Myostatin (GDF-8), a member of the TGF-beta 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 a negative regulator of myostatin in vivo. 28 kDa protein containing 243 amino acid residues of the human Myostatin Propeptide and 5 additional amino acid residues.