Double muscling

The MSTN gene provides instructions for making a protein called myostatin. This protein is part of the transforming growth factor beta (TGFβ) superfamily, which is a group of proteins that help control the growth and development of tissues throughout the body. Myostatin is found almost exclusively in muscles used for movement (skeletal muscles), where it is active both before and after birth. This protein normally restrains muscle growth, ensuring that muscles do not grow too large. Myostatin has been studied extensively in mice, cows, and other animals, and it appears to have a similar function in humans.

At least one mutation in the MSTN gene has been found to cause myostatin-related muscle hypertrophy, a rare condition characterized by increased muscle mass and strength, animals with such mutation are described a “double-muscled”.

These animals do not actually have two muscles, but each muscle is significantly larger than normal. Double-muscled animals have an incredibly muscular look even if they do not exercise. This mutation is of particular interest in species farmed for their meat (like cattle) but can occur in any mammal.

Myostatin mutation is studied in a line of mice called the “Mighty Mouse”. These mice were developed as a side effect of a line of research at the Johns Hopkins School of Medicine when they inactivated or “knocked out” a gene responsible for producing myostatin.

“Mighty mouse” next to common relative.

mightymouse_x600The myostatin mutation is particularly valued in cattle and is a recessive trait often found in the Belgian Blue, Piedmontese, and Marchigiana breeds–but present in many other breeds.


Confirmed mutation in MSTN found also in the whippet dog breed that results in a double-muscled phenotype known as the “bully” whippet (“A Mutation in the Myostatin Gene Increases Muscle Mass and Enhances Racing Performance in Heterozygote Dogs”, 2007). During this research study were screened also several mastiff type breeds (rottweiler, bulldog, Presa Canario, miniature bull terrier, American Staffordshire terrier, Staffordshire bull terrier, and bullmastiff) without any findings.