Osteoprotegerin (OPG), also known as OCIF, TR1, FDCR-1 or TNFRSF11B [TNF Receptor superfamily member 11B], is a disulfide-linked homodimer of two 55 kDa monomers. The mouse and human OPG proteins are approximately 85 percent and 94 percent identical with the rat protein, respectively. OPG is expressed in lung, heart, and kidney tissues, and high levels of OPG mRNA is also found in stomach, intestines, skin, and calvaria tissues. OPG inhibits osteoclast maturation and protects bone from normal osteoclast remodeling and ovariectomy-associated loss of bone substance.