Congenital adrenal hyperplasia (CAH) is a disorder of adrenal steroid synthesis. In more than 90% of cases CAH is caused by CYP21 (21-hydroxylase) deficiency leading to impaired cortisol and aldosterone synthesis and an increase in ACTH secretion. This then leads to stimulation of the adrenal gland and overproduction of androgens with virilisation of female external genitalia. The CYP21 enzyme consists of 495 amino acids and is encoded by the CYP21A2 gene located on chromosome 6p21.3 close to a 98% homologous pseudogene (CYP21p). The pseudogene contains several inactivating mutations that may be transferred to the active CYP21A2 gene by gene conversion (more than 60% of the affected alleles) or gene deletion (30% of the affected alleles). The severity of the disease depends on the degree of CYP21 deficiency. The diagnosis can be made by measuring levels of 17-hydroxyprogesterone and androstenedione in serum.

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