1. School of Mathematics, University of Leeds, Leeds LS2 9JT, UK.
2. Department of Biosystems Science and Engineering, ETH Zürich, Mattenstrasse 26, 4058 Basel, Switzerland and Swiss Institute of Bioinformatics, Quartier Sorge, Batiment Amphipole, 1015 Lausanne, Switzerland.
3. Corresponding author. Theoretical Biology and Biophysics, Theoretical Division, Los Alamos National Laboratory, Los Alamos 87545, NM, USA.