1. Whitehead Institute for Biomedical Research, Nine Cambridge Center, Cambridge, Massachusetts 02142, FAS Center for Systems Biology, Harvard University, 7 Divinity Avenue, Cambridge, Massachusetts 02138, and Department of Molecular and Microbial Systems, Centre of Microbial and Plant Genetics, K.U. Leuven, Kasteelpark Arenberg 22, B-3001 Leuven (Heverlee), Belgium