We have used a radioimmune assay to confirm our earlier findings of an unexpected immunoglobulin allotype in Igb-congenic BALB/c mice. Although these mice were bred to exclude the IgG2a allotype of BALB/c (Ig-la), an Ig-la-like antigen was detected in the 7S Ig fraction of two (of five) pooled serum samples, it represented 0.1--0.3% of the total 7S protein and was indistinguishable from a reference Ig-la. The detection of putative Ig-la in Igb-congenic mice is inconsistent with the notion that allotypes are products of allelic structural genes. It appears rather that expression of Ig-la is controlled by allelic regulator genes and that its low and transient production in Igb-congenic mice results from incomplete negative regulation.
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