Abstract: Objective To observe the selective killing of MCF-7 human breast cancer cells and vascular endothelial ECV304 cells by adenovirus (Ad)-mediated double suicide gene driven by KDR promoter. Methods The plasmid pAdEasy-KDR- CDglyTK was transfected into 293 packaging cells for amplification of the infectious Ad, which were then used to infect KDR-producing ECV304 and MCF-7 cells and LS174T cells that did not produce KDR. The infected cells were treated with 5-FC and/or GCV at different doses to observe the cell-killing and bystander effects of AdKDR-CdglyTK. The cell cycle changes were also detected by flow cytometry. Results The Ad at the titer of 2.0×10¹²pfu/ml was obtained after the amplification, whose infection rates of the cells were similar, but could be increased gradually with the multiplicity of infection (MOI) till reaching 100% with the MOI of 200. The infected cells exhibited different sensitivities to the two prodrugs: ECV304 cells and MCF-7 cells infected with Ad-KDR-CDglyTK showed similar high sensitivity to the prodrugs (P=1.00), whereas the infected LS174T cells appeared to be less sensitive (P<0.001). The killing effect of CD/TK fusion gene on the target cells was much stranger than that of either suicide gene (P<0.001), but all exhbiting considerable bystander effect. In addition, the cell cycle of MCF-7 cells was arrested at G₁ phase. Conclusions CD/TK fusion gene system driven by KDR promoter can selectively kill KDR-expressing vascular endothelial cells and MCF-7 cells.