1. CJF INSERM 94-02, Université RenéDescartes, 75270 Paris cedex 06, 1 and
2. Laboratoire des Virus Oncogènes, Institut Pasteur, Paris 75015, 2 France;
3. Max Delbrück Center for Molecular Medicine, 13125 Berlin, Germany 3 ; and
4. Gene Transcription Laboratory, Imperial Cancer Research Fund Oncology Unit, London W12 0NN, United Kingdom4