An enzymatic defect in purine metabolism like hypoxanthine-guanine phosphoribosyltransferase (HGPRTase) leading to Lesch–Nyhan syndrome, super activity of phosphoribosyl pyrophosphate synthetase enzyme (PRPP synthetase), familial juvenile hypouricemic nephropathy may also contribute to a defective function of ABCG2 gene or protein [48-50]. The gene discussed is HPRT1; the disease is Lesch-Nyhan syndrome.