PNP and hepatocellular carcinoma: As shown in a previous study, 0.5–1 μg/ml fludarabine killed 100% of PNP-expressing cells, with no detectable toxicity toward control cells, and expression of PNP in as few as 10% of HCC cells induced the efficient killing of most bystander cells because PNP metabolites are able to diffuse across cell membranes[28].