X-linked disorder resulting in a defect in formation of a creatine transporter protein due to mutations of the SLC6A8 gene at chromosome Xq28. 100% of the patients present with global developmental delay. 60% of patients will have seizures. Movement disorders, GI symptoms, and urogenital anomalies are also seen. MRS typically shows an absence of Creatine in males, and a diminished peak in females (As females have one working copy of the gene on one of their X chromosomes).