Hereditary xanthinuria is an autosomal recessive genetic disorder that results in excessive xanthine (a metabolic byproduct) in the urine. This increases the risk for formation of xanthine bladder or kidney stones and can cause significant kidney disease. Hereditary xanthinuria is a result of mutations in either xanthine dehydrogenase (XDH, type 1 xanthinuria) or molybdenum cofactor sulfurase (MOCOS, type 2 xanthinuria). Xanthinuria can also occur from non-genetic factors such as exposure to drugs that inhibit XDH (e.g. allopurinol). This is termed iatrogenic xanthinuria. We have identified the causative mutation in Toy Manchester Terriers (Type 2a). Genetic test results can be used to help guide medical management of affected dogs, identify dogs at risk even before they form stones, and to inform breeding decisions. We currently estimate that ~20% of Toy Manchester Terriers/English Toy Terriers are carriers of the mutation. Carriers do not develop the disease but can produce affected puppies if bred to another carrier. However, this does not mean that carriers need to be taken out of the breeding pool, as that could rapidly reduce breed diversity. As long as carriers are only bred to clear dogs, there is no risk of producing affected puppies... Xanthinuria leads to the development of urinary stones. This causes irritation that may manifest as straining to urinate, frequent urination, urgency with urination, blood in the urine, or life-threatening urinary obstructions. Microscopic crystals can also accumulate in the kidney and cause kidney disease. Patients with xanthinuria can present at virtually any age from a few months onwards. Though many patients have serious consequences, some remain asymptomatic. Males appear to be more likely to form stones than females..." [From University of Minnesota, College of Veterinary Medicine]