Classical xanthinuria is a rare, autosomal recessive metabolic disorder characterized by impaired xanthine dehydrogenase (XDH; EC 1.17.1.4) activity that results in a lack of uric acid (UA) formation and accumulation of its precursors, xanthine (X) and hypoxanthine (HX) [1,2]. The gene discussed is XDH; the disease is metabolic disease.