A virulent NDV strain (F48E9) selectively infects proliferating cells. Immunofluorescence analysis and quantification of NDV and BrdU staining in HeLa cells infected with the virulent NDV strain F48E9 (0.01 MOI) and labeled with BrdU (5 μM) at 12 hpi. Scale bar, 20 μm. Data are presented as the mean ± SD of triplicate samples from three independent experiments. **P < 0.01 by t-test.