Inhibition of ATM leads to partial rescue of the CSR defect in 53BP1^−/− B cells. (A) Representative flow cytometry experiment showing CSR to IgG1 by WT and 53BP1^−/− B cells 96 h after LPS, IL-4, and RP105 stimulation in the presence or absence of ATMi. (B) Graph summarizes CSR to IgG1 by WT and 53BP1^−/− B cells in four independent experiments in the presence or absence of ATMi. The means are shown as horizontal lines. p-values were calculated using a two-tailed Student’s t test. FSC, forward scatter.