Creatine deficiency syndromes are recently identified groups of inborn errors of creatine metabolism, including X-linked CTD (OMIM 300036) and two autosomal recessive defects of creatine synthesis (arginine–glycine amidinotransferase (AGAT, OMIM 612718) and guanidinoacetate methyltransferase (GAMT, OMIM 612736)). This evidence concerns the gene GAMT and cerebral creatine deficiency syndrome.