Figure 2. Steady states of (A) p53 and (B) active MDM2 (MDM2a) as a function of the intensity of ionizing radiation, ρ.

Local stability of these states is indicated as either stable (black curve: stable nodes; dark gray curve: stable spirals) or unstable (broken black curve: saddle points; light gray curve: limit cycles). Limit cycle oscillations whose amplitudes are shown by the dots arise from the unstable steady states (light gray curve) in the lower (A) or upper (B) branch of the steady state curves.