Cholesterol side-chain cleavage enzyme, mitochondrial (CYP11A1) normally catalyses the side-chain cleavage of cholesterol to form pregnenolone. Defects in CYP11A1 can cause Adrenal insufficiency, congenital, with 46,XY sex reversal (AICSR; MIM:613743). This is a rare disorder that can present as acute adrenal insufficiency in infancy with elevated ACTH and plasma renin activity and low or absent adrenal steroids. The severest phenotype is loss-of-function mutations associated with prematurity, complete under-androgenisation and severe, early-onset adrenal failure (Kim et al. 2008).