Mutation is armor that is only available in level 14 of the Human Campaign. It is an extended version of the Combat Armor, and Mutation gives 200% HP and uses up the Defensive slot.
200% Maximum HP
Human Campaign, Level 14
Mutation in real life, has another name. Cancer. Usually, it is caused by your DNA having copying errors, and instead of dying away, it spreads, and takes over your system, and might, and usually will, kill you.