Evidence for a G
restriction point. A. Cell cycle re-entry from G2 requires 6 hours of mitogen-stimulation. Serum-starved cells were stimulated by the addition of serum-containing medium. Subsequently, at the indicated times medium was replaced with serum-free medium containing Taxol for the last 9 hours. At 21 hours cells were harvested and fixed in 70% ethanol and mitotic entry was determined by MPM2 FACS staining. Error bars indicate the standard deviation for two experiments. B. Cell cycle re-entry from G2 requires protein synthesis. Serum-starved cells were serum-stimulated in the absence or presence of 50 μg/ml cycloheximide (CHX). Cells were fixed at 21 hours and mitotic entry was determined by MPM2 FACS staining. The level of MPM2 positivity in serum-stimulated cells at 21 hours is set at 100%.