The occurrence of heavy muscling in beef cattle, often called "double muscling," is the result of naturally occurring genetic mutations in the myostatin gene in the Belgian Blue and Piedmontese breeds. The deletions causing "double muscling" have occurred in other species as well. Myostatin generally inhibits muscle differentiation and growth. Inactivation of the myostatin gene leads to the increased muscling. This naturally occurring genetic mutation is not the result of any genetic engineering.
A recent invited presentation at the EAAP-ASAS-ADSA Growth and Development Symposium details the regulation of muscle growth and body composition in cattle (Hocquette, J.F., 2010. Endocrine and metabolic regulation of muscle growth and body composition in cattle. Animal 4:11, pp. 1797-1809).
