Osteoprotegerin (OPG), also called OCIF (osteoclastogenesis inhibitory factor) is a secreted 55-60 kDa protein that regulates bone density (1-3). As a member of the tumor necrosis factor receptor (TNFR) superfamily of proteins, it is designated TNFRSF11B (1-4). Mouse OPG cDNA encodes 401 amino acids (aa) including a 21 aa signal peptide and a 380 aa mature soluble protein with four TNFR domains, two death domains and a heparin‑binding region (4). The cysteine‑rich TNFR domains are essential for ligand interaction, while a cysteine at the C‑terminus mediates homodimerization (4). Mature mouse OPG shares 86%, 94%, 86%, 86% and 83% amino acid sequence identity with human, rat, equine, canine and bovine OPG, respectively. OPG is widely expressed and constitutively released as a homodimer by mesenchymal stem cells, fibroblasts and endothelial cells (1, 2, 5, 7). Regulation of its expression by estrogen, parathyroid hormone and cytokines is complex and changes with age (2). OPG has been called a decoy receptor for the TNF superfamily ligands, TRANCE (tumor necrosis factor‑related activation‑induced cytokine), also called RANK L (receptor activator of NF kappa B ligand), and TRAIL (TNF‑related apoptosis‑inducing ligand), which also bind TNF family receptors RANK and TRAIL receptors 1-4, respectively (2, 6). TRAIL decreases the release of OPG from cells that express it, while OPG inhibits TRAIL‑induced apoptosis (5, 6). Expression of RANK L on the cell surface, and thus its ability to stimulate osteoclastogenesis, is regulated by OPG by intracellular and extracellular mechanisms (7). Within osteoblasts, interaction of the basic domain of OPG with RANK L in the Golgi inhibits RANK L secretion (7). Extracellularly, OPG binding to RANK L results in clathrin‑mediated internalization and degradation of both proteins (7, 8). Binding of OPG by syndecan‑1 heparin sulfates on multiple myeloma cells also results in OPG internalization and degradation, contributing to bone loss (8, 9). OPG deficiency can cause juvenile Paget’s disease in humans, and insufficient OPG to balance with RANK L and RANK can produce osteoporosis and vascular calcification in both mice and humans (2, 10, 11).