1. P. H. Kussie, S. Gorina, and N. P. Pavletich are with the Cellular Biochemistry and Biophysics Program, Memorial Sloan-Kettering Cancer Center, New York, NY 10021, USA.
2. V. Marechal is with the Cervice de Microbiologie, Hopital Rothschild, F-75571, Paris 12, France.
3. B. Elenbaas and A. J. Levine are in the Department of Molecular Biology, Princeton University, Princeton, NJ 08544, USA.
4. J. Moreau is at the Institut Jacque Monad, Equipe d'Embryologie, 75251, Paris, France.