MOCOS and xanthinuria: Classically, Type I xanthinuria is caused by a mutation in XDH/XO gene mapped to chromosome 2p23.1, whereas Type II xanthinuria is caused by deficits of XDH/OX and aldehyde oxidase (AO) caused by mutations in molybdenum cofactor sulfurase gene (MOCOS) localized on chromosome 18q12.2 [3–6].