HA surface expression of A/HK/218847/06 (H1N1) and A/HK/218449/06 (H3N2) influenza viruses. MDCK cells were infected with either virus at m.o.i. = 1. The percentages of HA+ cells were measured by FACS using a specific anti-HA mAb. In the histograms, the gray area represents the negative control; the percentage of HA+ cells at 6 h p.i. (solid lines) and 8 h p.i. (dashed lines) are indicated. The bar graph shows the mean data from three independent experiments.