CYP21A2 and classic congenital adrenal hyperplasia due to 21-hydroxylase deficiency: The most common cause is a mutation in the CYP21A2 gene leading to 21-hydroxylase deficiency (21OHD), which results in impaired production of cortisol and increased production of adrenal androgens, leading to virilization of the external genitalia in 46,XX individuals (1).