We have cloned and sequenced the predominant germline V kappa gene segment expressed by B cells of strain A origin that synthesize antibodies with specificity for Ars. In hybridomas synthesizing anti-Ars antibodies, this V kappa gene segment (V kappa IdCR) has been found exclusively associated with the J kappa 1 gene segment without exhibiting junctional sequence variation. Sequence comparisons of the germline V kappa IdCR gene with expressed derivatives reveals that the latter frequently contain somatically introduced amino acid replacements. Taken together with results of previous structural analyses, these results show that the predominant population of IdCR+ V regions elicited in the secondary immune response is encoded by one or two combinations of V gene segments, has little junctional diversity, and is extensively diversified by somatic mutation in both heavy and light chains.

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