The structure of the genes has now been elucidated: P450 C11b contains nine exons and is located on chromosome 15; and P450 C17a also contains nine exons, and is located on chromosome 8. There are possibly two gene products of P450 C11b: one that uses 11-deoxycortisol as a substrate to produce cortisol, and the other being involved in the final synthesis of aldosterone. The P450 C17a gene is on chromosome 10 and contains eight exons; and the P450 C21 gene contains ten exons and is closely united to a pseudogene on chromosome 6 in the histocompatibility leucocyte antigen locus.
The steroid hydroxylases represent three gene families in the P450 superfamily. It seems likely that these genes have evolved from a common progenitor, several hundred million years ago, into distinct steroid hydroxylase gene families that now catalyse specific reactions.
