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Genetics Home Reference: your guide to understanding genetic conditions     A service of the U.S. National Library of Medicine®


Reviewed February 2010

What is the official name of the CYP21A2 gene?

The official name of this gene is “cytochrome P450 family 21 subfamily A member 2.”

CYP21A2 is the gene's official symbol. The CYP21A2 gene is also known by other names, listed below.

What is the normal function of the CYP21A2 gene?

The CYP21A2 gene provides instructions for making an enzyme called 21-hydroxylase, which is part of the cytochrome P450 family of enzymes. Cytochrome P450 enzymes are involved in many processes in the body, such as assisting with reactions that break down drugs and helping to produce cholesterol, certain hormones, and fats (lipids).

The 21-hydroxylase enzyme is found in the adrenal glands, which are located on top of the kidneys and produce a variety of hormones that regulate many essential functions in the body. 21-hydroxylase plays a role in producing hormones called cortisol and aldosterone. Cortisol helps maintain blood sugar levels, protects the body from stress, and suppresses inflammation. Aldosterone is sometimes called the salt-retaining hormone because it regulates the amount of salt retained by the kidneys. The retention of salt affects fluid levels in the body and blood pressure.

Does the CYP21A2 gene share characteristics with other genes?

The CYP21A2 gene belongs to a family of genes called CYP (cytochrome P450s).

A gene family is a group of genes that share important characteristics. Classifying individual genes into families helps researchers describe how genes are related to each other. For more information, see What are gene families? ( in the Handbook.

How are changes in the CYP21A2 gene related to health conditions?

21-hydroxylase deficiency - caused by mutations in the CYP21A2 gene

More than 100 mutations in the CYP21A2 gene have been found to cause 21-hydroxylase deficiency. Some of these mutations result from an exchange of genetic material between the CYP21A2 gene and a similar but nonfunctional piece of DNA called a pseudogene, which is located very close to the CYP21A2 gene on chromosome 6. This type of DNA exchange is called a gene conversion. The genetic material from the pseudogene contains errors that, when introduced into the CYP21A2 gene, disrupt the way the gene's instructions are used to make a protein. Other mutations that cause 21-hydroxylase deficiency change single protein building blocks (amino acids) in the 21-hydroxylase enzyme or delete or insert pieces of DNA in the CYP21A2 gene.

Researchers have described three forms of 21-hydroxylase deficiency. Individuals with a form of the disorder called the salt-wasting type have CYP21A2 mutations that result in a completely nonfunctional enzyme. People with the simple virilizing type of this condition have CYP21A2 gene mutations that allow the production of low levels of functional enzyme. Individuals with the non-classic type of this disorder have CYP21A2 mutations that result in the production of reduced amounts of the enzyme, but more enzyme than any of the other types. All types of 21-hydroxylase deficiency interfere with the production of cortisol and aldosterone. The substances that are usually used to form these hormones instead build up in the adrenal glands and are converted to androgens, which are male sex hormones. The excess production of androgens leads to abnormalities of sexual development in people with 21-hydroxylase deficiency.

Where is the CYP21A2 gene located?

Cytogenetic Location: 6p21.3

Molecular Location on chromosome 6: base pairs 32,038,316 to 32,041,670

(Homo sapiens Annotation Release 107, GRCh38.p2) (NCBI (

The CYP21A2 gene is located on the short (p) arm of chromosome 6 at position 21.3.

The CYP21A2 gene is located on the short (p) arm of chromosome 6 at position 21.3.

More precisely, the CYP21A2 gene is located from base pair 32,038,316 to base pair 32,041,670 on chromosome 6.

See How do geneticists indicate the location of a gene? ( in the Handbook.

Where can I find additional information about CYP21A2?

You and your healthcare professional may find the following resources about CYP21A2 helpful.

You may also be interested in these resources, which are designed for genetics professionals and researchers.

What other names do people use for the CYP21A2 gene or gene products?

  • CA21H
  • CAH1
  • CPS1
  • CYP21
  • CYP21B
  • Cytochrome P450 Family 21 Subfamily A Polypeptide 2
  • cytochrome P450, family 21, subfamily A, polypeptide 2
  • cytochrome P450, subfamily XXIA (steroid 21-hydroxylase, congenital adrenal hyperplasia), polypeptide 2
  • Cytochrome P450 XXI
  • Cytosteroid 21-Monooxygenase
  • P450c21B
  • steroid 21-hydroxylase
  • steroid 21-monooxygenase

See How are genetic conditions and genes named? ( in the Handbook.

What glossary definitions help with understanding CYP21A2?

acids ; adrenal glands ; aldosterone ; androgens ; cholesterol ; chromosome ; congenital ; cytochrome P450 ; deficiency ; DNA ; enzyme ; gene ; gene conversion ; hormone ; hyperplasia ; inflammation ; protein ; pseudogene ; stress ; wasting

You may find definitions for these and many other terms in the Genetics Home Reference Glossary.


  • Hughes IA. Congenital adrenal hyperplasia: 21-hydroxylase deficiency in the newborn and during infancy. Semin Reprod Med. 2002 Aug;20(3):229-42. Review. (
  • Huynh T, McGown I, Cowley D, Nyunt O, Leong GM, Harris M, Cotterill AM. The clinical and biochemical spectrum of congenital adrenal hyperplasia secondary to 21-hydroxylase deficiency. Clin Biochem Rev. 2009 May;30(2):75-86. (
  • Keen-Kim D, Redman JB, Alanes RU, Eachus MM, Wilson RC, New MI, Nakamoto JM, Fenwick RG. Validation and clinical application of a locus-specific polymerase chain reaction- and minisequencing-based assay for congenital adrenal hyperplasia (21-hydroxylase deficiency). J Mol Diagn. 2005 May;7(2):236-46. (
  • Krone N, Riepe FG, Grötzinger J, Partsch CJ, Sippell WG. Functional characterization of two novel point mutations in the CYP21 gene causing simple virilizing forms of congenital adrenal hyperplasia due to 21-hydroxylase deficiency. J Clin Endocrinol Metab. 2005 Jan;90(1):445-54. Epub 2004 Oct 13. (
  • Lee HH. Diversity of the CYP21P-like gene in CYP21 deficiency. DNA Cell Biol. 2005 Jan;24(1):1-9. Review. (
  • NCBI Gene (
  • New MI. An update of congenital adrenal hyperplasia. Ann N Y Acad Sci. 2004 Dec;1038:14-43. (
  • Nimkarn S, Lin-Su K, New MI. Steroid 21 hydroxylase deficiency congenital adrenal hyperplasia. Endocrinol Metab Clin North Am. 2009 Dec;38(4):699-718. doi: 10.1016/j.ecl.2009.08.001. Review. (
  • Torres N, Mello MP, Germano CM, Elias LL, Moreira AC, Castro M. Phenotype and genotype correlation of the microconversion from the CYP21A1P to the CYP21A2 gene in congenital adrenal hyperplasia. Braz J Med Biol Res. 2003 Oct;36(10):1311-8. Epub 2003 Sep 16. (


The resources on this site should not be used as a substitute for professional medical care or advice. Users seeking information about a personal genetic disease, syndrome, or condition should consult with a qualified healthcare professional. See How can I find a genetics professional in my area? ( in the Handbook.

Reviewed: February 2010
Published: February 1, 2016