In early onset severe forms clinical sequelae associated with MTHFR deficiency include microcephaly, hydrocephalus, seizures, hypotonia and global development delay, whereas in the adolescence onset form mental retardation and progressive encephalopathy are present (Arn et al 1998; Bishop et al 2008; Schiff et al 2011). The gene discussed is MTHFR; the disease is hyperinsulinemic hypoglycemia, familial, 4.