A minimum of four human class II alpha-chain genes are encoded in the HLA region of chromosome 6.

The major histocompatibility complex (MHC) in man, also called the HLA region, is located on the short arm of chromosome 6 and encodes antigens involved in immunological processes. The class II HLA antigens consist of two noncovalently associated polypeptide chains, one of molecular weight 34,000 (alpha) and the other of molecular weight 29,000 (beta). The extensive polymorphism of the beta chain(s) has allowed the genetic mapping of the corresponding beta gene(s) to the HLA-DR region. cDNA clones for the HLA-DR alpha chain have been used to map the non-polymorphic DR alpha-chain gene to chromosome 6 using mouse-human somatic cell hybrids. Similarly, the DR alpha-chain gene has been mapped to the short arm of chromosome 6 centromeric to the HLA-A, -B and -C loci by in situ hybridization experiments. We isolated a cDNA clone that is related to the DR alpha chain and encodes the class II antigen DC alpha chain. We describe here how this DC alpha clone was used to find two or three additional alpha-chain genes by cross-hybridization and how HLA-antigen loss mutants of a human lymphoblastoid cell line (LCL) were used to ascertain that these additional class II antigen alpha-chain genes are also located in the HLA region.