1. Department of Medicinal Chemistry, Isis Pharmaceuticals, Inc., 2292 Faraday Avenue, Carlsbad, California 92008, Department of Chemistry, McGill University, 801 Sherbrooke Street West, Montreal, PQ H3A2K6, Canada, and Beckman Coulter GmbH, Europark Fichtenhain B13, D-47807 Krefeld, Germany