1. Department of Mathematical Sciences, Clemson University, Clemson, South Carolina.
2. Institut Curie, 26 rue d'Ulm, Paris, F-75248 France.
3. INSERM, 11900, Paris, F-75248 France.
4. Hines Paris Tech, Fontainebleau, F-77300 France.
5. Mathematics and Statistics Department, Radford University, Radford, Virginia.
6. Virginia Bioinformatics Institute, Virginia Tech, Blacksburg, Virginia.