Osteoprotegerin (OPG), also known as OCIF and TNFRSF11B, is a secreted TNF superfamily receptor that regulates bone density. OPG is widely expressed and constitutively released as a homodimer by mesenchymal stem cells, fibroblasts, and endothelial cells. OPG functions as a decoy receptor for TRANCE/RANK L and TRAIL. OPG regulates the cell surface expression of TRANCE and its ability to stimulate osteoclastogenesis. Dysregulation of OPG/TRANCE interactions can lead to juvenile Pagetâs disease, osteoporosis, bone loss, and vascular calcification.