The CD9 (HI9a) antibody recognizes a 24-kDa type-III single-chain transmembrane protein spanning the membrane 4 times therefore called TM4. The CD9 antigen is mainly expressed on platelets (present in the alpha-granules), pre-B cells, monocytes, endothelia cells, epithelial cells and activated T cells. The CD9 antigen is a marker for 90% non T acute lymphoblastic leukemia cells and 50% acute myeloid leukemia. CD9 is not expressed by hematopoietic progenitor cells nor by resting mature T and B cells. CD9 antigen mediates platelet activation and aggregation playing an important role in cellular adhesion and migration.