In vitro fusion assays. RK13 cells were cotransfected with expression plasmids for PrV gB-008, gD, gL, selected gH variants, and EGFP. After 36 h, the areas of induced syncytia were determined by fluorescence microscopy. Mean sizes of 50 syncytia per mutant were calculated, and the percentages of mean wild-type sizes were plotted. Standard deviations (vertical lines) and significant differences from wild-type fusion activity are indicated (*, P < 0.05; **, P < 0.01; ***, P < 0.001).