In the future, direct demonstration of defective RNR activity in patient’s neutrophils and correction of HU-mediated inhibition in ADA-deficient MPH and neutrophil with PEG-ADA supplementation will help support the hypothesis that RNR dysfunction is the cause for neutrophil abnormalities in ADA deficiency. The gene discussed is NR2E3; the disease is hyperinsulinemic hypoglycemia, familial, 4.