Study participants were recruited from a group of patients with confirmed APRT deficiency enrolled in the National Institutes of Health supported APRT Deficiency Registry of the Rare Kidney Stone Consortium (RKSC, http://www.rarekidneystones.org/). Confirmation of APRT deficiency was based upon the determination of known biallelic pathogenic APRT mutations or absent APRT enzyme activity. Participants were eligible for inclusion if they a) were currently receiving allopurinol therapy (the currently recommended treatment for patients with APRT deficiency); b) were willing to interrupt their allopurinol treatment for a total of 3 weeks as outlined below and c) were at least 18 years of age. There were no other exclusion criteria if the above inclusions criteria were met.