1. Department of Medicinal Chemistry, Uppsala University, BMC, Box 574, 751 23 Uppsala, Sweden, Department of Cell and Molecular Biology, Uppsala University, BMC, Box 596, 751 24 Uppsala, Sweden, Division of Parasitology, National Institute for Medical Research, The Ridgeway, Mill Hill, London NW/ 1AA, UK, Medivir AB, Lunastigen 7, 141 44 Huddinge, Sweden, and Department of Biochemistry and Molecular Biology, University of Florida College of Medicine, P.O. Box 100245 Gainesville, Florida 32610-0245