Parental Vero si1 and si2 cells were infected at an MOI of 0.1 with recombinant viruses with the addition of the fusion inhibitor peptide z-fFG (10 μg/ml) to prevent syncytium formation. At indicated times after infection, cells were trypsinized, stained with an anti-F monoclonal antibody (Y503) (73) for 30 min at 4°C, centrifuged, washed, stained with the Alexa Fluor 647 anti-mouse IgG antibody (1:700; Invitrogen) for 30 min at 4°C, centrifuged, washed, fixed in 1% paraformaldehyde for 15 min at room temperature, centrifuged, and resuspended in PBS. Fluorescence signals were quantified with Accuri C6 (BD Biosciences).