1. Centre for Mathematical Medicine, School of Mathematical Sciences, University of NottinghamNottingham NG7 2RD, UK
2. Bioinformatics Group, Department of Computer Science, University College LondonGower Street, London WC1E 6BT, UK
3. Centre for Mathematical Biology, Mathematical Institute, University of Oxford24-29 St Giles', Oxford OX1 3LB, UK
4. Department of Animal and Plant Sciences, University of SheffieldWestern Bank, Sheffield S10 2TN, UK