The stability and fragility of four species' protein-protein interaction networks (PINs) are studied by investigating their robustness, i.e. their topological parameters retain a similar system behavior with respect to four different types of perturbations. Four types of perturbations are considered; that is (i) network nodes are randomly removed (failure), (ii) the most connected node is successively removed (attack), (iii) interaction edges are rewired randomly, and (iv) edges are randomly deleted. At most 50% of network nodes or edges are deleted or rewired. It is demonstrated that PINs are quite robust with respect to failure, attack, random rewiring and edge deletion, that is the average diameters for perturbed networks differ from the unperturbed cases have a difference less than 13%, which is relative small in comparison with WWW and the Internet results. These results suggest that PINs' network topologies are robust with respect to perturbations.
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