In 2012, Peretz et al. reported novel urinary biomarkers that can distinguish between xanthinuria type I [isolated xanthine dehydrogenase (XDH) deficiency] and type II [combined deficiency of XDH and aldehyde oxidase (AO)] (Peretz et al. 2012). Here, XDH is linked to hyperinsulinemic hypoglycemia, familial, 4.