In type II xanthinuria (MIM 603592) variants in the human molybdenum cofactor sulfurase gene (HMCS) cause combined XDH and aldehyde oxidase (AO) deficiency.2 The clinical manifestations of the two types of xanthinuria are indistinguishable, the most prominent being formation of xanthine calculi in the urinary tract in about 40% of the affected individuals.3 Both XDH and AO are drug‐metabolizing enzymes4 and their absence may lead to drug induced toxicity. The gene discussed is MOCOS; the disease is hereditary xanthinuria.