1. Division of Molecular and Cellular Biology, Eunice Kennedy Shriver National Institute of Child Health and Human Development, Bethesda, MD 20892
2. Intramural Research Program, National Library of Medicine, NIH, Bethesda, MD 20894
3. Laboratory of Molecular Biology, National Institute of Diabetes and Digestive and Kidney Diseases, Bethesda, MD 20892
4. Department of Biology, Massachusetts Institute of Technology, Cambridge, MA 02139
5. HHMI, Massachusetts Institute of Technology, Cambridge, MA 02139