The efficacy index of SFV and 5-FU combined treatment of 4 T1 tumor-bearing mice. The mice were treated with different doses of 5-FU and then i.t. inoculated with SFV/EnhLuc virus particles. The efficacy index reflects the correlation of tumor growth inhibition with the level of SFV expression in 5-FU treated mice. The calculations are described in the Methods section.