Higher virus-induced ERK activation leads to enhanced nuclear RNP export. MDCK cells were infected with H1N1 virus or H3N2 virus at m.o.i. = 1. RNPs were stained with anti-NP mAb and Alexa488-coupled goat anti-mouse Abs (green). The nucleus was counterstained with TO-PRO-3 (blue). Intracellular RNP localization was analyzed at indicated time points p.i. by multiphoton laser scanning microscopy. The merger of both channels is shown.