1. Division of Radiological Chemistry, Institute of Nuclear Medicine, Department of Radiology, University Hospital, Petersgraben 4, CH-4031 Basel, Switzerland, Departamento de Bioquímica e CNC, Universidade de Coimbra, Coimbra, Portugal, Faculdade de Medicina, Serviço de Biofísica, Universidade de Coimbra, Coimbra, Portugal, and Division of Cell Biology and Experimental Cancer Research, Institute of Pathology, University of Berne, PO Box 62, Murtenstrasse 31, CH-3010 Berne, Switzerland