Coexistence of clonally related IgG and IgM isotypic variants among NP-specific memory B cells. V-genes from memory B-cell pool were sequenced as described in “V_H gene analysis.” Sequence comparison reveals shared CDR3 among 2 V-D-J rearrangements in IgG and IgM clones as indicated. Shared clonal origin was further confirmed by similar mutation patterns, including L to W at position 33.