Complexity in the major histocompatibility complex.

The human major histocompatibility complex (MHC) is one of the most intensively studied regions of the human genome, containing over 70 known genes and spanning about 4 million base pairs (4 Mbp) of DNA on chromosome 6p21.3 (Klein, 1986). It can be divided up into three regions: the class I region (telomeric), the class II region (centromeric), and the class III region (between class I and II), which includes the complement component genes C2, C4, and Bf (Trowsdale & Campbell, 1988). The MHC has been mapped in detail using pulse field gel electrophoresis (PFGE) and by cloning in yeast artificial chromosome (YAC) and cosmid vectors, revealing long stretches of DNA between the regions as well as between individual class I and class II genes. Novel genes, that have no sequence relationships with class I, class II or complement components, have recently been found in these areas, and we will present an update on these after reviewing the more established loci.