SF3B1 is required for an efficient DDR. A, Clonogenic survival assays demonstrating that depletion of SF3B1 (two independent siRNAs) induces sensitivity to IR in 293T cells. Mean surviving fraction of three independent experiments is plotted ± SEM. B, Representative immunofluorescence staining of γH2AX-marked DNA damage in untreated U2OS cells depleted of SF3B1, and 1 and 24 hours following 2 Gy IR. White scale bar, 20 μm. C, Quantification of three independent experiments described above (≥200 cells were scored/experiment). Mean percent cells containing ≥5 γH2AX foci is plotted ± SEM. D, Quantification of GFP-positive cells by flow cytometry from a DR-GFP reporter cassette stably integrated in U2OS cells, to assess HR. Mean % GFP-positive cells normalized to siSCR-transfected cells is plotted ± SEM. E and F, Long-term cellular survival assays demonstrating that depletion of SF3B1 (two independent siRNAs) induces sensitivity to etoposide and olaparib in U2OS cells and that SF3B1 and BRCA1 are epistatic. Mean surviving fraction of three independent experiments is plotted ± SEM. *, P < 0.05; **, P < 0.01; ***, P < 0.001; ****, P < 0.0001; NS, nonsignificant.