The sera of C57BL/6 mice transgenic for a mu a allotype heavy (H) chain and kappa light chain gene contained endogenous nontransgene immunoglobulin (IgM) (mu b allotype) and IgA molecules which carried the idiotype expressed by the transgenically encoded IgM (mu a) molecule. Serological analysis demonstrated that the presence of the transgenic idiotype on endogenous IgM and IgA was caused by the secretion of chimeric molecules that carried both chains encoded by the mu a transgene and products of endogenously rearranged Ig mu b or alpha genes. These and other results suggest that allelic exclusion of Ig gene rearrangement in mu, kappa transgenic mice is not absolute, that B cells can secrete Igs composed of more than a single (H) chain type, and that endogenous isotype switching does not result in a complete silencing of transgene expression.

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