D54 tumor cells transduced with either EcPNP (D54/EcPNP, filled circles) or TvPNP (D54/TvPNP, open circles) were incubated in triplicate with 10 μM [3H]F-araA. A portion of the culture medium was removed at various time points following addition, and analyzed for F-araA or F-Ade by reverse phase HPLC (3). The amount of F-araA converted to F-Ade and activated to F-Ade nucleotides intracellularly was determined by measuring loss of radioactivity from the medium. At the end of each experiment, cells were harvested using Tryple Express (Gibco, Grand Island NY) and cell numbers determined using a Coulter Counter. Each value represents the mean and standard deviation of three measurements.