Abnormal persistence of memory B cells with no or only few V_H186.2 gene somatic mutations in bim^−/− mice. Wild-type and bim^−/− mice were immunized with NP-KLH, and after 28 days, single antigen-specific IgG1⁺NP⁺ B cells with a memory B cell phenotype (CD38^high) were sorted by flow cytometry. The cDNAs of the V_H186.2 genes of these cells were amplified by PCR, sequenced, and the frequencies of mutations in wt and bim^−/− B cells determined by comparison with the germ-line V_H186.2 gene sequence. Distribution of somatic mutations in the V_H186.2 genes of wt and bim^−/− cells. Column height represents the total number of sequences, and the black portion indicates sequences with mutations giving rise to a tryptophan to leucine exchange at amino acid position 33 (W33L), which is known to lead to enhanced affinity for NP.