The human major histocompatibility complex (MHC) class III region contains 57-60 structural genes spanning 654-759 kb of genomic DNA. Analysis of the sequence identities of the human and mouse genomic regions between NOTCH4 and complement C2 yields important information on the locations of the coding and regulatory sequences. It also provides insights into the relationship between protein function and level of sequence conservation, and on the clustering of genes with related functions.