1. Louis-Jeantet Research Laboratories (V.L., K.R., P.A.H., J.-C.I.), University Medical Center, 1211 Geneva 4, Switzerland;
2. Department of Biochemistry and Molecular Biology and the Howard Hughes Medical Institute, University of Chicago (G.W., D.F.S.), Chicago, Illinois 60637
3. Department of Physiology (A.M.), University Medical Center, 1211 Geneva 4, Switzerland;