1. Institute of Biochemistry, University of Vienna, Dr. Bohrgasse 9/3, A-1030 Vienna, Research Institute of Molecular Pathology, A-1030 Vienna, Austria, Université Louis Pasteur, URA 1386 du CNRS, F-67400 Strasbourg-Illkirch, France, and Boehringer Ingelheim Research and Development Vienna, Dr. Boehringergasse 5-11, A-1121 Vienna, Austria