R. Milo1, S. Shen-Orr1, S. Itzkovitz1, N. Kashtan1, D. Chklovskii2, U. Alon1,*1 Departments of Physics of Complex Systems and Molecular Cell Biology, Weizmann Institute of Science, Rehovot, Israel 76100. 2 Cold Spring Harbor Laboratory, Cold Spring Harbor, NY 11724, USA.