We report the complete DMA sequence of a c-DNA clone of the heavy chain μb allele of the C57BL/6 mouse. Comparisons have been made with the nucleotide sequences of the germ line BALB/c μa and the plasmacytoma TEPC-183 μa alleles reported elsewhere over the entire length of the coding and the 3' untranslated region. In contrast to the extensive differences between the γ2a a and b alleles we have reported earlier we see a very high degree of homology between the μ alleles. Only one of the nucleotide differences between C57BL/6 μb and BALB/c μa leads to an amino acid substitution. This single amino acid exchange must form the allotyplc determinant of the μb allele. A comparison of four different DHA sequences indicates that they are all distinct IgM alleles.
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