(a, b) Assays of DSB repair mediated by homologous recombination (HR) or non-homologous end-joining (NHEJ) in U2OS-GFP cells (illustrated in Supplemental Fig. 1). The number of FACS sorted GFP+ cells indicates DSB repair efficiency and is normalized to control I-SceI or NHEJ reporter-only transfected cells. OE: overexpression (mean ± SEM, *P<0.05, **P<0.01, unpaired t-test). (c, d) Primary cortical neurons were transfected with shRNAs together with pre-digested NHEJ reporter. The ratio of GFP+ cells to total mCherry+ cells indicates the repair efficiency. AU, arbitrary units. Scale bars: 8μm (mean ± SEM, n = 60–80, *P<0.05, unpaired t-test). (e, f) Neurons transfected with indicated shRNAs together with mCherry (white arrows) were labeled for γH2AX after 1 h etoposide treatment (5 μM) (mean ± SEM, n = 16–83, ***P<0.001, ns: no significant difference, unpaired t-test). Scale bar: 8 μm. (g, h) Primary cortical neurons transduced with lentivirus carrying indicated shRNAs were labeled with the anti-53BP1 antibody after 1 h etoposide treatment. White arrows, 53BP1 foci. The percentage of cells with more than five 53BP1 foci upon etoposide treatment was quantified (mean ± SEM, n = 402–619, **P<0.01, unpaired t-test). Scale bar: 4 μm. (I, j) Lentivirus-transduced neurons were treated with vehicle (DMSO) or etoposide for 1 h and harvested for comet assays (mean ± SEM, n = 13–18, *P<0.05, unpaired t-test). Scale bar: 100 μm.