Time course for IgG1 CSR in Ku70^−/−H/HL⁺, Ku70^−/−Lig4^−/−H/HL⁺, and WT-H/HL⁺ B cells stimulated with αCD40 plus IL-4. (a) Representative FACS plots for IgG1 CSR quantification in B cells from the indicated backgrounds stimulated with αCD40 plus IL-4. IgG1 FACS was recorded at day 2.5, 3.5, and 4.5. AID, AID^−/− B cells used as negative controls for CSR. Percentage of B220⁺IgG1⁺ cells that have undergone CSR is shown for each genotype at each time point. Data shown are representative of three independent B cell stimulation experiments, with one mouse of each genotype per experiment. (b) Quantification of CSR levels at days 2.5, 3.5, and 4.5 from the beginning of the in vitro stimulation. Data are based on at least three mice per genotype used in three separate experiments. Table S1 contains the summary of all FACS experiments.