The rationale for supplementing sapropterin dihydrochloride, a synthetic BH4 analogue, is based upon the defective biosynthesis or recycling of this essential cofactor for the aromatic L‐amino acid hydroxylases (AADC) in all types of BH4 deficiencies. The gene discussed is DDC; the disease is hyperinsulinemic hypoglycemia, familial, 4.