Xanthinuria type I (caused by XDH gene defects, OMIM#278300) or type II (MOCOS gene defects, OMIM#603592) as well as purine nucleoside phosphorylase deficiency (PNP gene defects, OMIM#613179) are forms of hypouricemia resulting from impaired synthesis pathways. The gene discussed is PNP; the disease is hyperinsulinemic hypoglycemia, familial, 4.