CYP21A2 and congenital adrenal hyperplasia: Of note, ~95% of CAH patients have a deficient 21-hydroxylase (21OH) due to the mutation of CYP21A2. Since adrenocorticotropic hormone (ACTH) is partially downregulated by glucocorticoid and mineralocorticoid through a feed-back mechanism, reduced levels of glucocorticoid and mineralocorticoid are accompanied by elevated level of ACTH, which in turn results in not only accumulation of precursors in adrenal androgenesis but also exacerbates CAH (2).