1. Department of Cell Biology and Molecular Genetics, Institute for Physical Science and Technology, University of Maryland, College Park, Maryland 20742;
2. Howard Hughes Medical Institute, Department of Biomedical Engineering and Center of Synthetic Biology, Boston University, Boston, Massachusetts 02215
3. Wyss Institute, Harvard University, Boston, Massachusetts 02115
4. Boston University School of Medicine, Boston, Massachusetts 02118
5. Department of Biology, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139;