CD9 is a member of the transmembrane 4 superfamily, which is also known as the tetraspanin family. CD9 is a cell surface glycoprotein with 4 hydrophobic domains that are described as complex with integrins and other transmembrane 4 superfamily members. It is found expressed on the surface of the exosomes. 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 is found on the membrane of the oocytes and also appears to intervene in maintaining the normal shape of oocyte microvilli.