CD9 antigen is a glycoprotein expressed on the surface of developing B lymphocytes, platelets, monocytes, eosinophils, basophil, stimulated T lymphocytes and by neurons and glial cells in the peripheral nervous system. CD9 belongs to a family of membrane proteins termed tetraspanins which transverse the membrane four times. In pre B cells and platelets, CD9 antigen regulates cell activation and aggregation possibly through an association with the integrin CD41 / CD61 (GPIIb / GPIIIa). CD9 is involved in cell motility, osteoclastogenesis, neurite outgrowth, myotube formation, and sperm-egg fusion, plays roles in cell attachment and proliferation and is necessary for association of heterologous MHC II molecules on the dendritic cell plasma membrane which is important for effective T cell stimulation. CD9 functions in many cellular processes including differentiation, adhesion, and signal transduction, and expression plays a critical role in the suppression of cancer cell motility and metastasis. CD9 is also considered as metastasis suppressor in solid tumors.