SLC6A8 and neurodevelopmental disorder: Creatine deficiency syndromes are a group of rare monogenic neurodevelopmental disorders caused by impaired creatine synthesis or transport, involving two enzymes, arginine:glycine amidinotransferase (AGAT) and guanidinoacetate methyltransferase (GAMT), as well as the creatine transporter (SLC6A8).1, 2, 3, 4