CSR is reduced in splenic B cells from mice with reduced levels of APE1, APE2 or both APE1 and APE2. CFSE-stained B cells from WT littermates and ape1+/−, ape−/− or ape1+/− ape2−/− mice were cultured for 3 days with LPS+ interleukin (IL)-4 (IgG1 CSR), LPS+interferon-γ (IFNγ) (IgG2a CSR), LPS+dextran sulphate (IgG2b CSR), LPS+anti-IgD-dextran (anti-δ-dex)(IgG3 CSR), or with LPS+IL4, IL5, transforming growth factor (TGF)β, and anti-δ-dextran (IgA CSR). After culture, cells were fixed and stained for surface isotype expression and analysed by flow cytometry (Schrader et al. 1999). Shown are the average per cent switching relative to WT littermates, set at 100% (+s.e.m.), for the different isotypes. *p<0.05, **p<0.01 as determined by the one-sample t-test. Black bars, ape1+/−; dark grey bars, ape2Y/−; light grey bars, DBL. Originally published in Guikema et al. (2007).