Osteoprotegerin (OPG)/osteoclastogenesis inhibitory factor (OCIF) is a member of the TNF receptor superfamily and is now referred to as TNFRSF11B. It is an osteoblast secreted decoy receptor that binds specifically to osteoprotegrin ligand (OPGL/TNFSF11) to inhibit osteoblast maturation. It is also a decoy receptor for TRAIL and inhibits TRAIL-induced apoptosis. Human and mouse OPG share approximately 84% and 94% amino acid sequence identity, respectively, with rat OPG. Natural OPG exists predominantly as disulfide-linked dimers.