(a) Log-log plot of C(k) against k from metabolic and regulatory networks in E. coli. k is the outgoing degree of each node. C(k) is defined as the average clustering coefficient (C) of all the nodes of a given k. The dashed lines correspond to C(k) ~ k-1. Colors: (orange) metabolic network, (blue) regulatory network. (b-d): C(k) against k from the regulatory networks in S. cerevisiae, H. sapiens and P. falciparum.