1. Department of Mathematics, Politecnico Torino, Corso Duca degli Abruzzi 24, 10129 Torino, Italy
2. Centre for Mathematical Biology, Mathematical Institute, 24-29 St.Giles', Oxford OX1 3LB, UK
3. Oxford Centre for Integrative Systems Biology, Department of Biochemistry, South Parks Rd, Oxford OX1 3QU, UK