You ever heard that cockroaches would survive a nuclear holocaust? Apparently it's kinda true of a Bomb too, and they have the scientists investigating the Zones pulling their hair, because these bitches will actually live in the deep zone and not become aberrations, or at least not acquire aberrant traits.

In fact they can be found so deep into the zone that you have seen at least one scientist fistfight (more of a slapfight) where one of the guys was drunkenly shouting that they had to revise what being an aberration means

Some scientists will insist that they are clearly aberrant because they can survive in the deep zone, even if they don't mutate. And other scientists will go that no, that's not what aberrant means.