For example, 21-hydroxylase deficiency is involved in >90% of congenital adrenal hyperplasia cases, which are associated with an inherited deficiency of 21-hydroxylase.[13] However, we did not detect any mutations in the CYP21A2 gene, which makes it unlikely that the patient had undiagnosed 21-hydroxylase deficiency. The gene discussed is CYP21A2; the disease is hyperinsulinemic hypoglycemia, familial, 4.