CD51 (Cluster of Differentiation 51), known as vitronectin receptor ? chain, or integrin ?V is a type I integral membrane glycoprotein. CD51 forms a heterodimer with integrin ?1 (CD29), ?3 (CD61), ?5, ?6, or ?8. CD51 contains two disulfide-linked subunits of 125 kD and 24 kD. Many extracellular matrix proteins with RGD-motifs are CD51 ligands. In association with its ? chains, CD51 binds vitronectin, von Willebrand factor, fibronectin, thrombospondin, osteopontin, fibrinogen, and laminin. CD51, as an adhesion molecule, plays important roles in leukocytes homing and rolling, mediates bone absorption and angiogenesis. CD51 is expressed on endothelial cells, fibroblasts, macrophages, platelets, osteoclasts, neuroblastoma, melanoma, and hepatoma cells.