THE CLASS-I REGION OF THE MHC GENES IS ON E OF THE MOST COMPLEX IN THE WHOLE HUMAN GENOME

The human major histocompatibility complex (MHC) is a 4-Mb genomic seg ment located on chromosome 6, in the distal portion of the 6p21.3 band . The MHC is a well described region of the human genome and represent s a cluster of genes intensively studied because of the important func tions of the gene products in the immune system. The total number of g enes located within the MHC is now approximately 100, but more can be predicted. The density of genes in the Class II and Class III regions has been known for several rears, but the high density of genes in the Class I region did only recently become apparent. This class I region spans 2,000 kb on the telomeric half of the MHC, and contains six cla ss I genes, three of which, the classical human leukocyte antigen gene s HLA-A, HLA-B and HLA-C, encode membrane-anchored cell surface glycop roteins that present the endogenous antigenic peptides to the T cell r eceptors and are recognized by NK cell receptors. These genes are char acterized by a high polymorphism and a nearly ubiquitous expression. T he biological function of the three nonclassical class I genes HLA-E, HLA-F, and HLA-G still remains uncertain. In addition to these six gen es, the MHC contains a number of class I pseudogenes and gene fragment s whose close relationship indicates a common origin from a typical HL A class I ancestor gene. This region appears also to harbour various n on-HLA single copy genes such as OTF3, TCF19, S gene, TUBB, HSR1, B30. 2, ZNF173, HCG V: HCG-VII, MOG or OLFR2 as well as eight recently desc ribed multigene families named P5.1, 3.8-1, PERB11, BAT1, HCGII, HCGIV : HCGVIII and HCGIX containing a number of sequences displayed through out the HLA Class I region. The presence of all these coding sequences and multigene families suggests that the gene density of the HLA Clas s I region is comparable to that of the class II and class III regions , i.e. one gene per 20 or 30 kb. We present here a new map compiling t he update knowledge of the structure of this Class I region.