Cre recombinase induces efficient CSR to IgG1 independently of AID and 53BP1. (A) Schematic representation of the IgH^I-96k allele (top) and the Cre-induced recombinant that encodes IgG1 (bottom). LoxP sites are indicated as red triangles, and I-SceI sites are indicated as blue circles. (B) Representative flow cytometry experiments showing CSR to IgG1 of IgH^I-96k B cells infected with retroviruses encoding Cre or catalytically inactive Cre*. IgG1 expression was analyzed at 72 and 96 h after LPS and IL-4 stimulation. (C) Experiment as in B but for IgH^I-96kAID^−/− and IgH^I-96kAID^−/−53BP1^−/− B cells analyzed at 96 h after LPS and IL-4 stimulation. (D) Graph shows the results of three independent flow cytometry experiments measuring CSR to IgG1 after Cre infection of IgH^I-96AID^−/− and IgH^I-96kAID^−/−53BP1^−/− B cells. The means are shown as horizontal lines. FSC, forward scatter.