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CYP21A2 gene

cytochrome P450 family 21 subfamily A member 2

Normal Function

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 (glucose) 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.

Health Conditions Related to Genetic Changes

21-hydroxylase deficiency

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.

More About This Health Condition

Other Names for This Gene

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

Additional Information & Resources

Tests Listed in the Genetic Testing Registry

Scientific Articles on PubMed

Gene and Variant Databases


  • Hughes IA. Congenital adrenal hyperplasia: 21-hydroxylase deficiency in the newborn and during infancy. Semin Reprod Med. 2002 Aug;20(3):229-42. doi: 10.1055/s-2002-35387. Citation on PubMed
  • 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. Citation on PubMed or Free article on PubMed Central
  • 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. doi: 10.1016/S1525-1578(10)60550-8. Citation on PubMed or Free article on PubMed Central
  • Krone N, Riepe FG, Grotzinger 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. doi: 10.1210/jc.2004-0813. Epub 2004 Oct 13. Citation on PubMed
  • Lee HH. Diversity of the CYP21P-like gene in CYP21 deficiency. DNA Cell Biol. 2005 Jan;24(1):1-9. doi: 10.1089/dna.2005.24.1. Citation on PubMed
  • New MI. An update of congenital adrenal hyperplasia. Ann N Y Acad Sci. 2004 Dec;1038:14-43. doi: 10.1196/annals.1315.009. Citation on PubMed
  • 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. Citation on PubMed
  • 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. doi: 10.1590/s0100-879x2003001000006. Epub 2003 Sep 16. Citation on PubMed

The information on this site should not be used as a substitute for professional medical care or advice. Contact a health care provider if you have questions about your health.