In two human studies involving the H1N1 influenza virus (23, 24), 17 out of 46 and 24 out of 49 antibodies had at least one AGY Ser to Arg amino acid replacement resulting from SHM (Figure 3A). Arg replacement mutations in CDR sequences accounted for 2.9 and 3.1% of all V-region gene missense mutations (CDRs and FRs) in the two studies, with replacements at germline AGY codons comprising most of these (2 and 2.23%). A similar trend was observed in antibodies against hepatitis A, B, and C, rhino, dengue, avian influenza, and West Nile viruses. CDR Arg mutations accounted for 2.4–9.4% of all missense mutations in V-region genes for these antibodies, most of which (1.5–6.6%) occurred at germline CDR AGY codons (Figure 3B; Table 1).