The prognosis of these diseases can be greatly improved by starting specific medication in the early neonatal period [1,2,3,4,5,6,7], as observed through the management of two siblings affected by methylenetetrahydrofolate reductase (MTHFR) deficiency, which is described below. This evidence concerns the gene MTHFR and hyperinsulinemic hypoglycemia, familial, 4.