Living cells were analyzed using BioStation ID system (GE Healthcare). Confluent MDCK cells were infected with the NAdeficient influenza virus at the multiplicity of infection (MOI) of 0.0001 in the presence or absence of 1 mg/ml TPCK-trypsin. At 24 hours post infection, culture dishes containing infected cells were set into the chamber of BioStaion ID system, which was maintained at 37uC under 5% CO 2 and 95% humidity. Then, images were acquired during next 24 hours at interval with 1 hour. The excitation wavelength was controlled by a manual filter wheel equipped with filters suitable for enhanced green fluorescence protein (EGFP).