Among 24 reviewed adolescent/adult onset patients with MTHFR deficiency in the literature, clinical manifestations included gait disorder (96%, from motor central or peripheral origin), cognitive decline (74%), epileptic syndromes (50%), encephalopathy (30%), psychotic symptoms (17%), and thrombotic events (21%). The gene discussed is MTHFR; the disease is hyperinsulinemic hypoglycemia, familial, 4.