Myostatin HMP, also known as growth differentiation factor 8 (GDNF-8) is a protein encoded by MSTN gene. Myostatin HMP is primarily synthesized in skeletal muscle cells and acts in muscle tissues. Myostatin HMP belongs to TGF-beta super-family and negative feedback regulator of skeletal muscle mass. In circulation, myostatin exists as latent myostatin propeptide that binds and inhibit myostatin in vitro. In vivo, myostatin propeptide is a negative regulator of myostatin. Recombinant myostatin propeptide contains 244 amino acids with a molecular mass of 27.8 kDa. Myostatin contains two identical subunits that consist of 109 amino acids. Myostatin occurs as inactive form and requires protease cleavage in NH2-terminal to be activated.