Modelling reveals that the network exhibits a hysteresis at the G1/S transition.

A Simulated time course showing the concentrations of CDKA, KRP, un-phosphorylated RBR (uRBR) and active E2FA. The transition is clearly visible at the point when E2FA is suddenly released from RBR inactivation. B Bifurcation diagram showing KRP steady states. At low levels of CDKA, there exists a unique stable steady state of high KRP activity. Eventually, this state is eliminated in a saddle node bifurcation, and the system switches to a new stable state of low KRP activity. Specifically, (36) subject to CDKA/B : CYCBT = MYB3R3T = MYB3R4T = SMRT = FBP = SCF = APC = 0. Parameter values are given in Methods section 5.5.