HPRT1 and Parkinson disease: Given the role of HPRT1 in hypoxanthine conversion, the decreased HPRT1 expression in PD may contribute to the increased abundances of xanthines by inhibiting the processing of hypoxanthine and its precursor inosine through this branch of the conversion pathway (see Fig. 6, right side), and thereby indirectly increase the levels of the alternative hypoxanthine conversion product xanthine (see Fig. 6, left side).