1. Department of Molecular Biology, Massachusetts General Hospital, 185 Cambridge Street, Boston, MA 02114, USA.
2. Harvard-MIT Division of Health Sciences and Technology, E18-435, 77 Massachusetts Avenue, Cambridge, MA 02139, USA.
3. Howard Hughes Medical Institute and Whitehead Institute for Biomedical Research, 9 Cambridge Center, Cambridge, MA 02142, USA.
4. Department of Molecular Cell Biology, Research Institute for Microbial Diseases, Osaka University, 3-1 Yamada-oka, Suita-shi, Osaka 565-0871, Japan.
5. Department of Biology, Massachusetts Institute of Technology, Cambridge, MA 02139, USA.