1. Present address: Institute of Veterinary Biochemistry, University of Zürich, Winterthurerstr. 190, CH-8057 Zürich, Switzerland
2. Institute for Molecular Biology, University of Zürich, Winterthurerstr. 190, CH-8057 Zürich, Switzerland
3. Present address: Department of Medicine, Division of Cardiology, Albert Einstein College of Medicine, Bronx, NY 10461, USA