Comparison of (A) bioluminescence signals detected by the CCD camera and (B) tumor volumes in GCV-treated mice (n= 7, M1 – M7) harboring HSV-TK-Luc tagged U87MG glioma xenografts. GCV (60 mg/kg per day) was administered for 14 days starting at day 7 post tumor implantation. All mice were imaged at least on days 7, 15, 22, 29, and 56 post tumor implantation. BLI signals and tumor volumes at the beginning of therapy were set as 100%. Identical symbols in both graphs correspond to identical animals.