In vitro growth characterization of parental and mutant viruses. (a, b) For growth kinetics, NBL-6 cells were infected at MOI’s of 1 or 0.01. Infected cells and supernatants were collected and virus titers were determined at the indicated times pi. The data presented are means ± SD of triplicate measurements. The asterisk indicates a P < 0.05 for means when compared to the parental viruses. (c, d) Means ± SD of diameters of 50 plaques measured for each virus are shown. The plaque diameter of parental viruses was set to 100%. The asterisks indicate a P < 0.05 for means when compared to the parental viruses.