[The genetic map of HLA].

The major histocompatibility complex (MHC) gene products are known to play a fundamental role in foreign antigen presentation to the cellular immune system. The human major histocompatibility complex (HLA) spans about 4 million base pairs (4 Mbp) of DNA at chromosome position 6p21.3 and is one of the most intensively studied regions of the human genome, containing over 70 known genes. The HLA can be divided up into three regions: the class I sequences at the telomeric end of the complex; the class II loci at the centromeric end; and, between these, the class III genes including those for the complement components. The analysis of HLA has focused primarily on their structure and function. However, the quantitative differences in gene expression are also important, since quantitative variations of cell surface HLA influences immunoregulation and the expression of immunologically mediated diseases. Recently, biochemical and genetic studies have revealed the existence of tissue-specific cis-acting regulatory gene elements and trans-acting protein factors, capable of binding to the cis-acting regions, that govern HLA gene expression.