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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.

The cluster of differentiation (CD) system is commonly used as cell markers in Immunophenotyping. Different kinds of cells in the immune system can be identified through the surface CD molecules associating with the immune function of the cell. There are more than 320 CD unique clusters and subclusters have been identified. Some of the CD molecules serve as receptors or ligands important to the cell through initiating a signal cascade which then alter the behavior of the cell. Some CD proteins do not take part in cell signal process but have other functions such as cell adhesion. 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.