After 6 days of incubation, the culture medium was removed and 2 ml of 4 % paraformaldehyde was added (15 min, 5°C). The slides were washed with PBS and incubated overnight with 2 % goat serum at 5°C. Next, 100 μl of the primary antibody (mouse anti-MV-N monoclonal antibody [# MAB8906; Chemicon, Temecula, CA, USA]), diluted 1:200 in PBS-2 % goat serum-0.1 % saponin, was added to each well (60 min, room temperature). After washing twice with PBS, 100 μl of the secondary antibody (goat anti-mouse F(ab′) 2 -Cy3 [# LU1513615; Jackson ImmunoResearch, West Grove, PA, USA]) diluted to 1:500 in PBS-2 % goat serum-0.1 % saponin, was added to each well. After incubation (45 min, room temperature), coverslips were washed twice with PBS, rinsed with water, mounted on a slide using Vectashield DAPI mounting medium (Vector Laboratories, Peterborough, UK) and inspected under fluorescence microscope. Positivity or negativity was checked by the presence or absence of fluorescence, respectively.