1. Program in Systems Biology, Department of Biochemistry and Molecular Pharmacology, University of Massachusetts Medical School (UMMS), 368 Plantation Street, Worcester, MA 01605–0103, USA.
2. Institute for Medical Engineering and Science, and Department of Physics, Massachusetts Institute of Technology (MIT), Cambridge, MA 02139, USA.
3. Program in Biophysics, Harvard University, Boston, MA 02115, USA.