Osteoprotegerin (OPG) is a secretory glycoprotein that is a family member of the TNF receptor (TNFR) superfamily. Osteoprotegerin protects bone by blocking osteoclastogenesis and increasing bone density. Unlike other TNFRs, osteoprotegerin lacks a transmembrane domain as well as any apparent cell-associated signals. High levels of osteoprotegerin mRNA are found in specialized tissues such as lung, heart, kidney, and placenta. There is evidence that osteoprotegerin plays a role in degenerative arterial disease.