Myostatin
A protein that limits muscle growth; altered versions of its gene cause double muscling.
Also: MSTN, GDF8
Myostatin is a protein that tells muscle cells to stop multiplying. It is made from the MSTN gene, and when that gene is broken or altered the brake weakens and the animal builds unusually large muscles. In cattle, different variants of the gene have been found in the Belgian Blue, the Piedmontese, the Parthenaise and other breeds, and they explain double muscling. Similar variants occur in some sheep, in whippets and in other animals. Breeders can test for myostatin variants and either use them on purpose in beef crosses or avoid them where easy calving matters more.