The Secondary Mutation is a phenomenon in which an existing mutant gains additional powers, or a change in appearance. The first known subject is Polaris. Beast is actually the second and it is he who named the phenomenon.
Secondary mutation is noted as the appearance of new powers, or an increase in existing powers.
It has been shown that secondary mutation could be created by unnatural means, like Beast's mutations coming from his initial experimentation on himself, or Emma Frost's diamond body, created somehow by Cassandra Nova.
The Secondary Mutation, like the release of the Extinction Gene, has been induced by the critical point reached by the world mutant population in 2001. Since the M-Day, there are no new Secondary Mutation subjects.
List of Secondary Mutation subjects