PP4 is a serine/threonine phosphatase, which regulates a number of crucial cellular physiology. We have previously reported that genetic ablation of PP4 in mature B cells by CD23 promoter-mediated cre-loxp system impairs immunoglobulin (Ig) class switch recombination (CSR). However, the mechanism behind PP4 for CSR is unknown. In those mutant mice, the basal levels of serum Igs of all isotypes including IgM were strongly reduced. To address whether the impaired CSR in PP4-deficient B cells was derived from the reduced IgM production, we generated PP4 conditional knockout mice by deaminase activation-induced deaminase (AID) promoter-mediated cre. As expected, the basal level of serum IgM in AID/cre/PP4 F/F mice was comparable to control mice. Despite of that, the basal levels of all switched Ig isotypes were drastically reduced in AID/cre/PP4 F/F mice. After challenged with T cell-independent antigen TNP-Ficoll, AID/cre/PP4 F/F mice failed to mount antigen-specific IgG3 and IgG1. The results revealed that with normal mature B cells and serum IgM, CSR remained defect in AID/cre/PP4 F/F mice. Upon stimulation by LPS plus IL-4, PP4-deficient B cells exhibited increased ATM phosphorylation on S1987, suggesting severe DNA damage during CSR. Consistently with this, PP4-deficient B cells had increased p53-foci and phospho-p53-foci on S15 upon stimulation by DNA damaging drug etoposide in vitro. Although ATM-p53 axis was activated in PP4-deficient B cells, upon stimulation by etoposide the mutant B cells showed reduced gammaH2AX-foci in intensity and duration. We thus propose that PP4 is required for CSR through efficient recruitment of gammaH2AX, which is essential for S-region repair.