1. J. Hodgkin is at the Medical Research Council Laboratory of Molecular Biology, Cambridge, CB2 2QH, UK.
2. H. R. Horvitz is in the Department of Biology, Howard Hughes Medical Institute, Massachusetts Institute of Technology, Cambridge, MA 02139, USA.
3. J. Kimble is in the Department of Biochemistry, Howard Hughes Medical Institute, University of Wisconsin, Madison, WI 53706, USA.