1. Department of Biology, Massachusetts Institute of Technology, Cambridge, MA 02139, USA.
2. Program in Molecular Biology, Cell Biology, and Biochemistry, Boston University, Boston, MA 02215, USA.
3. Howard Hughes Medical Institute, Department of Biomedical Engineering, and Center for BioDynamics, Boston University, Boston, MA 02215, USA.
4. Wyss Institute for Biologically Inspired Engineering, Harvard University, Boston, MA 02118, USA.