PNP and neoplasm: Here we show that fusion of the mutant human PNP to the anti-HER2/neu scFv via an α-helical linker (hDM-αH-C6.5 MH3B1) results in an active protein that can be targeted to tumor cells, where it can cleave a relatively non-toxic prodrug to a cytotoxic drug, resulting in the inhibition of tumor cell proliferation.