Adrenal insufficiency in paediatric patients is mostly due to congenital adrenal hyperplasia (CAH), a severe monogenic disease caused by steroid 21-hydroxylase deficiency (21-OHD, encoded by the CYP21A2 gene) in 95% of cases. The gene discussed is CYP21A2; the disease is classic congenital adrenal hyperplasia due to 21-hydroxylase deficiency.