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Growth kinetics of mutant viruses in vitro. A549 (A) or DF-1 (B) cells were infected with the indicated viruses at an MOI of 0.001. The supernatants of the infected cells were collected at the indicated time points. Virus titers were determined by use of a plaque assay in MDCK cells. The virus titers are means ± the SD (n = 3). * and **, P < 0.05 and P < 0.01, respectively, according to a one-way ANOVA, followed by a Dunnett's test.
