Comparison of infectivity between HPV-16 pseudovirions carrying GFP prepared from intracellular assembly with HPV-16 pseudovirions carrying GFP generated from in vitro assembly. HPV-16 pseudovirions carrying GFP (HPV-16/GFP psV) were prepared using two different methods (intracellular assembly versus in vitro assembly) as described in the Materials and Methods. For the characterization of infectivity, 1 × 105 of 293TT cells were seeded to each well of a 24-well plate the day before the infection. HPV-16/GFP psV (3 × 109 psV particles containing ~30 ng of pcDNA3-GFP plasmids) prepared from intracellular assembly or in vitro assembly were diluted with 1 ml of medium, added to each well and incubated at 37°C. After 72 hours, the cells were harvested, and GFP-positive cells were analyzed by flow cytometry analysis using a FACSCalibur flow cytometer and analyzed with CellQuest software. Uninfected cells served as a negative control. A. Representative flow cytometry data. B. Graphical representation of the percentage of GFP+ cells. Data expressed as means ± standard deviations (SD) are representative of at least two different experiments.