Myostatin （GDF-8），a member of the TGFbeta superfamily，is a potent and specific negative regulator of skeletal muscle mass. In serum，myostatin circulates as part of a latent complex containing myostatin propeptide and/or follistatin-related gene. The myostatin propeptide is known to bind and inhibit myostatin in vitro. This interaction is relevant in vivo，with a majority （>70%） of myostatin in serum bound to its propeptide. The myostatin propeptide is negative regulator of myostatin in vivo.
