The major histocompatibility complex (MHC) is a fascinating region of the human genome. More is known about this 4 Mb of DNA (0.1% of the genome) on the short arm of chromosome 6 than about any other region of similar size. Among the 80 or so MHC genes found so far are several clusters with related functions in antigen processing and presentation. In addition to its importance in immunology, the MHC is a useful model for investigating gene organization, polymorphism, linkage disequilibrium and recombination. A large number of diseases, many of the autoimmune type, are associated with the region.