felt had gotten out of hand after forty years of publishing. (Reported in Newsrama Q&A with Joe Quesada)
The Celestials experimented with DNA on Earth:
They developed three humanoid species, one with fantastic variability. The variable species would ultimately lead to the existence of the Deviants (and to a lesser extent the Inhumans, a similar experiment performed by the Kree hoping to unlock their own genetic potential). The Deviants would have incredible abilities but these were tied to a complete inconsistency of form. No two Deviants ever looked the same or bred true in terms of superhuman ability.
The second, with powerful and stable superhuman abilities called the Eternals, who while maintaining the Human form, developed a stable and extraordinary set of physical and mental abilities. However, the Celestials would deem the Eternals a failure due to the development of a uniform and limited power set throughout the species. While the Unimind was the most amazing psychic achievement of the Eternals, it would pale in comparison to the mutant potential embedded in Homo Sapiens.
The third would offer fantastic range and capabilities but were completely unable to determine what mutant abilities would be the result of interactions between its members. Homo Sapiens, could potentially produce the subspecies Homo Superior; a breed of beings whose potential powers were far in excess of anything the Deviants or Eternals could produce.
Homo Sapiens was also capable of spontaneous mutation due to environmental conditions. Such mutates included: The Hulk (the result of gamma radiation exposure), Spider-man (the bite of a radioactive spider), the Fantastic Four (exposure to cosmic radiation) and Daredevil (exposure to a radioactive isotope).
While there was no mutagen or catalyst for X-gene capabilities, mutates, humans whose genetic capacities could be activated by outside sources, radiation, toxins, or other environmental conditions were an aspect of the Celestial process allowing for environmental conditions to produce possible mutates whose abilities could spontaneously provide the development of other mutants.
Contenders to the throne of Earth's most powerful mutants include members of the Summers clan, Jean Grey (whose interactions with the galactic life force called the Phoenix gave her fantastic capabilities, Nate Grey (from an alternate reality, his powers allowed him vast psionic capabilities, and Gabriel Summers [Vulcan] whose energy manipulation abilities made him one of the most powerful mutants ever known.
Franklin Richards abilities gave him the ability to alter reality making his desired result of any use of his vast psionic and reality-altering potential, real. It is suspected this is the power the Celestial sought in their experiments.
Franklin Richard's power was so great he eventually resurrects the star-god Galactus, and makes Galactus his herald against the threat of the Celestials (Fantastic Four #603).
There are many other mutants and scientists who studied mutant development and a number may have engaged in morally questionable experiments on mutants over the centuries and in parallel realities. Such experimentation, while it may have yielded fantastic results are inconsequential compared to the manipulations done by the Celestials for tens of thousands of years of direct genetic manipulations. These luminaries include:
En Sabah Nur: Also known as Apocalypse who performed a variety of experiments developing and enhancing mutant abilities for over a thousand years.
Nathaniel Essex: Known as Mr. Sinister, he was a powerful mutant who enhanced his own mutant abilities after experimenting on mutants for over two hundred years while he perfected his technology for recognizing the "Essex Factor" in humanity.
Hank McCoy: Also known as the Beast, he is one of the modern Marvel Earth-616 leading authorities on mutant abilities, the mutant genome and the Legacy Virus, a mutant-specific infection.
Charles Xavier: Until his recent death, Xavier was the leading authorities on mutant powers, psychology, education and origins on Marvel Earth-616. He collaborated with Hank McCoy, Moira McTaggart and Max Eisenhart (Magneto) on the origins of mutants, sociology of mutants and human society.