APRT and glioblastoma: To test this hypothesis, we depleted the five known enzymes that are responsible for these reactions, hypoxanthine phosphoribosyl transferase 1 (HPRT1), adenine phosphoribosyl transferase (APRT), orotate phosphoribosyl transferase (OPRT/UMPS), uracil phosphoribosyl transferase (UPRT), and quinolinate phosphoribosyl transferase (QPRT), in U87 and MES28 GBM cells.