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)). The gene discussed is GATM; the disease is cerebral creatine deficiency syndrome.