The final purified virus preparation was concentrated by ,5fold, and adsorbed for 1 min on a 300-mesh copper EM grid coated with a carbon film, followed by washing with deionized water for 5 s and staining with 1% uranyl acetate for 1 min. The grid was air dried, and was inspected on a 200 kV Tecnai F20 G2 transmission electron microscope equipped with a field emission gun.