1. Institute of Virology, Philipps University, Marburg, Germany
2. M. P. Chumakov Institute of Poliomyelitis and Viral Encephalitides, Moscow, Russia
3. Laboratory of Molecular Carcinogenesis, National Institutes of Environmental Health Sciences, Research Triangle Park, North Carolina
4. Roche Products, Ltd., Welwyn Garden City, Hertsfordshire, United Kingdom