1. Department of Biological Chemistry, The Alexander Silverman Institute of Life Sciences, The Hebrew University of Jerusalem, Givat Ram, Jerusalem 91904, Israel, SUGEN, Inc., 515 Galveston Drive, Redwood City, California 94063-4720, and Max-Planck Society, Research Group “Growth Factor Signal Transduction” Medical Faculty, Friedrich-Schiller University, Drackendorfer Str. I D 07747, Germany