CD61, also known as GPIIIa and Integrin beta-3, is a glycoprotein encoded by the ITGB3 gene.  CD61 heterodimerize with other cluster of differentiation molecules resulting in transmembrane integrins, facilitating cell to cell interactions and signal transduction.  CD61 combines with CD41 to form platelet glycoprotein IIb/IIIa that serves as a receptor for fibrinogen and fibronectin.  Platelet activation causes a conformational change in the glycoprotein to bind soluble fibrinogen to from a platelet plug.  Alternative, CD61 can heterodimerize with CD51 to form Integrin alpha V beta 3, a receptor for vitronectin and von Willebrand factor.  CD61 is expressed in the membrane and cytoplasm of platelets and megakaryocytes across diverse tissue types.  Endothelium is also stained.  Most cases of acute megakaryoblastic leukemia (AML M7) are positive for CD61.