A member of the TNF superfamily, OPG is expressed in many different tissues, including the heart, liver, lung and bone marrow although it main role is to mediate bone density. When expressed, the protein may form dimers or stay in its monomeric form. The three dimensional structure of OPG can be divided into seven distinct domains, four of which correspond to the extracellular region of TNFR proteins and constitutes the soluble form of OPG.