Steroid 21-hydroxylase (CYP21A2) specifically catalyses the 21-hydroxylation of steroids which is required for the adrenal synthesis of mineralocorticoids and glucocorticoids. Defects in CYP21A2 can cause adrenal hyperplasia 3 (AH3; MIM:201910), a form of congenital adrenal hyperplasia (CAH) where cortisol synthesis is defective. This results in increased ACTH levels, causing overproduction and accumulation of cortisol precursors, particularly 17-hydroxyprogesterone (17HPROG). The resultant excessive production of androgens causes virilization. 21-hydroxylase deficiency accounts for more than 90% of CAH cases and ranges from mild to complete loss of activity (White et al. 2000, White & Bachega 2012).