Proliferating cells enhance NDV replication. HeLa cells were cultured with different concentrations of FBS for 24 h and then stained with BrdU (A), after which cell cycle distribution (B) and apoptosis (C) were analyzed by flow cytometry. Cells were pretreated with different concentrations of serum for 24 h and then infected with NDV. At 24 hpi, viruses were quantified by qRT-PCR based on M gene expression (D) and titration with plaque assay (E). Data are presented as the mean ± SD of triplicate samples from four independent experiments. **P < 0.01 by t-test.