Scatter plot of CD44v protein expression on AML leukemic cells and normal myeloid cells. Normal myeloid cells: CD34⁺ cells (very immature BM hematopoietic cells). GP, granulocytic precursors (CD34⁻, CD15⁺ BM cells); PMN, polymorphonuclear cells; MP, monocytic precursors (CD34^neg, CD14⁺ BM cells); Monocytes: 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. CD34⁺ cells, GP, and MP were identified by double-staining with (1) an FITC-conjugated MoAb directed to CD34, CD15 for GP and CD14 for MP, respectively, (2) unconjugated MoAbs directed to variant epitopes 3v, 6v, and 9v plus PE-goat anti-mouse (PE-GAM). Leukemic cells were identified by specific electronic gating.37 Flow cytometric analysis was performed as described in Materials and Methods. The staining intensity of variant CD44 isoforms was evaluated by measuring the percentage of labeled cells over the background (cells incubated with isotype-matched control antibodies). Each symbol (▪) refers to the percent CD44v cells per patient. Expression of CD44v was considered as negative (−) when less than 5% of the cells were labeled, low (+) when 5% to less than 20% of cells were labeled, and high (++) when more than 20% of the cells were CD44v⁺.