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Virus morphology. H18N11-WT (A), H18N11-NA-F144C/T342A (B), and A/WSN/33 (C) viruses were purified, stained with 2% uranyl acetate, and air dried. Virions were observed and representative images were acquired with a Tecnai F20 transmission electron microscope (FEI Company, Eindhoven, Netherlands) at 200 kV. Bars, 50 nm.
