Type I (OMIM 278300) is caused by a human XDH/XO deficiency, an enzyme which catalyzes the hypoxanthine oxidation to xanthine, and the formation of uric acid from xanthine, due to mutations in the XDH/XO gene localized on chromosome 2p23 [2], [3], [4], [5], [6] (Figure 1). This evidence concerns the gene XDH and hyperinsulinemic hypoglycemia, familial, 4.