CYP21A2 and classic congenital adrenal hyperplasia due to 21-hydroxylase deficiency: Large cohort studies of patients with CAH due to 21-OHD worldwide have revealed and confirmed that approximately 15% of CAH patients carry at least one CAH-X allele, a contiguous deletion disrupting both CYP21A2 and TNXB tandem genes [10,11,12,13].