HPRT1 and neoplasm: Therefore, its toxicity is exerted in tumour cells, where ePNP GDEPT is directed: in fact, 6-methylpurine (MeP), the product of MeP-dR phosphorolysis, is activated to the cytotoxic compound by cellular HPRT and/or APRT activities (see Section 6) and then incorporated both in RNA and DNA.