CD9 antigen is also known as tetraspanin-29 (TSPAN29), 5H9 antigen, Leukocyte antigen MIC3 (MIC3), Motility-related protein, is a multi-pass membrane protein which belongs to the tetraspanin (TM4SF) family or the transmembrane 4 superfamily. CD9 is a cell surface glycoprotein that is known to complex with integrins and other transmembrane 4 superfamily proteins. TSPAN29 is found on the surface of exosomes. MIC3 Involved in platelet activation and aggregation, regulates paranodal junction formation and also Involved in cell adhesion, cell motility and tumor metastasis. CD9 antigen also seems to be a key part in the egg-sperm fusion during mammalian fertilization.