CYP21A2 and classic congenital adrenal hyperplasia due to 21-hydroxylase deficiency: 21-hydroxylase deficiency (21-OHD), the most common form of congenital adrenal hyperplasia, is an autosomal recessive disease caused by mutations in CYP21A2 and has an incidence of 1:15,000-18,000 births (1, 2).