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. It 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). It also regulates cell motility in a variety of cell lines, and appears to be an important regulator of Schwann cell behaviour in peripheral nerve.