Osteoprotegerin (OPG) is a cytokine, which can inhibit the production of osteoclasts. It is a member of the tumor necrosis factor (TNF) receptor superfamily. It is a basic glycoprotein comprising 401 amino acid residues arranged into 7 structural domains. It is found as either a 60 kDa monomer or 120 kDa dimer linked by disulfide bonds.Osteoprotegerin inhibits the differentiation of osteoclast precursors into osteoclasts and also regulates the resorption of osteoclasts in vitro and in vivo. Osteoprotegerin is a RANK homolog, and works by binding to RANKL on osteoblast/stromal cells, thus blocking the RANKL-RANK ligand interaction between osteoblast/stromal cells and osteoclast precursors. This has the effect of inhibiting the differentiation of the osteoclast precursor into a mature osteoclast.