– CD9, also known as 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 with 4 hydrophobic domains that is described to complex with integrins and other transmembrane 4 superfamily members. The protein takes part in cellular signal transduction events and thus play a role in the regulation of cell development and activation, growth and motility. Besides, CD9 seems to be a key role in the egg-sperm fusion during the mammalian fertilization processes. CD9 also seems to be a key part in the egg-sperm fusion during mammalian fertilization.