A comment on one of my answers mentioned that:
Even if they lost their powers they still possess the mutant X-gene. I would say they are still technically mutants
This refers to (at least in part) the two mutants (Magneto and Polaris) in that list that had their powers removed by:
Wanda Maximoff during the House of M shenanigans
But I was thinking, what about all the mutants who lost their powers. Were their powers simply stripped, leaving the X-Gene, or was the X-gene reverted to a less advanced state?