Increased DNA end resection in the absence of 53BP1 is dependent on ATM. (A) Representative flow cytometry experiment showing CSR to IgG1 by IgH^I-96kAID^−/− B cells infected with an I-SceI–encoding retrovirus in the presence or absence of ATMi. (B) Graph shows the results of three independent flow cytometry experiments, with each dot representing an individual experiment. The means are shown as horizontal lines. (C) Representative ethidium bromide–stained argarose gels showing the PCR amplification products after I-SceI–induced recombination of IgH^I-96kAID^−/− and IgH^I-96kAID^−/−53BP1^−/− B cells in the presence or absence of ATMi. (D) Bar graph showing the frequency of I-SceI–induced recombination products running ≤300 nt for IgH^I-96kAID^−/− and IgH^I-96kAID^−/−53BP1^−/− B cells in the presence or absence of ATMi. Error bars indicate standard deviation. p-values were calculated using a two-tailed Student’s t test (three independent experiments). (E) Frequency of direct I-SceI joins, reconstituting an I-SceI site, determined by sequencing of individual molecular products of I-SceI–infected IgH^I-96kAID^−/− and IgH^I-96kAID^−/− 53BP1^−/− B cells in the presence or absence of ATMi. Error bars indicate standard deviation. p-values were calculated using a two-tailed Student’s t test (three independent sequencing experiments). FSC, forward scatter.