Subcellular localization of NS1 in infected cells. Human A549 alveolar epithelial cells, mouse embryonic fibroblasts (MEF), duck embryonic fibroblasts (DEF) and chicken embryonic fibroblasts (CEF) were infected at a MOI = 3 with different strains of influenza virus. The cells were fixed, stained with a rabbit anti-NS1 polyclonal antibody and a secondary FITC-labelled anti-rabbit antibody and imaged with a confocal microscope. Shown are representative pictures obtained from cells fixed 4 hpi. The C-terminal amino acid sequence of NS1 is indicated under the name of each viral strain. The basic amino acids identified by Melen et al. as defining the mammalian NoLS are present in the NS1 of Udorn and are underlined.