In homozygous mice bearing I regions derived from haplotype k, only a single type of Ia molecule bearing the alloantigenic specificities Ia.7 and Ia.22 was found using techniques of sequential immune precipitation and tryptic peptide analysis. As suggested at the fourth Ir Gene Workshop (Sachs 1978), Ia.7 is considered here to be an antigenic determinant associated with I-E-subregionencoded molecules, i. e., it is excluded from the I-C subregion. The I-C subregion is currently defined mainly by functional traits. It is now known that the I-E molecules are composed of an α chain encoded in the I-E subregion, and a β chain encoded in the I-A subregion. Since the I-C subregion is not involved with the determination of these Ia molecules, and since in homozygotes there is apparently only a single type of molecule bearing both specificities Ia.7 and Ia.22, the term "I-E/C" molecule should probably be dropped in favor of the simpler designation I-E.
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