A. MEF, PAK1−/− MEF and PAK2−/− MEF at 24hpi with wtPRV or US3null PRV, stained for active caspase-3 (green) and nuclei (blue) B. Percentage of active caspase-3 positive MEF, PAK1−/− MEF and PAK2−/− MEF that were either mock infected or infected with wt PRV or US3nullPRV (24 hpi). Three independent replicates of each experiment were performed, data shown are means and standard deviations, and statistical analysis was performed using the SPSS software. Means were compared with an analysis of variance and a least significant difference post hoc test for a multiple comparison of means (α=0.05). Different letters indicate significant differences at the 0.05 level.