Stimulation of cell cycle progression in Go/G1-arrested cells by IL-2–displaying retroviruses. F7 cells, arrested in Go/G1 by an overnight IL-2 deprivation, were stimulated for different time periods with 2 ng/mL of recombinant IL-2 (rIL-2), viral supernatants containing retroviruses coated with amphotropic envelopes (A), or viral supernatants containing retroviruses coated with IL-2 chimeric envelopes (IL2-SU). The percentage of cells in the different phases of the cell cycle (Go/G1; S; G2/M) was measured by FACS analysis after propidium iodide staining. At the onset of stimulation, 72.33% of F7 cells were in the Go/G1 phase. Experiments were performed in duplicate and the means are shown.