SLC6A8 and hyperinsulinemic hypoglycemia, familial, 4: Cerebral creatine deficiency disorders (CCDD) are a group of rare diseases comprising L-arginine: glycine amidinotransferase (AGAT) deficiency (OMIM #612718), guanidinoacetate-N-methyltransferase (GAMT) deficiency (OMIM #612736) and creatine transporter (CRTR) deficiency (OMIM #300352) [16].