Hell on Earth: Gimme Shelter
The Ghost Rock-infused radiation that blanketed the world after Judgment Day brought many changes, and some were on a cellular level. Conventional radiation usually only causes illness and death, but the malevolent power of ghost rock allowed this new type of radiation to cause rapid cellular growth and alteration. Unfortunate individuals who were bathed by the bombs' fire (and who were not killed outright) often mutated, seeing their bodies and physiologies undergo serious change within months, days, or sometimes even minutes.
Sometimes, these changes caused a mental shift as well, creating violent, bestial mutants now commonly known as "Trogs". In most cases, however, the mutant retained their personality. That can be a fine line for many "normals" in the wastes these days, and mutants often find themselves excluded from human settlements simply because of a few extra digits or scales.
This perception of danger is stoked by the Cult of Doom, a fanatical group of mutants who believe that the time for humanity is over. They seek to wipe out all remaining "unevolved" to hurry along the process of the mutants inheriting the earth. Increasing numbers of mutants, disenfranchised by the bigotry of their former kinsfolk, are joinging the Cult for protection, revenge, or simply a sense of belonging.