CD123 expression is significantly elevated in CD34⁺/CD38^– cells of CP-CML and BC-CML patients compared with normal donors. The percentage of CD123⁺ cells within CD34⁺ (A) and CD34⁺/CD38^– (B) subpopulations from normal donors (N), CP-CML, BC-CML, and AML patients has been determined by multicolor flow cytometry. **P < .01; ***P < .001 (by Mann-Whitney test compared with normal donors). (C) The percentage of CD123⁺ cells within CD34⁺ and CD34⁺/CD38^– subpopulations of matched chronic and blast phase patient samples is plotted. (D) IL-3 levels in normal donor and in CP-CML patient plasma, with matched samples taken at diagnosis and following TKI treatment (on average 6 months on either imatinib or nilotinib), were measured by bead array.