Adenoviral-mediated HSV1-sr39tk gene expression repetitively imaged in two mice. Each mouse was tail-vein injected with 1.0 × 109 pfu adenovirus in which the HSV1-sr39tk reporter gene is driven by the CMV promoter. Each mouse was imaged in a microPET system at the days indicated 1 h after tail-vein injection of 250 μCi [18F]-labeled penciclovir molecule (FHBG). The scale represents the accumulation of FHBG measured as percent injected dose per gram of tissue (%ID/g). (a) The Swiss Webster mouse shows a decrease in gene expression over time, and (b) the nude mouse shows persistence of gene expression in the liver. These differences are likely due to the immune system being fully competent in the Swiss Webster mouse.