Mutants, sometimes called 'mutes' or 'mutts', are people who have been mutated either by exposure to Shade, toxins or radiation during gestation, or who has developed mutations during their life because of excess exposure. 'Mutt' is a derogatory term to refer to someone who is physically and cosmetically mutated.
Viable mutation is only caused by exposure to shade, a toxic corruption left behind in nature by the Cataclysm. Mutations because of toxins or radiation commonly lead to death, rather than effective alterations. The corruption behind Shade has an almost sapient purpose to its mutations and a large number of specific mutations have been identified.
Shade also has the power to trigger mutation in adults, although the vast majority of mutants were mutated before birth.
In the Shadowmark, mutants are second-class citizens and are often treated as less valuable than "purebreed" wastelanders and are sometimes enslaved. This doesn't hold true for all settlements, but there is widespread discrimination of mutants all over the Shadowmark.
Mutants in Fusion City
With the public perception of mutants as little more than third-class citizens, they are ever only hired for the dirtiest, most dangerous jobs, if at all. People find mutants repulsive, ugly and even though mutations are non-contagious, people avoid them as if they were carrying the plague. There are certain employers who specifically look for mutants to hire and usually mutants with specific mutations, such as toxin or radiation resistance, great strength or general resilience. Mutants often find solace in gangs, where people don't care as much.
Not that there aren't mutants who are successful. But the prospering mutants are rare but avoid persecution and discrimination through their status and large amount of money.
The vast majority of mutants in Fusion City were born there, as getting from the Shadowmark into the city is a challenge not easily overcome. Mutants are most common in families living in Perimeter, the area surrounding the wall to the wastelands outside. Scientists believe that proximity to the wastelands, even protected from it by the wall, is enough to trigger mutations in gestating infants.