CYP21A2 and classic congenital adrenal hyperplasia due to 21-hydroxylase deficiency: <b>Background:</b> Congenital Adrenal Hyperplasia (CAH) is a group of autosomal recessive disorders caused by impaired adrenal steroidogenesis, most frequently due to pathogenic variants in the <i>CYP21A2</i> gene leading to 21-hydroxylase deficiency (21-OHD).