Human OPG (Osteoprotegerin), also known as OCIF (Osteoclastogenesis inhibitory factor), is a member of the TNF receptor superfamily. Recombinant human OPG/OCIF specifically acts on bone tissues and increases bone mineral density and bone volume associated with a decrease of active osteoclast number. Recombinant human OPG is a 20 kDa protein, comprising of 174 amino acid residues.