Humans with mutations in AGAT, GAMT, or SLC6A8 display cerebral creatine deficiency syndromes (CCDSs), which are rare diseases that are characterized by the disruption of the synthesis or transfer of creatine (Wyss and Kaddurah-Daouk, 2000; Braissant et al., 2011). Here, GAMT is linked to cerebral creatine deficiency syndrome.