Congenital adrenal hyperplasia due to 21-hydroxylase deficiency (21OHD) is a common autosomal recessive disorder caused by mutations in the CYP21A2 gene, which encodes 21-hydroxylase (an enzyme involved in aldosterone and cortisol biosynthesis). The gene discussed is CYP21A2; the disease is classic congenital adrenal hyperplasia due to 21-hydroxylase deficiency.