Viral plaque assays. Figure 1 shows representative plaques for the WT and H275Y mutant strains of A/Brisbane/59/2007 (H1N1) viruses at each time point. The average plaque radius of each strain over time, calculated by averaging three independent experiments of three such wells at each time point, is shown in Figure 2 . The plaque growth is characterized by an initial delay where no growth is observed, followed by a period of linear increase of the plaque radius over time. After 60 h, the rate of plaque growth declines and the linear approximation is no longer valid. The growth attenuation could be due to a number of factors including a hardening of the overlay, a depletion of nutrients required for viral production and cell maintenance, and the widespread destruction of the cell monolayer leading to holes and irregularities disrupting and limiting further growth.