1. P. K. Maini is at the Center for Mathematical Biology, University of Oxford, Oxford OX1 3LB, UK, and the Oxford Center for Integrative Systems Biology, University of Oxford, Oxford OX1 3QU, UK.
2. R. E. Baker is at the Center for Mathematical Biology, University of Oxford, Oxford OX1 3LB, UK.
3. C.-M. Chuong is in the Department of Pathology, University of Southern California, Los Angeles, CA 90033, USA.