Simulation of ERBB receptor signaling in response to addition of epidermal growth factor and heregulin. Network-free stochastic simulation of the rule-based model illustrated in Figure2 with NFsim (see Methods). The simulation results demonstrate the capability of the rule-based modeling approach to represent site-specific phosphorylation kinetics. For each time course, phosphorylation level is normalized to the maximum level. Time courses are clustered by similarity (see Methods).