New Brevoort Formspring

About my previous question on the X-gene. The difference is that it has never been established. The X-gene is what make's someone a Mutant and it has always appeared once a child starts going thru puberty and it's a question that came up in the store when we had a convo about Hope and the AvsX Event and we couldn't find one instance where it has been stated which is what exactly happened so I decided to ask you the Guru of Marvel Continuity