Abstract: The present invention provides a method of killing replicating or non-replicating, transfected or transduced mammalian cells and bystander cells, comprising: (a) transfecting or transducing mammalian cells with a nucleic acid encoding a non-human purine cleavage enzyme; and (b) contacting the transfected or transduced cells with an effective amount of a substrate for the purine cleavage enzyme, wherein the substrate is non-toxic to mammalian cells and is cleaved by the enzyme to yield a purine toxic to the targeted mammalian cells and bystander cells, to kill the mammalian cells expressing the enzyme and the bystander cells. Further provided is a vector comprising a DNA sequence coding for a non-human purine nucleoside phosphorylase protein and the vector is capable of replication and/or expression in a host which comprises, in operable linkage: a) optionally, an origin of replication; b) a promoter; and c) a DNA sequence coding for said protein.