1. European Molecular Biology Laboratory Meyerhofstrasse 1 D‐69117 Heidelberg Germany
2. Present address: Netherlands Cancer Institute Plesmanlaan 121 1066 CX Amsterdam The Netherlands
3. Division for Molecular Biology of Mitosis, German Cancer Research Center Im Neuenheimer Feld 280 D‐69120 Heidelberg Germany
4. European Molecular Biology Laboratory Grenoble Outstation F‐38042 Grenoble France
5. Swiss Federal Institute of Technology Zürich, Institute of Biochemistry 8092 Zürich Switzerland