GLUT1 deficiency syndrome-1 ( is a neurologic disorder showing wide phenotypic variability. The most severe ‘classic’ phenotype comprises infantile-onset epileptic encephalopathy associated with delayed development, acquired microcephaly, motor incoordination, and spasticity. Onset of seizures, usually characterized by apneic episodes, staring spells, and episodic eye movements, occurs within the first 4 months of life. Other paroxysmal findings include intermittent ataxia, confusion, lethargy, sleep disturbance, and headache. Varying degrees of cognitive impairment can occur, ranging from learning disabilities to severe mental retardation. Hypoglycorrhachia (low CSF glucose, less than 40 mg/dl) and low CSF lactate are essentially diagnostic for the disorder. As more cases with GLUT1 deficiency syndrome were described, the phenotype was broadened to include individuals with ataxia and mental retardation but without seizures, individuals with dystonia and choreoathetosis, and rare individuals with absence seizures and no movement disorder. The disorder, which results from a defect in the GLUT1 glucose transporter causing decreased glucose concentration in the central nervous system, is part of a spectrum of neurologic phenotypes resulting from GLUT1 deficiency. GLUT deficiency syndrome-2 (612126) represents the less severe end of the phenotypic spectrum and is associated with paroxysmal exercise-induced dystonia with or without seizures. Correct diagnosis of GLUT1 deficiency is important because a ketogenic diet often results in marked clinical improvement of the motor and seizure symptoms (reviews by Pascual et al., 2004 and Brockmann, 2009).