PROTOCOL 670 | VOL.7 NO.4 | 2012 | NATURE PROTOCOLS INTRODUCTION Current high-throughput techniques such as yeast two-hybrid screens for protein interaction partners produce great volumes of experimental data that can be integrated and explored to gain insight into biological processes performed by interacting molecules 1-6. Furthermore, structural biologists study the interactions of residues in protein structures to understand complex protein structure-function relationships 7-11. Commonly, large-scale interaction data are represented as networks and initially analyzed by graph-theoretic methods to characterize the topological network structure and its global and local interaction properties 12-18. A number of software tools are available for the visual exploration and computational analysis of networks 19-21. General software libraries for network analysis are the Java framework JUNG 22 , the C + + library LEDA 23 , the Python package NetworkX 24 and R packages such as igraph 25 , statnet 26 , sna 27 , tnet 28 and QuACN 29. However, they cannot be applied by users without programming expertise. In contrast, sophisticated free software platforms such as Pajek 30 , VisANT 31 , ONDEX 32 and BIANA 33 provide graphical user interfaces for the analysis of biological networks. In addition, the free and stand-alone Cytoscape platform has gained considerable interest in recent years because of its open-source code development and its rapidly growing community of users and developers 34,35. In particular, its functionality is easily extendable by additional plug-ins that support specific network analysis tasks. For example, software protocols are already available for cluster analysis with the TransClust and ClusterExplorer plug-ins 36 , as well as for the integration of physical and genetic interactions into module maps with the PanGIA plug-in 37. Here we demonstrate how to apply two of our Cytoscape plug-ins, NetworkAnalyzer 38 and RINalyzer 39 , for the standard and advanced analysis of network topologies. NetworkAnalyzer performs a comprehensive analysis of network topologies without requiring advanced knowledge in graph theory or programming expertise 38. In particular, it supports the characterization of molecular networks in terms of scale-free and small-world properties, modularity and hierarchical structure 5,12,13,40 , the identification of important network nodes and edges based on topological parameters 11,41-43 , and the comparison of networks with regard to their topology 44-47. Since its initial release in 2007, NetworkAnalyzer has been extended by additional features and topological parameters and is widely used in academia and industry as indicated by thousands of software downloads. Recently, this plug-in became an integral part of each standard installation of Cytoscape, and its source code was published under the GNU Lesser General Public License. Basically, NetworkAnalyzer efficiently computes a number of topological parameters, including node degree, clustering and topological coefficient, charact...