The Extinction Gene, or E-Gene, is a hominid gene discovered by Doctor Hank McCoy in September 2001. According to him, it could turn on among the humans (Homo sapiens sapiens) and cause the extinction of the human race, in four generations; This race would be completely replaced by the mutants (Homo sapiens superior) because the number of this latter reached, at the beginning of the 21st century, a very important threshold.
Cassandra Nova thought that this gene was responsible for the disappearance of Neanderthals (Homo sapiens neanderthalis), thirty thousand years earlier, when they were replaced by the humans. Not wanting to see the disappearance of the whole of Humanity, she decided to commit the Genosha Genocide to decrease the number of mutants on Earth (of more than sixteen million people).
This genocide isn't seen to have been sufficient to prevent the release of the E-Gene. On the contrary, this release seemed to be prevented only by the transformation of millions of mutants into humans by the Scarlet Witch in December of 2005. Until the number of mutants rises again it is unclear if the E-Gene will reappear in the human genome.