The d120 and R7041 virus infections induced apoptosis in U937 cells. The percentage of apoptotic cells was analyzed at 24h p.i. with Annexin V/propidium iodide double staining and flow cytometry. A) Percentage of apoptotic U937 cells. The level of apoptosis was significantly increased in d120-infected cells when compared to its parental virus HSV-1(KOS)-infected (1 and 5 moi) cells. Also, the level of apoptosis was significantly increased in R7041-infected cells when compared to its rescue virus R7306 (1 and 5 moi) and HSV-1(F)-infected (1 and 5 moi) cells. The bars represent the mean level of apoptotic cells ± SD from three independent experiments. The statistical significances of the differences in the level of apoptosis in comparison of the deletion viruses versus the parental viruses are marked as * (*:p<0.05, **:p<0.01) and in comparison with uninfected cells as # (#:p<0.05, ##:p<0.01). B) Representative flow cytometry dot plots showing Annexin V/propidium iodide double staining of uninfected, d120, its parental virus HSV-1(KOS), R7041, its rescue virus R7306 and HSV-1(F)-infected cells. The apoptotic cell population with positive staining for Annexin V and negative staining for propidium iodide is gated from the dot plots.