Like most Tyranid strains, purestrain Genestealers are bilaterally symmetrical hexapods. They are bipeds that walk with a hunched posture, although they would stand about two meters tall if fully upright. They have two pairs of arms, one ending with fighting claws and the other ending with manipulative hands.
Genestealers can thrive in a multitude of extreme climates and can live for hundreds of years. Purestrain Genestealers have some telepathic abilities, but they are not sapient in and of themselves.
Genestealers are able to infect sapient beings with their DNA, causing them to give birth to Hybrids. Infected parents are totally devoted to their new, "perfect" offspring. Successive generations of Genestealer hybrids look more like the host race, becoming virtually indistinguishable after four generations. Fourth generation hybrids, however, actually spawn new purestrain Genestealers to renew the cycle.
Purestrain Genestealers, hybrids, and infected hosts organize as cults that are devoted to expanding their numbers and gaining power. When a Tyranid hive fleet approaches, these cults will rise up to fight for the Tyranid hive mind, disrupting efforts to defend the planet against the Tyranid invasion.