Scatter plots of cell cycle and serum variation. Colored scatter plots of model variables at time t = 50 versus serum variation were produced, and are here reported for CycD and E2F in panles A and B respectively. This graphs show CycD and E2F configurations at equilibrium, further emphasizing the role of serum variation. In panel A, CycD levels increase with serum concentration making evident the positive monotonic relationship between serum and CycD. In panel B, instead, the low E2F state is characterized by black-blue points (low serum), while the high state by red-green points (high serum), thus revealing that E2F distribution exhibits bimodal dependence on serum concentration.