Reduced A/T transitions during SHM in Fanca^−/− mice. (A) Distribution of mutations in the J_H4 intronic region (506 bp) that was amplified from Peyer’s patch PNA^high B cells isolated from WT and Fanca^−/− mice. (B) Proportion of sequences with numbers of mutations per clone (the central circle shows the total number of analyzed sequences) from WT (n = 5) and Fanca^−/− mice (n = 5). (C) The spectrum of base substitutions is expressed as a percentage of the total number of mutations (left), and the frequency of mutation (right) was corrected for base composition. Gray boxes denote a significant decrease in mutation frequency compared with WT (the χ² test was applied according to the SHMTool algorithm; *, P < 0.05; **, P < 10⁻³; ***, P < 10⁻⁴). Data for the SHM are from five independent experiments. (D) Differential expression of Polη in FANCA- and FANCG-deficient cells. Whole cell extracts were prepared from the indicated human lymphoblast cell lines and analyzed by immunoblotting for the expression of Polη, MSH2, FANCA, FANCG, and Vinculin (asterisks indicate nonspecific bands). Representative data from two independent experiments are shown.