In a cell or microorganism the processes that generate mass, energy, information transfer, and cell fate specification are seamlessly integrated through a complex network of various cellular constituents and reactions 1. However, despite the key role these networks play in sustaining various cellular functions, their large-scale structure is essentially unknown. Here we present the first systematic comparative mathematical analysis of the metabolic networks of 43 organisms representing all three domains of life. We show that, despite significant variances in their individual constituents and pathways, these metabolic networks display the same topologic scaling properties demonstrating striking similarities to the inherent organization of complex non-biological systems 2. This suggests that the metabolic organization is not only identical for all living organisms, but complies with the design principles of robust and errortolerant scale-free networks 2-5, and may represent a common blueprint for the large-scale organization of interactions among all cellular constituents. An important goal in biology is to uncover the fundamental design principles that provide the common underlying structure and function in all cells and microorganisms 6-13. For example, it is increasingly appreciated that the robustness of various cellular processes is rooted in the dynami
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