Multicycle virus growth kinetics were performed in MDCK cells (12-well plate format, 5 X 10s cells/well) infected with the indi¬cated viruses (triplicates) at multiplicity of infection (MOI) of 0.001. Virus titers in the tissue culture supernatants were determined by immunofocus assay (fluorescent focus-forming units, FFU/ml) (Nogales et al„ 2014b) using mouse monoclonal antibodies (MAbs) against IAV (HT103) (O’Neill et al„ 1998) or IBV (B017; Abeam) NP, and a fluorescein isothiocyanate (FITC)-conjugated anti-mouse sec¬ondary antibody (Dako). Mean value and standard deviation (SD) were calculated using Microsoft Excel software.