1. Howard Hughes Medical Institute and Department of Molecular, Cellular and Developmental Biology, University of Colorado, Boulder, CO 80309, USA
2. Howard Hughes Medical Institute and Division of Basic Sciences, Fred Hutchinson Cancer Research Center, Seattle, WA 98109, USA
3. Present address: Laboratory of Molecular Biology, University of Wisconsin, Madison, WI 53706, USA
4. Howard Hughes Medical Institute and Department of Biology, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
5. Present address: Department of Continuing Education, Harvard University, Cambridge, MA 02138, USA