Yield of rescued PR8 virus obtained using different plasmid proportions. The total amount of plasmid transfected per well was set to 0.8 μg in a 24-well plate format for the whole experiment. A proportion of this amount was dedicated to the viral rescue plasmids, while the rest of the amount was dedicated either to the Env-expressing plasmid (black symbols) or the B1 inert plasmid (grey symbols), referred to as "other plasmids" in the chart. Each point shows the average yield from three replicate transfections and error bars indicate standard deviation. Negative results (0 p.f.u./mL) are plotted as 1 p.f.u./mL on the logarithmic scale. **, p < 0.01 at the 0.2 μg level.