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Compared with the MHC of typical mammals, the chicken MHC is smaller and simpler, with only two class I genes found in the B12 haplotype. We make five points to show that there is a single- dominantly expressed class I molecule that can have a strong effect on MHC function. First, we find only one cDNA for two MHC haplotypes (B14 and B15) and cDNAs corresponding to two genes for the other six (B2, B4, B6, B12, B19, and B21). Second, we find, for the B4, B12, and B15 haplotypes, that one cDNA is at least 10-fold more abundant than the other. Third, we use 2D gel electrophoresis of class I molecules from pulse-labeled cells to show that there is only one heavy chain spot for the B4 and B15 haplotypes, and one major spot for the B12 haplotype. Fourth, we determine the peptide motifs for B4, B12, and B15 cells in detail, including pool sequences and individual peptides, and show that the motifs are consistent with the peptides binding to models of the class I molecule encoded by the abundant cDNA. Finally, having shown for three haplotypes that there is a single dominantly expressed class I molecule at the level of RNA, protein, and antigenic peptide, we show that the motifs can explain the striking MHC-determined resistance and susceptibility to Rous sarcoma virus. These results are consistent with the concept of a "minimal essential MHC" for chickens, in strong contrast to typical mammals. [ABSTRACT FROM AUTHOR]