Myostatin is a genetic protein that affects muscle growth in humans and animals. Scientists have learned a lot about this protein from a noticeable myostatin mutation common in the Whippet dog breed. Whippets with one mutant copy of the gene are faster, so these are desirable for racing dog breeders. But selective breeding has caused increased instances of both copies of the myostatin genes mutating, which results in "double muscle" Incredible Hulk dogs!?