Belgian Blue bulls look like they are made of muscle because they have a mutation in the gene that codes for the protein myostatin. In humans, as in other types of cattle, myostatin normally limits the number of muscle fibers that form before birth and then limits the growth of those fibers later on. Courtesy of Se-Jin Lee and Alexandra McPherron/PNAS hide caption

itoggle caption Courtesy of Se-Jin Lee and Alexandra McPherron/PNAS