1. Centre for Mathematical Medicine, School of Mathematical Sciences, University of Nottingham, Nottingham NG7 2RD, England
2. Bioinformatics Unit, Department of Computer Science, University College London, Gower Street, London WC1E 6BT, England
3. Centre for Mathematical Biology, Mathematical Institute, University of Oxford, 24-29 St Giles, Oxford OX1 SLB, England