1. Whitehead Institute for Biomedical Research, Nine Cambridge Center, Cambridge, MA 02142, USA.
2. Department of Biology, Massachusetts Institute of Technology, Cambridge, MA 02139, USA.
3. Program in Molecular Biology University of Southern California, Los Angeles, CA 90089–1340, USA.
4. Corning, Inc., Corning, NY 14834, USA.
5. Howard Hughes Medical Institute, Massachusetts Institute of Technology, Cambridge, MA 02139, USA.