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Modularity, motifs and other structural features
The notion of clustering in a network model corresponds intuitively with the idea of functional modules in regulatory networks [90]. It has been suggested that functional modules are an important level at which to consider biological organisation for a number of reasons [54]. Modules involve a small fraction of network components working together in a relatively autonomous fashion and, as such, they represent a possible route to reducing the complexity of regulatory networks. Furthermore, empirical evidence suggests that such independent control substructures may actually exist [138]. It has also been suggested that functional modules may be one of the units on which evolution operates [54].
Several statistical properties of networks have been identified that provide a potential means if identifying and measuring modularity in systems. Ravasz et. al. describe a hierarchically structured network, in which small functional modules combine in a hierarchical fashion into progressively larger units [98]. They acknowledge however, that more work is still required to be able to accurately and usefully characterise modularity in network structure.
A second approach to the investigation of modularity in networks is the identification of ``regulatory motifs'', small, repeated patterns of interaction that occur with greater regularity than would be expected in a random network [86] (see Figure 13).
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Next: Further reading Up: Network models Previous: Small world and scale-free Nic Geard 2004-05-06