The chicken B locus is a minimal essential major histocompatibility complex

Here we report the sequence of the region that determines rapid allograft r ejection in chickens, the chicken major histocompatibility complex (MHC). T his 92-kilobase region of the B locus(1-4) contains only 19 genes, making t he chicken MHC roughly 20-fold smaller than the human MHC5. Virtually all t he genes have counterparts in the human MHC, defining a minimal essential s et of MHC genes conserved over 200 million years of divergence between bird s and mammals. They are organized differently, with the class III region ge nes located outside the class II and class I region genes. The absence of p roteasome genes(5,6) is unexpected and might explain unusual peptide-bindin g specificities of chicken class I molecules. The presence of putative natu ral killer receptor gene(s)(5,7) is unprecedented and might explain the imp ortance of the B locus in the response to the herpes virus responsible for Marek's diseases(8-10). The small size and simplicity of the chicken MHC al lows co-evolution of genes as haplotypes over considerable periods of time, and makes it possible to study the striking MHC-determined pathogen-specif ic disease resistances(8-10) at the molecular level.