The constantly recycled air, water and food of the hive have an inevitable effect on its inhabitants. Pollutants and toxins build up in the bio-system causing genetic instability and mutation. The effects of this are worse further down the hive where toxins are more concentrated. Because mutation is so common minor deformities are tolerated to a degree, even in the Spire and Hive City. However, conspicuous mutants are rooted out and destroyed according to the strict laws of House and Hive. Only those who flee downhive can hope to escape and start new lives, losing themselves amongst the ever-shifting population of the Underhive.
Underhive inhabitants are more tolerant of mutants than other hivers. So long as mutants keep a low profile, and are not obviously or grossly deformed, they can live peacefully even inside a big settlement. Underhivers are not inclined to ask questions or look too closely at their neighbours, and are more sympathetic to mutants in general.
Of course not all are equally tolerant. House Cawdor especially are quick to turn upon mutants. Other fanatical groups such as the Redemptionists hate all mutants no matter how minor their deformities. Their creed preaches the uncompromising destruction of all deviants.
For those too mutated to live in the big settlements the lawless expanse of the badzones provides ample opportunity to hide. Mutants of the most heinous varieties, scaly many-limbed monstrosities, will naturally gravitate towards the deeper parts of the hive and the Hive Bottom. They are outcasts even from the badzones.
Not all mutants are physically grotesque. Some appear normal, but actually have psychic powers of one kind or another. These mutants are more readily accepted into Underhive than conspicuous deviants. Some are even welcomed and protected because their abilities are useful, such as psychic healers and precognostics.
Though not native to Necromunda many Abhumans have found their way here. Abhumans are stable mutant strains of humans some are tolerated in the Imperium, many are shunned and others are hunted. Some of the most common Abhumans are:
Many of the outcast mutants will band together in gangs known as scavvies. Some scavvie leaders can end up carving up their own small badland kingdoms like King Redwart the Magnificent. One of the more stable strains of necromundian mutants are the Scalies. Scalies are nigh on Abhuman classification, however they are still outlawed by the Imperium Of Man, they often lend their muscle to the Scavvies with there strength in melee combat or their ability to use thier own heavy weapons like Spear Guns and Scatter Cannons.
All the previous example of mutants can be considered benign compared to the mutations that the forces of chaos can bestow. To the followers of the chaos powers these mutations are considered a gift from their dark gods. The mutations of the chaos gods run the gambit from subtle psychic abilities to turning a follower to a gibbering chaos Spawn.