Osteoprotegerin (OPG) or osteoclastogenesis inhibitory factor (OCIF) is a secretory glycoprotein belonging to the TNF super family. OPG consists of 401 amino acid residues; it has a molecular weight of 60 kDa as a monomer and 120 kDa as a disulfide-linked dimer and is produced in a wide range of tissues, e.g. bone, skin, liver, stomach, intestine, and lung. Osteoprotegerin inhibits the binding of RANK to RANKL (TRANCE, osteoprotegerin ligand, OPGL, osteoclast differentiation factor, ODF), thus inhibiting the recruitment, proliferation and activation of osteoclasts. Since OPG exhibits an inhibitory effect on osteoclasts, it acts as a soluble factor in the regulation of bone mass.^1,2,3 Osteoclast formation activity may be the cause of bone loss in many imbalances in bone metabolism such as osteoporosis, osteopetrosis, hypercalcemia, metastatic osteolytic lesions and rheumatic bone degradation.^2,4