Increased percent of γH2AX-positive cells in AMLs after irradiation. (A) Immunofluorescence microscopy of ionizing radiation-induced foci of γH2AX in irradiated splenocytes (2 Gy) isolated from a WT control and leukemic mouse transplanted with 1586 cells, 6 hours postirradiation. Nuclei were stained with 4,6 diamidino-2-phenylindole. Cells were viewed with a Zeiss Axioplan epifluorescence microscope and images were processed using Adobe Photoshop software. Original magnification, ×787.5. (B) The percentage of positive cells (≥3 γH2AX positive foci) is shown at 0 to 12 hours postirradiation in splenocytes isolated from a WT control or leukemic mice transplanted with 1586 or 4525 cells. A minimum of 100 cells were counted per time point. (C) Distribution of the number of foci per cell at 6 and 12 hours (50 cells were enumerated for each time point). An increased percentage of γH2AX-positive cells, as well as an increased number of foci per cell in AMLs compared with WT controls, is consistent with the hypothesis that DNA DSBs persist in the AMLs and contribute to genomic instability.