Because patients with urinary hypouricemia tend to show very low serum uric acid levels (lower than 2 mg/dl), candidate causes in this range of serum uric acid levels may be the decreased production of uric acid caused by certain conditions, such as xanthinuria, purine nucleoside phosphorylase deficiency, xanthine oxidase inhibitors, and liver diseases. The gene discussed is XDH; the disease is hyperinsulinemic hypoglycemia, familial, 4.