Autosomal recessive disorder caused by a deficiency of AGAT resulting in a creatine deficiency. AGAT catalyzes the combination of arginine and glycine to form guanidinoacetic acid (GAA). GAA is methylated to form creatine. These patients present with global developmental delay, hypotonia or gross motor delay . They may also have febrile seizures, muscle weakness or myopathies. They may present from the newborn period to young adulthood. MRS may show an absent Creatine peak.