In the allopurinol loading test, oxipurinol is detected in serum and urine of type I xanthinuria patients after administration of allopurinol, as conversion of allopurinol to oxipurinol is catalyzed by XOR and AO, while oxipurinol is not detected in the case of type II xanthinuria. This evidence concerns the gene AOX1 and xanthinuria type I.