Myostatin is a protein that limits muscle growth in tissues where it can be found in high concentrations. Proteins like myostatin that act as signaling molecules between cells are called growth factors. Myostatin was formerly known as Growth and Differentiation Factor 8. Myostatin is made in muscle cells, then carried to other muscle cells through blood flow, attaching to specific receptors on their cellular membranes. Although the exact mechanism is unknown, myostatin is somehow able to limit the development of muscle stem cells, cells responsible for differentiating into additional muscle cells, leading to a cap in muscle size.

Myostatin was discovered in 1997 when geneticists McPherron and Se-Jin Lee created "Mighty Mice" -- mice lacking a certain gene and hence unable to produce myostatin, possessing muscles about twice the size of those in normal mice. The researchers commented that they "look like Schwarzenegger mice." The gene associated with myostatin has been found in the genomes of humans, mice, and zebrafish. Certain types of cattle have been selectively bred to have defective myostatin genes, leading to an increase in total size.

In 2001, Se-Jin Lee was able to make mice with large muscles but an intact myostatin gene by inserting mutations that caused the release of myostatin-suppressing substances. In 2004, a German boy was found to have mutations in both copies of his myostatin-producing gene, meaning his body was unusually large. Researchers have begun looking into methods to suppress the production on a wider scale with livestock. This has raised numerous bioethics questions.

Lower concentrations of myostatin in the blood are responsible for muscle growth in youthful animals. Hormones trigger decreases in the release of myostatin. When an animal enters the adult phase, the production of myostatin resumes in full. The discovery of myostatin has lead to questions about the presence of similar signalers and their associated genes in other types of tissue, for example brain tissue. Myostatin manipulation has also been cited as an approach to combating muscular dystrophy.