The world wide web and protein interaction networks have been found to be scale-free. Scale-free networks, in contrast to random networks, display a distribution of connectivity that decays with a power law tail. Most proteins in the cell, for instance play, a very specific role, interacting with only one or two other proteins. A few proteins, however, interact with a huge number of other proteins. Such scale-free networks have been successfully generated by growth models in which new nodes connect preferentially to the more highly connected nodes. Scale-free topology is therefore strongly dependent on a network's history or evolution.