C6 cells transduced by the pCLp16 virus did not form tumors after stereotaxic implantation in rat brains. 1 × 105 C6 cells were infected with the pCL parental virus 24h prior to bilateral stereotaxic implantation in the caudate putamen of Wistar rats. A. Typical tumor formed, 4× objective lens, B. same tumor, 10× objective lens. 1 × 105 C6 cells were infected with the pCLp16 virus 24h prior to implantation in the same manner as the controls. C. Typical result, 4× objective lens, D. same sample, 10× objective lens. Tumors were allowed to form for 45–60 days post injection. Paraffin embedded sections were cut in 5 μm sections and stained with hemotoxilin and eosin.