CYP21A2 and classic congenital adrenal hyperplasia due to 21-hydroxylase deficiency: The most common form of CAH results from mutations in the cytochromep450 family 21 subfamily A member 2 gene (CYP21A2), which leads to 21-hydroxylase deficiency (21-OHD) and may be life-threatening due to a reduced adrenal production of cortisol and aldosterone.