CYP21A2 and classic congenital adrenal hyperplasia due to 21-hydroxylase deficiency: 21-hydroxylase deficiency (21OHD)(OMIM: 201910) is an autosomal recessive disorder caused by pathogenic variants in the CYP21A2 gene, accounting for over 90% of congenital adrenal hyperplasia (CAH) cases (1).