Abstract
A comparison between the cloned mouse DNA segments that were found to code for the lambda and kappa light chains of immunoglobulins established that there were seven short nucleotide sequences, two of which matched 6 out of 7, two 7 out of 8, two 8 out of 9, and one 9 out of 10 bases; these sequences were located either at homologous amino acid positions or at positions displaced by four amino acids or less. They all occurred in the framework regions (FRs), five next to the complementarity-determining regions (CDRs). Three of these were unique and did not occur elsewhere in the immunoglobulin nucleotides sequenced thus far or in DNA's of phage phi X174, phage G4, or simian virus 40. Five could serve as sites of joining by recombination or insertion of CDR to FR segments, and the invariant tryptophan that is the first residue of the second FR might serve as a sixth. These sites are consistent with the mini-gene or insertional hypotheses for the generation of antibody diversity but could also serve as points of recognition for a mutator enzyme or could serve to limit somatic mutation to the CDRs.