2D Colony Formation Assay.

To perform the 2D colony formation assay, cells infected with viruses were first selected for 4 to 6 d and then were plated in a 6-well plate (2,000 cells per well) in 1.5 mL growth medium. Medium was changed every 3 d for all wells. After growing for 2 to 3 wk, cells were incubated with fixation buffer (5% acetic acid, 5% methanol) for 30 min and then stained with 0.5% crystal violet solution for 30 min. After discarding crystal violet solution, pure water was added to wash away residual crystal violet.