1. Department of Medicine, Columbia University College of Physicians & Surgeons, 630 West 168th Street, New York, NY 10032, USA.
2. Department of Clinical Biochemistry, Cambridge Institute for Medical Research, Addenbrookes Hospital, Cambridge, CB2 2XY, UK.
3. Institute of Biochemistry, University of Kiel, D-24098 Kiel, Germany.
4. Center for C. elegans Anatomy, Albert Einstein College of Medicine, Bronx, NY 10461, USA.