1. Department of Biological Sciences, University of Maryland Baltimore County, Baltimore, Maryland 21250
2. Graduate Program in Macromolecular and Cellular Structure and Chemistry, The Scripps Research Institute, La Jolla, California 92037
3. Novartis Agricultural Discovery Institute, Inc., San Diego, California 92121
4. Department of Molecular Biology, Massachusetts General Hospital and Department of Genetics, Harvard Medical School, Boston, Massachusetts 02114