Genestealers infest colonies and cities belonging to other creatures, repopulating with their peoples and attempting to establish a Genestealer Cult, eventually leading a Tyranid hive fleet to the planet to consume it in its entirety.
Genestealer Acolytes are the result of generations of implanted genes into humanoids, resulting in a genestealer-human hybrid that appears like a human, but with noticeable differences, such as several arms, more rows of teeth, hunched postures, and other traits similar to a mutant.