Gene map of the extended human MHC

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Abstract
The gene map for the extended major histocompatibility complex (xMHC) comprises 421 loci (excluding RNA genes) in a sequence length of 7.6 Mb — extending the previous gene map of the classical MHC, which was 3.6 Mb long and contained 224 loci. All 421 xMHC loci have been assigned definitive and approved gene symbols. About 50% of the xMHC gene loci are present in clusters or superclusters that are not restricted only to immune genes. The two largest clusters, comprising histone and tRNA genes are the largest of their type in the genome. Transcription hotspot analysis indicates that it is just as likely that the classical MHC is hitch-hiking with gene clusters of the xMHC as the reverse. About 22% of the expressed xMHC genes show a higher than average number of non-synonymous coding polymorphisms. About 28% of the xMHC genes can be associated with immune system function. About 10% of the xMHC genes are currently known to be disease-causing or disease-associated. About 20% of the xMHC genes have putative paralogues elsewhere in the genome, indicating considerable potential for functional redundancy. The gene map of the xMHC provides an invaluable resource for the study of the most important genetic region of the human genome in relation to infectious, inflammatory and autoimmune diseases.