Primary hypouricemia may arise from xanthine oxidase deficiency or a defect in the renal tubular transport of UA, with decreased UA reabsorption leading to very low serum levels of this molecule (≤2.0 mg/dL) (Martín & García-Nieto, 2011; Iso & Kurabayashi, 2015). The gene discussed is XDH; the disease is hyperinsulinemic hypoglycemia, familial, 4.