 By conducting correlation-based studies of how multiple genes and proteins are associated with each other, and how these are connected to specific biological processes, systems biology research has revealed common patterns in biological networks such as scale-free network architectures and multi-level hierarchies (Ravasz et al. 2002). Systems biologists often distinguish between two major classes of networks based on their connectivity distribution (how components, or nodes, in the network are connected to other nodes). Exponential and scale-free network structures are illustrated on  Figure 1  (for details on the differences, see Albert et al. 2000). An exponential network is largely homogeneous with approximately the same number of links per node, and it is unlikely to find nodes with many links. In contrast, scale-free networks are inhomogeneous. Most nodes have only a few links but some nodes (called hubs) have a large number of  connections.[3]