A common inherited disorder of steroidogenesis in man, 21-hydroxylase (21-OH) deficiency, is linked to the HLA major histocompatibility complex (MHC)1, and is associated in particular with certain allotypes of the HLA-linked complement proteins2,3. Recently, this disorder was demonstrated to result from a defective structural gene for the 21-OH enzyme, also termed cytochrome P-450C21 (ref. 4). The human (HLA) and murine (H-2) MHCs are homologous in overall organization and in the structures of their component genes5. To determine whether 21-OH genes are located in the H-2 complex, we have now used a bovine adrenal complementary DNA clone encoding part of 21-OH6 to examine a cluster of overlapping cosmid clones derived from the S region of the BALB/c mouse7. We found that there are two 21-OH genes in this region, located immediately 3′ to the C4 and Slp genes. © 1984 Nature Publishing Group.