CD61 is a 110 kD integrin beta chain also known as beta3 integrin or gpIIIa. It associates with the integrin alphav chain (CD51) to form the vitronectin receptor. In addition, CD61 can associate with the integrin alphaIIb chain (CD41) to form the gpIIb/IIIa complex. CD61 is expressed on platelets, megakaryocytes, endothelium, smooth muscle, a subset of B cells, myeloid cells, osteoclasts, and mast cells. CD61, in conjunction with CD41 or CD51, mediates adhesion to fibronectin, fibrinogen, vitronectin, thrombospondin, and von Willebrand factor. Leukocyte-endothelial adhesion is mediated by the binding of alphav/beta3 integrin or vitronectin receptor to CD31 (PECAM-1).