I'm reading the Disassembled story arc and came across something I hadn't seen before. In Spectacular Spider-Man v2 #19 they talk about using a nuke at Bikini Atoll to test the effects of radiation on servicemen with "...the M and X genes...". I know about the X-Gene, but what is the M-Gene?
You will find very little, if any, real information on the M-gene because Marvel hasn't shared much information about it, officially.
Unofficially, I suspect the M-gene is the genetic potential to exhibit metahuman capability in normal humans who are NOT mutants. In the Marvel Universe, there are a number of ways a person can come to exhibit metahuman potential:
- Being a Mutant: They can have a distinct genetic element within their DNA of an artificial nature which gives them, upon the activation of puberty, metahuman capabilities of an unknown origin. These powers may be physical or psychic in their operation and can range from inconsequential to cosmically powerful in scope.
- Being an Inhuman: A person could have DNA related to a series of experiments by the Kree upon a subset of the Human species in order to create metahuman abilities artificially jumpstarting gene sequences similar to but not exactly the same as the X-gene through a process called terregenesis. Once exposed to the Terrigen Mists, a mutagenic catalyst, a person with sufficient Inhuman DNA will undergo terregenesis, exhibiting a transformation similar in range and scope to that of a mutant entering puberty. While the scope of powers are similar, mutations under terregenesis can be wide and unexpected even with the best genetics theories available.
Using Magic: A Human whose mental and spiritual strength is sufficient for manipulating magical forces could with proper discipline and training harness three different forces for magical power. Mental and Spiritual energies for mind control, astral travel or other feats of mental capacity such as telepathy. These draw their power from the inner fortitude of the mage and have a limited effectiveness since they draw their power from the mage's lifeforce. The other two sources of power draw from other dimensional realms, tapping into the near-limitless power of those realms.
These dimensions are dangerous since the mage acts as both funnel between dimensions and as a fuse, capable of burning out if they lapse in concentration. The last source of magical power comes from avocation: The summoning of elderitch beings who loan the magus their power for their own reasons. Beings such as the Vishanti or the Occtessence, immensely powerful magical beings lend their power to magi in order to maintain relevance and control in the world of the living. While few possess the ability to wield magic, it has never been revealed to be a strictly genetic disposition in the Marvel Universe.
- Being a Mutate: Once upon a time, the Marvel Universe had beings of power who were not mutants. They gained their abilities by activation of a catalyst which should have killed them, but instead released incredible abilities similar to mutant powers in almost every conceivable way. These powers were considered genetic but the source of their activation was unknown. The only thing that was certain: these powers were NOT related to the X-gene. It is likely, these powers are what would be mysteriously dubbed, the M-gene abilities. M-gene subjects would include beings such as: The Fantastic Four, the Incredible Hulk, Spider-Man, and Daredevil, just to name a few.
The mutates and M-gene theory tie together nicely with the image above because it implies someone knew it was possible for radiation to activate both latent effects and more operant effects due to the exposure to radiation. Spider-Man, Daredevil and the Hulk all achieved their powers through an interaction with a radiation source that should have sickened or even killed them. Yet, instead of dying slowly, losing their hair from the effects of radiation poisoning, they gained superhuman abilities.
Out of Universe: The reason mutates (people randomly getting superpowers due to exposure to dangerous radiations or toxic chemicals) were replaced with mutants (people born with unique and unusual powers) was the idea you could not keep randomly exposing people to deadly toxins or strange liquids, or radioactive fruit and explaining they got superpowers.
Instead, by making mutation something that happened naturally (with the influence of the cosmic space gods, the Celestials manipulating Human DNA) we could make new mutants appear randomly in the Marvel universe and explain it away with the word, mutant.
In light of Marvel's current issues with mutants and licensing, new mutants have been supplanted with Inhumans, with almost the exact same effect (spontaneous superpowers) and no difficulties with any other licensing. We may see the M-gene getting more traction if the X-gene cannot be utilized by Marvel for the foreseeable future.
Note: There are several Human subspecies who have metahuman capabilities as a matter of course, requiring no additional activation of their powers and whose range of abilities while impressive don't quite equal the peak range of the mutant subgroup typified by the Omega-level mutants. This tampering, however, is the source of what is believed to be the metahuman potential in Humanity (as well as several other species, formerly the Kree, the Skrull, the Dire Wraiths and many others) caused by the Celestials for as-yet unknown reasons.
These two subspecies on Earth were called the Eternals and the Deviants. Both groups have members who are born with their metahuman abilities but both groups have significantly reduced birthrates due to the near-immortality of some of their more powerful members. In the case of the Deviants, their divergent forms may make reproduction...difficult or impossible.