Xanthinuria type II (XAN2) is an autosomal recessive inborn error of metabolism resulting from a defect in the synthesis of the molybdenum cofactor, which is necessary for the 2 enzymes that degrade xanthine: XDH (607633) and AOX1 (602841). Most individuals with type II xanthinuria are asymptomatic, but some develop urinary tract calculi, acute renal failure, or myositis due to tissue deposition of xanthine. Laboratory studies show increased serum and urinary hypoxanthine and xanthine and decreased serum and urinary uric acid (summary by Ichida et al., 2001). Two clinically similar but distinct forms of xanthinuria are recognized. In type I xanthinuria (XAN1; 278300), there is an isolated deficiency of xanthine dehydrogenase resulting from mutation in the XDH gene; in type II, there is a dual deficiency of xanthine dehydrogenase and aldehyde oxidase. Type I patients can metabolize allopurinol, whereas type II patients cannot (Simmonds et al., 1995).