Xanthinuria is classified into three types: type I, caused by XOR deficiency; type II, due to mutations affecting both XOR and aldehyde oxidase (AO) [67]; and type III, which involves mutations in molybdenum cofactor sulfurase (MOCOS), impacting XOR, AO, and sulphite oxidase (SO) activity (all molybdenum containing enzymes). Here, MOCOS is linked to xanthinuria type I.