Induction of cell cycle arrest by Vpr or C81 is dependent on the cell type. (A) HeLa, HepG2, and 293 T cells were transfected with pME18Neo encoding Flag-tagged wild-type Vpr or C81, or the control pME18Neo-Flag, together with the GFP expression plasmid pEGFP-N1. At 48 h post-transfection, cells were fixed and stained with PI. GFP-positive cells were analyzed by flow cytometry using CELL Quest for acquisition and ModFit LT for quantitative analysis of DNA content. Arrows indicate the peaks of cells at the G1 and G2/M phases. The G2/M:G1 ratios are indicated at the upper right in each graph. (B) HeLa, HepG2, and 293 T cells were transfected with pME18Neo encoding Flag-tagged wild-type Vpr or C81, or the control plasmid pME18Neo-Flag, together with pSV-β-galactosidase. At 48 h post-transfection, the cells were collected and lysed. Lysates with equal β-galactosidase activity were subjected to Western blot analysis using a cyclin B1 specific antibody.