Signal generation is both dependent on presence of tumors and gusA -encoding rVACV. A) Confirmation of successful tumor colonization after low-dose injection of 1 × 105 pfu GLV-1h68. In the left panel time dependent 4-MU fluorescence is shown for individual sera of rVACV injected tumor bearing mice. The sera of two mice (#482 and #486) resulted in generation of significant amounts of 4-MU. Viral titer analysis of tumors from the same mice revealed that only those mice (#482 and #486) had virus colonized tumors which also resulted in generation of 4-MU. B) Tumor bearing (black) and non-tumor bearing (grey) control male (left panel) and female (right panel) mice were injected with 5 × 106 pfu GLV-1h68 into the retro-orbital sinus vein. Analysis of sera revealed conversion of the fluorigenic compounds FDGlcU (solid lines) and 4-MUG (broken lines) in all mice. Significant differences (p < 0.05) between tumor bearing and control mice was observed after 9 dpi. Results show average plus standard deviation of fluorescein and 4-MU fluorescence.