GDF-8 / Myostatin / MSTN is a member of the bone morphogenetic protein (BMP) family and the TGF-beta superfamily. This group of proteins is characterized by a polybasic proteolytic processing site which is cleaved to produce a mature protein containing seven conserved cysteine residues. The members of this family are regulators of cell growth and differentiation in both embryonic and adult tissues. GDF-8 / Myostatin / MSTN is highly expressed in skeletal muscle, and myostatin loss-of-function leads to doubling of skeletal muscle mass. Experiments in mice have improved that GDF-8 / Myostatin / MSTN is a key regulator of mesenchymal stem cell proliferation and differentiation, and mice lacking Myostatin encoding gene show decreased body fat and a generalized increase in bone density and strength. The increase in bone density is observed in most anatomical regions, including the limbs, spine, and jaw, and myostatin inhibitors have been observed to significantly increase bone formation. GDF-8 / Myostatin / MSTN is also expressed in the early phases of fracture healing, and GDF-8 / Myostatin / MSTN deficiency leads to increased fracture callus size and strength. Together, these data suggest that GDF-8 / Myostatin / MSTN has direct effects on the proliferation and differentiation of osteoprogenitor cells and that GDF-8/Myostatin/MSTN antagonists and inhibitors are likely to enhance both muscle mass and bone strength.