Mutation of human Moco sulfurase gene is responsible for classical type II xanthinuria, due to the failure of the mechanism responsible for inserting the essential sulfur atom into the active center of hAOX1 and XO.16 The presence of nsSNPs in hAOX1 leading to loss of the Moco insertion may also be related with the presence of type II xanthinuria disease conditions and should be further investigated. The gene discussed is XDH; the disease is hereditary xanthinuria.