DNA damage measured by γ-H2AX phosphorylation. CD34⁺ cells from (A) normal donors (n = 5) and (B) CML CP patients (n = 5) were exposed to 2 Gy of γ-radiation, and phosphorylation of histone γ-H2AX was measured by flow cytometry at different times after irradiation and is shown as ratio of mean channel fluorescence (MCF) for phosphorylated γ-H2AX compared with isotype control. **P < .01, ***P < .001 (differences in γ-H2AX phosphorylation in CML CD34⁺ cells compared with normal CD34⁺ cells exposed to 2 Gy at 2 and 24 hours after irradiation, respectively). Data are mean ± SEM of multiple experiments. (C) Flow cytometric plots showing phosphorylated γ-H2AX versus cell cycle based on DNA labeling with DAPI in CML and normal CD34⁺ cells with and without exposure to radiation. The gates were drawn based on the isotype controls that are shown in the top panels. Separate gates were used for the DAPI low (G₀/G₁ populations) and DAPI high (S/G₂/M populations) because of differences in “nonspecific” fluorescence of the 2 populations.