Characterisation of the phase space in the bistable switch model (1). A: Phase space diagram for the deterministic model of the bistable switch. Black lines are nullclines for the variables x and y in the deterministic switch model (1), with their intersections corresponding to equilibria of the switch. I and III are stable equilibrium points, II is an unstable one. Trajectories converge to either I or III, depending on the initial condition, as shown for the sample trajectories plotted as light blue lines. B: Schematic illustration of the configuration space for the Markov process (5) describing the cell transformation process. Circular nodes below the dashed line correspond to possible configurations (X; Y)T of the switch, and the arrows between the nodes correspond to transitions in the configuration due to reactions. The configurations above the dashed line are collapsed into the on state, which is assumed to be irreversible due to subsequent transformation processes.