**2.3 Major Histocompatibility Complex**

One the most studied regions of the vertebrate genome is the Major Histocompatibility Complex (MHC), a region central to the vertebrate immune response. In humans, the MHC is a large, gene dense region, spanning 3.6Mb and containing 224 genes divided into three regions; Class I, II and III (MHC Sequencing Consortium, 1999). Classes I and II encode genes involved in endogenous and exogenous antigen presentation respectively. Class III contains immune genes, involved in the inflammatory, complement and heat-shock responses, as well as a number of non-immune genes. This organization is in stark contrast to the chicken MHC consisting of only 19 genes within a 92kb region (Kaufman et al., 1999), making it difficult to establish the evolutionary history of the MHC. The position of marsupials and monotremes in vertebrate phylogeny ideally situates them to bridge the gap between chicken and eutherian mammal divergence and trace the evolutionary history of this important region. BAC clones have, once again, played an essential role in the study of the MHC organization and sequencing in marsupials and monotremes.
