1. Department of Tumor Biology, Karolinska Institute, Box 60400, S-104 01 Stockholm, Sweden.
2. Department of Immunology, Karolinska Institute, Box 60400, S-104 01 Stockholm, Sweden.
3. Molecular Therapeutics, Inc., West Haven, CT 16516.
4. Department of Biology, Yale University, New Haven, CT 06510.
5. Virology Laboratory, Jerome H. Holland Laboratory, American Red Cross, Rockville, MD 20855.