Enligt Wikipedia-artikeln på Decimation -händelsen förlorade de sin X-gen.
In X-Men Legacy" #259, Polaris tests positive of the X-gene, meaning that Apocalypse recreated both her mutant abilities and X-genes with his nano technology.
Feral (Deceased) and Thornn were not actually repowered, they merely had their physical mutations restored, but not their X-gene, as shown in Wolverine #55.
As a side effect, no new mutants are born. This leads into the "Endangered Species" storyline, and later, the "Messiah Complex", that deals with the birth of Hope Summers, the only new mutant born since the Decimation, Messiah War and the "Second Coming" storyline, in which Cerebra detects the activation of the X-Gene in five as-yet unknown individuals, leading into the upcoming "The Five Lights" storyline in Uncanny X-Men.
It has been revealed in the "Endgangered Species" storyline that latent mutants, such as Charlotte Jones, have also lost their X-gene after M-Day.