CYP21A2 and congenital adrenal hyperplasia: Congenital adrenal hyperplasia (CAH) is a group of autosomal recessive disorders characterized by defective adrenal steroidogenesis resulting in deficiency of the glucocorticoid cortisol and in many cases the mineralocorticoid aldosterone.1 Over 90% of cases are due to 21-hydroxylase deficiency secondary to inactivating mutations in CYP21A2.2,3 The resulting lack of glucocorticoid (and mineralocorticoid) activates the hypothalamic-pituitary-adrenal (HPA) axis causing excessive synthesis of adrenocorticotrophic hormone (ACTH) and adrenal androgens.