The major histocompatibility complex (MHC) is a large genomic region that plays an important role in the immune system. MCH proteins are expressed on the surface of cells and display fragments of molecules from invading microbes or tumor cells to T lymphocytes that may kill or co-ordinate the killing of the microbe or tumor cell. The major histocompatibility complex consists of four closely related genes with as many as fifty alleles possible. Two people chosen at random are very unlikely to have the same array of alleles.