Pools of rat C6 glioma cells transfected with pUB were mixed with cells stably transfected with pUB:HSVTK (□), pUB:mgmk/HSVTK (■), pUB:30 (∆) and pUB:mgmk/30 (▲) at a ratio of 95:5 (vector:suicide genes, respectively). Mixed cells were used to seed tumors in nude mice (n=5 for each group). When tumor size reached 3-4mm (day 0), (A) GCV (1mg/kg) or (B) PBS was administered twice a day for 8 days. During this period, tumor growth was measured every other day. Tumor volume was calculated using the formula 4/3π ((Width × Length × Height)/2), plotted and analyzed for statistical significance using Student's t-test. Asterisks denote statistical significance (P ≤ 0.05) in tumor sizes between mice harboring 5% of MGMK/30-expressing tumor cells and harboring either 5% of HSVTK-, mutant 30-, or MGMK/HSVTK-expressing tumor cells in the presence of 1mg/kg GCV.