The major histocompatibility complex (MHC) of mammals number of mostly duplicated contains a large genes. In the HLA system (the MHC of man), which is by far the best‐studied major histocompatibility system so far, roughly 20 genes have been defined and mapped. They code for three classes of proteins: HLA‐A, ‐B and ‐C (Class I), HLA‐DP, ‐DQ and ‐DR (Class II) and serum complement components C2, C4 and Bf (Class III). Furthermore, the region contains genes for 21‐hydroxylase (21‐OH) and tumor necrosis factor (TNF).