The cerebral creatine deficiency syndromes, inborn errors of creatine metabolism, include two autosomal recessive creatine biosynthetic disorders [guanidinoacetate methyltransferase (GAMT) deficiency (MIM 601240) and L-arginine:glycine amidinotransferase (AGAT) deficiency (MIM 602360)] and the X-linked creatine transporter mutation (MIM 300036) affecting creatine uptake1. The gene discussed is SLC6A8; the disease is hyperinsulinemic hypoglycemia, familial, 4.