Osteoclastogenesis inhibitory factor or osteoprotegerin(OPG/OCIF) is a novel secreted glycoprotein involved in the regulation of bone density. It is a novel member of the TNF receptor superfamily. Mature OPG/OCIF contains 7 domains and can be divided into 3 regions(TNF receptor cysteine-rich region; death domain region and heperin binding region). The OPG/OCIF gene is a single-copy gene consisting of five exons and four introns. It is located in 8q23～24. The factors involved in the regulation of bone formation and resoption(eg. TGF-β1,1,25(OH)₂VD₃ and TNF-α)regulate the expression of OPG/OCIF gene. The mechanisms by which OPG/OCIF inhibits bone resoption can be concluded that: 1) OPG/OCIF inhibits survival of osteoclasts and induces apoptosis of osteoclasts; 2) OPG/OCIF inhibits formation of osteoclasts.