Effect of adding an EGFR siRNA to gefitinib on cell growth inhibition. Lung cancer cells were treated with gefitinib 24 h before the EGFR siRNA transfection. After transfecting the siRNA, the cells were incubated for another 48 h, after which cellular growth was assessed by the MTS assay. (A) shows the combination effect of gefitinib and siRNA. *P < 0.05 and **P < 0.01 compared to controls with only siRNA transfection. "black triangle" P < 0.05 and "double black triangles" P < 0.01 compared to agents plus control siRNA. Median-effect plot (B), dose-effect curve (C), Fa-CI plot (D), and normalized isobologram (E) were computationally generated by Biosoft CalcuSyn program with EGFR siRNA and combined to gefitinib in H358 cells. Similar additive effect was found in all the combinations in all the five cell lines (data not shown).