CD51 is a type I integral membrane glycoprotein, known as vitronectin receptor alpha chain, or integrin alphaV. It forms heterodimer with integrin beta1 (CD29), beta3 (CD61), beta5, beta6, or beta8. CD51 contains two disulfide-linked subunits of 125 kD and 24 kD, and is expressed on endothelial cells, fibroblasts, macrophages, platelets, osteoclasts, neuroblastoma, melanoma, and hepatoma cells. Many extracellular matrix proteins with RGD-motifs are CD51 ligands. In association with its beta 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.