Figure 11 shows the bifurcation diagram for the regulatory module of the mammalian G1/S transition. The bifurcation parameter is the strength of mitogenic stimulation, Fm . The quantity of primary interest is the level of E2F1, which is a transcription factor targeting genes underlying cell cycle progression. For sufficiently low level of Fm, the level of E2F1 stays at a constant low value. At a critical value of Fm a transcritical point is reached, above which the level of E2F1 starts to increase with Fm . As the level of Fm increases further, a saddle-node (SN1) is reached and the level of E2F1 undergoes a discontinuous jump upwards, thereby triggering the S phase of the cell cycle.