1. Max Planck Research Unit in Growth Factor Signal Transduction, Medical Faculty, University of Jena, 07747 Jena, Germany.
2. Ludwig Institute for Cancer Research, London W1 P 8K1T, UK.
3. Department of Pharmacology and Toxicology, University of Ulm, 89069 Ulm, Germany.
4. Institute of Biochemistry II, Medical Faculty, University of Jena, 07747 Jena, Germany.
5. Institute of Pharmacology, Freie Universität Berlin, 14195 Berlin, Germany.
6. Department of Molecular Biology, Max Planck Institute of Biochemistry, 82152 Martinsried, Germany.
7. Ludwig Institute for Cancer Research, London W1 P 8BT, UK, and Department of Biochemistry and Molecular Biology, University College, London WCE 6BT, UK.