Genetic studies in several species have demonstrated that myostatin (MSTN) normally functions to limit skeletal muscle mass. In a recent study, Mosher et al. reported that a mutation in the canine MSTN gene is responsible for the double-muscling phenotype seen in 'bully' whippets. Furthermore, they show that loss of even one functional MSTN allele seems to confer a competitive advantage to racing whippets, providing the first definitive evidence that loss of myostatin function can enhance athletic performance.