Bioluminescence imaging of nude mice carrying HSV-TK-Luc expressing U87MG gliomas. (A) Serial images from a mouse with 4 s.c. xenografts treated with GCV from day 7 to day 21 post tumor implantation and (B) saline treated control mouse, sacrificed on day 35 post tumor implantation due to massive tumor growth. The largest tumor in this mouse on the upper right back has already become necrotic. The day 35 image is also displayed at a broader grayscale range for better visualization of tumor localization. Note that control tumors also showed decreased light emission and a reduction in tumor size within the first 3 weeks post tumor implantation.