Gamma World D6 adapts TSR's classic Gamma World to West End Games' D6 System. Concepts from the fan-created D20 Omega World also are incorporated, and ideas derived from a variety of other sources are used. The game is notable for having started as a play-by-chat-room experience which was codified and standardized into a set of HTML rules.
From the Introduction:
If you've played Gamma World before, the big difference between the published works and this version is simply no mutated animals. There's only Purebred Humans and Mutated Humans in the world.