War never changes
Mutants, also known as twists or chem-dregs, can be found on almost any human world in the galaxy, and are often a fact of life that Imperial authorities (and most Imperial citizens) would rather ignore. In it’s most basic sense, mutation is the changing of the divine physical form of mankind into something unnatural or even alien. In fact mutation can range from something as minor as a blemish to something as extreme as additional limbs or massive size. Nobody can be sure what causes mutation, although the warp, radiation, toxins and extreme conditions are all believed to be possible causes by many scholars. What is known is that the offspring of a mutant will likely have mutation of their own.
In the Calaxis Sector, as with the Imperium as a whole, mutants are dealt with in one of two ways. If they are not exterminated, they are oppressed and controlled. Most worlds, hive worlds in particular, have sizable mutant underclasses that are employed as serfs or indentured labourers. They are often responsible for handling of jobs no one else will take, and work in hellish conditions with little in the way of rest or recompense.
Mutants are so despised and poorly treated by the Imperium that they are easily swayed to the side of chaos and there are entire armies made up mutants.