Mutants are creatures bearing some form of severe genetic deviance from the normal members of their species. Some are born mutants, while others have suffered more spontaneous mutation through the influence of the Warp, whether by worshiping it or being exposed to its energies. Persecuted and hunted by Imperial society, Mutants often make up the forces of Chaos.
Mutation among humanity
Mutation among humanity is especially meaningful: as mankind undergoes a long process of evolution into a new, psychic race, the birth of mutant humans accompanies it; and as the metamorphosis slowly approaches its distant conclusion, mutation becomes increasingly common. This constant and random warping of the human gene pool threatens to destroy the human race before it can reach its final form. The only response is to root out those manifesting mutation. There are many Imperial organisations which go about this, some in less sophisticated ways than others.
Human mutants are often terribly misshapen creatures, abominably deviating from the normal human form. On the other hand, not all mutants are degenerated beasts; many only bear a few extraordinary traits setting them apart from the rest of humanity.
The Imperium considers mutation a crime and a physical sign of spiritual corruption.
The position of mutants within the Imperium varies from world to world. On primitive worlds they are often slain at birth. On more advanced worlds they may be allowed to live, but rarely enjoy the benefits of other humans and are usually segregated from the normal population. In general terms, mutants form a huge downtrodden portion of the Imperium's population. Their dissatisfaction sometimes erupts into rebellion, sometimes allowing the mutants to take temporary control of an entire world. Retaliation, however, is usually swift and effective.
The laws of Imperial worlds normally forbid such creatures from possessing armaments; those weapons that mutants do possess are usually limited to crude shotguns or blunderbusses, heavy duty revolver-stubguns, chains, whips and clubs which can be easily made and concealed.
Navigators are considered mutants, but were actually intentionally created in the distant past. Because they are a vital part of the Imperium, they are the only tolerated form of mutant.
Mutation among other species
Chaos and mutation also affects other alien species, but mutation is especially prevalent among mankind. The Kroot have the ability to evolve through controlled mutation by consumption of their enemies.