1. Laboratory of Molecular Biology (S.-Y.C.), Center for Cancer Research, National Cancer Institute, National Institutes of Health, Bethesda, Maryland 20892;
2. Department of Cellular and Molecular Physiology (J.L.L.), University of Massachusetts Medical School, Worcester, Massachusetts 01655;
3. Signal Transduction Laboratory (P.J.D.), Ordway Research Institute, Inc., Albany Medical College, Albany, New York 12208