DNA break repair in preleukemic BM pre-B cells. (A) Kinetics of H2AX phosphorylation after irradiation (1 Gy) in BM pre-B cells from preleukemic Eμ-Myc^T/+, Eμ-Myc^T/+Parp-1^−/−, and Eμ-Myc^T/+Parp-2^−/− mice. (B-D) Representative microscopic images showing immunofluorescence staining of Rad51 (B), 53BP1 (C), and phospho-RPA (pRPA) (D) foci in BM pre-B cells derived from mice of the indicated genotype (left) and high-throughput microscopy of Rad51 (B), 53BP1 (C), and pRPA (D) fluorescence levels per individual nucleus (right). Red indicates Rad51 or 53BP1 or pRPA; blue indicates 4′,6-diamidino-2-phenylindole. At least 2000 nuclei were quantified per condition. Horizontal lines represent median values for each genotype. Original magnification: x63. (E) Representative flow cytometric dot plots of phosphorylated Chk-1 (pChk-1) in BM pre-B cells of each genotype. (F) Graph showing the percentage of pChk-1⁺ cells in different cell cycle phases. Bars represent mean ± SEM obtained from at least 6 mice per genotype. (G-H) Large-scale gene expression and GSEA showed a significant enrichment for DNA replication (G) and ATR (H) pathways in pre-B cells from Eμ-Myc^T/+Parp-2^−/− mice compared with Eμ-Myc^T/+ wild-type (WT) control cells. At the top is the GSEA enrichment plot; at the bottom, the normalized gene expression heatmap of genes involved in those pathways. *P < .05, **P < .01, ***P < .001. a.u., arbritary units.