The major histocompatibility complex (MHC) proteins display peptides on the surface of an antigen-presenting cell (APC). Since T lymphocytes recognize antigen only when the antigen is bound to MHC proteins, the latter play a central role in the acquired immune response. The MHC genes are categorized into three classes on the basis of the structure and function of the proteins they encode. Genes encoding the MHC class I products are located at both ends of the MHC, whereas the genes encoding the MHC class II and class III proteins are located between the two class I regions. In one recent study, this experimental system allowed a tumor-specific human cytotoxic T-lymphocyte clone to be studied in a tumor-bearing mouse that possessed a human MHC transgene. In this study, the antigenic peptide bound to the human MHC proteins and the T-cell antigen receptor (TCR) that recognized the antigen were similar to the peptides and TCR that might work during actual tumor rejection in a human patient. The different types were designated by the particular human leukocyte antigen (HLA) class I protein followed by a number, given out in nonsequential order. Thus, serologically distinct HLA-A alleles could be classified as HLAA1, -A2, -A3, -A9, etc. However, as more antibody reagents were developed, it became clear that some of the HLA specificities could be further subdivided into a related set.