The human major histocompatibility complex (the HLA complex) encodes two classes of cell surface heterodimeric glycoproteins which regulate the immune response to foreign antigens. Molecular analysis of the HLA class I and class II genes has yielded considerable information about their organization and structure. The class I genes consist of the polymorphic A, B and C genes and the non-polymorphic Qa, Tla-like genes. The HLA class II region contains multiple alpha and beta genes and pseudogenes which map into three subregions: DR, DQ and DP. The class II genes share similar gene structures, but vary in their degree of polymorphism.