Cytotoxic and bioluminescent activity in U87MG glioma cells transiently transfected with different amounts of HSV-TK-Luc plasmid. (A) Cytotoxic activity as measured by MTT assay and (B) luciferase activity as determined luminometrically in cell lysates. Results are displayed as counts per second (cps)/ μg protein. (C) Linear regression analysis of cytotoxic activity plotted against luciferase activity for the different amounts of plasmid DNA. Results from 3 independent experiments were used.