Somatic hypermutation (SHM) and class-switch recombination (CSR) of Ig genes are dependent upon activation-induced cytidine deaminase (AID)-induced mutations. The scaffolding properties of proliferating cell nuclear antigen (PCNA) and ubiquitylation of its residue K164 have been suggested to play an important role organizing the error-prone repair events that contribute to the AID-induced diversification of the Ig locus. We generated knockout mice for PCNA (Pcna⁻/⁻), which were embryonic lethal. Expression of PCNA with the K164R mutation rescued the lethal phenotype, but the mice (Pcna⁻/⁻tgK¹⁶⁴R) displayed a meiotic defect in early pachynema and were sterile. B cells proliferated normally in Pcna⁻/⁻tgK¹⁶⁴R mice, but a PCNA-K164R mutation resulted in impaired ex vivo CSR to IgG1 and IgG3, which was associated with reduced mutation frequency at the switch regions and a bias toward blunt junctions. Analysis of the heavy chain V186.2 region after NP-immunization showed in Pcna⁻/⁻tgK¹⁶⁴R mice a significant reduction in the mutation frequency of A:T residues in WA motifs preferred by polymerase-η (Polη), and a strand-biased increase in the mutation frequency of G residues, preferentially in the context of AID-targeted GYW motifs. The phenotype of Pcna⁻/⁻tgK¹⁶⁴R mice supports the idea that ubiquitylation of PCNA participates directly in the meiotic process and the diversification of the Ig locus through class-switch recombination (CSR) and somatic hypermutation (SHM).