Are anti-arsonate antibody N-segments selected at both the protein and the DNA level?

Abstract The V-D junctional residue at position 99 of AIJ anti-arsonate antibodies with a major cross-reactive idiotype is invariably a serine. This serine is not encoded in the germline of either the V H or D H gene segments nor can it be generated by intracodonic recombination between VH and DH. It must, therefore, be generated somatically (N segment addition) and selected by antigen. Sequence data indicate that the serine is frequently encoded by the uncommon TCG triplet. Here J. D. Capra and his colleagues discuss several explanations for the repeated appearance of this unusual codon at this position. They conclude that whatever the mechanism, N segment additions are selected at both protein and DNA levels.