1. J. D. Axelrod is in the Department of Genetics, Harvard Medical School, 200 Longwood Avenue, Boston, MA 02115, USA, and in the Department of Pathology, Brigham and Women's Hospital, Boston, MA 02115, USA.
2. K. Matsuno and S. Artavanis-Tsakonas are at the Howard Hughes Medical Institute and in the Departments of Cell Biology and Biology, Boyer Center for Molecular Medicine, Yale University School of Medicine, New Haven, CT 06536, USA.
3. N. Perrimon is at the Howard Hughes Medical Institute and in the Department of Genetics, Harvard Medical School, 200 Longwood Avenue, Boston, MA 02115, USA.