Generalized recombination is a phenomenon dependent on recognition of homologous DNA and may be responsible for generating a mouse γ2b-γ2a hybrid immunoglobulin heavy chain1. The genetic element, Chi, has been shown to enhance generalized recombination in prokaryotes2-10. Sequence comparison predicted Chi to be present in the CH3 domain of the mouse γ2b gene, and using plaque size in the appropriate selective conditions as a measure of Chi activity2,3, a clone containing the γ2b gene11 was shown to confer Chi activity on the γ vector which carried it. Thus Chi, if functional in eukaryotic systems, could have promoted the recombinational event which produced the γ2b-γ2a hybrid immunoglobulin. We report here that the frequency of Chi in the mouse genome is similar to that in other eukaryotic unique sequences but lower than in immunoglobulingenes. The positions at which Chi occurs in immunoglobulin genes and the fact that the γ2b-γ2a hybrid variants isolated from mouse myeloma cell line MPC11 may represent an aberrant class switch suggest that homologous recombination may be broadly applicable to the heavy-chain class switch.
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