Congenital adrenal hyperplasia (CAH) due to 21-hydroxylase deficiency is one of the most technically challenging monogenic conditions for molecular diagnosis, owing to the complex genomic architecture of the <i>CYP21A2</i> locus and the extensive homology between the functional gene and its pseudogene. This evidence concerns the gene CYP21A2 and classic congenital adrenal hyperplasia due to 21-hydroxylase deficiency.