Stochastic simulation of the model of intrinsic apoptosis pathway under intrinsic noise. (A) Stochastic model under intrinsic noise is simulated using the modified Gillespie stochastic simulation algorithm as described in the Methods. Simulation results of 150 time trajectories of CEA with [c9p, c3p] =[ 104,10], representing the stochastic response under intrinsic noise in 150 cells, are plotted at different number of CC molecules. It is evident that under intrinsic noise perturbation the time trajectories of CEA present cell-to-cell variability when the number of CC molecules is below 1000. (B) Histograms of the steady-state CEA values of the 150 cells simulated with [c9p, c3p] =[ 104,10] and at varying input CC level. They exhibit bimodal distribution around two steady states, one at CEA = 960 (number of molecules) and the other at CEA = 55 (number of molecules). The bimodal distribution implies that the stochastic model under intrinsic noise is bistable.