The major histocompatibility locus (MHC) contains some 224 genes and is one of the most gene-dense regions of the human genome. It comprises the HLA class I and II genes, and contained within the class I region are the MHC class I chain-related (MIC) genes, of which there are two functional coding genes in man, MICA and MICB. All these genes are highly polymorphic—some having hundreds of alleles within the human population, and a large number function in the immune system. The entire MHC region is highly conserved between chimpanzees (Pan troglodytes) and humans, but these closely related species react quite differently to various pathogens, the most notable being HIV. The entire human MHC genomic sequence is available, and in the June 24 PNAS, Tatsuya Anzai and colleagues at the Tokai University School of Medicine use probes derived from this to isolate chimpanzee genomic library clones and to derive the genomic sequence of half of the chimpanzee MHC region in order to discover the molecular basis for these immunologic differences (PNAS, 100:7708-7713, June 24, 2003).