1. Department of Microbiology and Molecular Genetics, Harvard Medical School, Boston, MA 02115.
2. NSF Center for Biological Timing.
3. Department of Biochemistry, Molecular Biology, and Cell Biology, Northwestem University, Evanston, IL 60208
4. Department of Neurobiology and Physiology, Northwestem University, Evanston, IL 60208.