Pro-apoptotic properties of the Vpr-eGFP mutants. Cells expressing either the wild type Vpr-eGFP or mutant Vpr-eGFP were selected by fluorescence cytometry, using the eGFP fluorescence. The percentage of cells undergoing apoptosis was assessed by the number of cells labeled with cells with Cy5 alone, or with both Cy5 and PI. Statistical analysis was achieved using the multi-factorial ANOVA test and the Dunnett analysis. Three independent measurements were performed for each assay.