Temporal parameters of the G
to S transition in resting mammalian fibroblasts stimulated to proliferate by serum. NIH3T3 cells, made quiescient by serum starvation, were stimulated with 10% serum. For restriction point determination (panel A) cells were serum-starved again after different time periods and the fraction of BrdU-positive cells determined 14 hours after serum stimulation. Values of BrdU-positive cells at the end of the 14 hours (total time) are plotted as a function of the simulation time in the presence of serum. Fraction of BrdU-positive cells (B) was determined at different time point after serum stimulation. Experimental points are shown by colored symbols. Restriction point overcoming (panel A) or S phase state (panel B) are shown as on/off binary states by the dashed lines. Data for the population assuming a half-life of growth factor/cell interaction of 2 hours are shown as solid black lines fitting experimental data.