OPG is a member of the TNF receptor superfamily and also known as osteoclastogenesis inhibitory factor (OCIF), TNFRS member 11B (TNFRS11B), and TR1.20–22,59 It was identified as a secreted glycoprotein synthesized by several kinds of cells including osteoblasts, B lymphocytes, and articular chondrocytes.20,22,59 An in vivo study with Opg transgenic mice revealed that overexpression of OPG led to profound yet nonlethal osteopetrosis because of decreased numbers of osteoclasts. The gene discussed is TNFRSF11B; the disease is osteopetrosis.