Foreign gene expression cassette removal derivatives kill tumor cells in culture more efficiently than their parental virus GLV-1h68. GI-101A cells were infected with each virus strain at an MOI of 0.01. Cell viability was determined using CytoTox 96 Non-Radioactive Cytotoxicity Assay kit (Promega, Madison, WI, USA). Cell survival before infection was set at 100%. The values are the mean of quadruplicate samples, and the bars indicate SD. The data represents two independent experiments. Statistical analysis was performed using two-way ANOVA. *, **, and *** indicate P < 0.05, 0.01, and 0.001, respectively, when compared with the GLV-1h68 group.