Nuclear localization of M protein promotes the cytopathogenicity of NDV. The subcellular localizations of M protein in DF-1 cells infected with rSS1GFP or rSS1GFP-M/NLSm were examined by immunofluorescence staining (A) and Western blotting (B). The relative levels of the M protein were compared with the control Lamin B1 or Tubulin expression. N represents the nucleus and C represents the cytoplasm. C The CPE and GFP expression were observed in rSS1GFP- or rSS1GFP-M/NLSm-infected DF-1 cells at the indicated time points. Original magnification was 1 × 200. D The syncytium formation in plasmids-transfected DF-1 cells. The number of syncytia (cells containing more than three nuclei) was counted in 10 random areas of the well (mean ± SD).