Xanthinuria is characterized by very low concentration of uric acid in blood and urine and high concentration of urinary xanthine. Xanthinuria is classified into 2 groups, types I (XAN1) and II (XAN2). Patients with XAN1 lack only xanthine dehydrogenase/xanthine oxidase (XDH/XO) activity, while patients with type II lack both XDH/XO and aldehyde oxidase activities. It has been suggested that the mutation in the molybdenum cofactor sulfurase gene is responsible for XAN2.