Integrin alpha V beta 3 (aVb3) is a heterodimeric adhesion protein that is highly expressed on endothelial cells and osteoclasts. Active cell surface aVb3 binds to the extracellular matrix proteins Vitronectin, Fibronectin, Fibrinogen and Thrombospondin. It is essential for the maturation of osteoclasts and their bone resorption activity. Loss of aVb3 function during osteoclastogenesis causes osteoporosis. In addition, aVb3 enhances VEGF R2 mediated angiogenesis, IGF-1 mediated tumor cell invasiveness, and cell entry by several viruses.