S107 V_H gene sequences from class-switched B cells contain somatic mutations. The S107 V_H germ line sequence is shown aligned with 4 S107 sequences cloned from the PCR product derived from sorted B220⁺ IgM^- IgD^- κ⁺ spleen cells pooled from 2 VavP-Bcl2 mice. Boxes indicate CDR regions 1 and 2. Bracketed bases in the consensus represent mutational hot spots. Boxed residues indicate mutations: those in black boxes encode amino acid changes (replacement mutations); those in gray boxes represent silent mutations.