• Glucose Transporter Type 1 Deficiency (GLUT1) deficiency is caused by heterozygous, loss of function variants in in the SLC2A1 gene. • Features include epilepsy that tends to be unresponsive to antiepileptic mediations, acquired microcephaly, complex movement disorder. Milder phenotypes include the absence of seizures and paroxysmal events triggered by exercise, exertion, or fasting. Here, SLC2A1 is linked to hyperinsulinemic hypoglycemia, familial, 4.