CAH-X is a hypermobility-type Ehlers–Danlos syndrome connective tissue dysplasia affecting approximately 15% of patients with 21-hydroxylase deficiency (21-OHD) congenital adrenal hyperplasia (CAH) due to contiguous deletion of CYP21A2 and TNXB genes. This evidence concerns the gene CYP21A2 and classic congenital adrenal hyperplasia due to 21-hydroxylase deficiency.