1. Department of Antiviral Therapy, Schering-Plough Research Institute, Kenilworth, NJ 07033.
2. Present address: PBL Biomedical Laboratories, Piscataway, NJ 08854.
3. Present address: Pfizer-PGRD, Ann Arbor, MI 48105.
4. Present address: Avance Pharma, Quebec H7V 5B7, Canada.