Gene locus and gene structure

21-Hydroxylase is encoded by the CYP21 gene
(formerly called CYP21B and 21-OHB), a member
of the cytochrome P450 oxidase gene
family. This gene is located within the class III
genes of the major histocompatibility complex
on the short arm of human chromosome 6. It is
part of a tandem paired arrangement of three
other genes: active C4A and C4B genes and a
96–98% homologous inactive CYP21P gene, a
pseudogene due to intragenic deletions resulting
in stop codons (formerly called CYP21A or
21-OHA). These genes originated from a duplication
event in evolution. The CYP21 (21-OHB)
gene consists of 10 exons spanning almost 6 kb
of genomic DNA (the actual distance of 30 kb to
the C4 A and C4 B genes is not shown to scale).