Schematic illustration of the protein criticality characterization method. System components may exert different impacts when perturbed. Color gradient corresponds to amplitudes, with empty color representing zero effect. (A) Circumstance that the impact is within a limited area near the epicenter (i.e. the perturbed component, marked as "1"). (B) Circumstance that the impact spreads throughout the network, with many distant spots severely affected (marked as "2"). (C) A stable equilibrium (X
) attracts its neighboring trajectories. (D) An unstable equilibrium repels the trajectories. Stable equilibriums may collapse due to perturbations on critical system components. Components whose perturbations exert large impacts, or cause qualitative changes in system dynamics (upon perturbation) are regarded as critical.