Scatter plot of total CD44 expression on normal myeloid cells and on AML leukemic cells. Normal Myeloid Cells: CD34⁺ cells (very immature hematopoietic cells); GP, granulocytic precursors (CD34⁻, CD15⁺ BM cells); PMN, polymorphonuclear cells; MP, monocytic precursors (CD34⁻, CD14⁺ BM cells); Mono, mature monocytes (circulating CD14⁺ cells). AML Leukemic Cells: from AML patients with the following FAB types.34,35 M1/M2, myeloblastic AML; M3, promyelocytic AML; M4, myelomonocytic AML; M5, monoblastic AML. Mature monocytes and PMN from PB were identified according to their forward angle and side scatter.37CD34⁺ cells and MP were double-stained with (1) an FITC-conjugated MoAb directed to either CD34, which is specific for very immature hematopoietic cells, or CD14, a monocyte-specific antigen and (2) a PE-conjugated anti-CD44 MoAb F10-44-2, which binds to an epitope located in the constant part of the CD44 molecule. Negative controls were cells labeled with FITC and PE-conjugated isotype-matched control antibodies. The MFI was determined by flow cytometry, relative to cells labeled with PE-conjugated IgG2a (negative controls), as described in Materials and Methods. Each symbol refers to the MFI value per patient, and the horizontal bars indicate the mean MFI value in each category of patients. The normal BM cells and the leukemic cell populations were isolated as described in Materials and Methods. The gating of leukemic cells is shown in Fig 1.