Induction of cell cycle arrest and cell apoptosis by 3b protein expression. p3b/EGFP-N1 plasmid was transfected into COS-7 cells, and the DNA contents of cells were measured by flow cytometry. EGFP expression positive and negative cells were gated with forward scatter (on the left row). The middle and right rows were the assay of cell cycle in p3b/EGFP-N1 negative and positive cells. In p3b/EGFP-N1 positive cells, sub-G1 phase was changed from 11.80% to 53.50%, 48.36% at 24 h or 36 h, 48 h separately. The proportion was decreased to 23.34 and 24.85% at 60 and 72 h respectively. However, in EGFP negative cells, the changes of sub-G1 phase were not obvious.