XDH and hereditary xanthinuria: Xanthinuria, which was first described by Dent and Philpot in 1954 [36], is another asymptomatic primary hypouricemia caused by a genetic defect in xanthine dehydrogenase (XDH/XOR) [37] for xanthinuria type 1 (XAN1; Mendelian Inheritance in Man (MIM) 278300) [38], or of the molybdenum cofactor sulfurase (MOCOS) for xanthinuria type 2 (XAN2; MIM 603592) [39].