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