Cerebral creatine deficiencies are a small group of inherited disorders that result from defects in creatine biosynthesis and utilization. Commonly affected tissues include the brain and muscles. There are three distinct CCDs. The most common is creatine transporter defect (CTD), an X-linked disorder caused by pathogenic variants in SLC6A8. The main symptoms of CTD are intellectual disability and developmental delay, and these are caused by a lack of creatine in the brain, due to the defective transporter. There are also two enzymatic defects of creatine biosynthesis, arginine:glycine amidinotransferase deficiency (AGAT deficiency), caused by variants in GATM and guanidinoacetate methyltransferase deficiency (GAMT deficiency), caused by variants in GAMT. The single enzyme defects are both