Figure 2.

A toy example of a biological network model. This network has undirected edges which are generally used to display co-expression or binding relationships. The connectivities described in the text are illustrated here. The red node is a hub, intermediate nodes are shown in black, and peripheral nodes are shown in teal. Hubs play an important role in maintaining network integrity and in information propagation. This makes targeting hubs complicated. Peripheral nodes tend to have limited influence on signal transduction because of their limited connectivity. Even if a peripheral node sends a message to a hub, that message is tempered by all of the other signals the hub is receiving which diminishes the chances that that message will be propagated. Intermediate nodes strike a fine balance between hubs and peripheral nodes making promising drug targets. In addition to hubs, intermediate, and peripheral nodes, a bridging nodes is shown in orange and a bottleneck node is shown in blue. Bridging nodes connect subnetworks that would otherwise be isolated. Bridging nodes have low connectivity but high centrality and they tend to be independently regulated. As such they may characterize disease conditions making them good drug targets. Bottleneck nodes are positioned in paths that are well-traveled. They generally have high connectivity and high centrality. They could be classified as hubs; however, they differ from hubs in that they are independently regulated. These properties make bottleneck nodes promising drug targets as well.