1. Institute of Pharmaceutical Sciences, Swiss Federal Institute of Technology Zurich (ETH Zurich), and Institute of Anatomy, University of Zurich, Zurich, Switzerland, Institut de Recerca Biomèdica de Barcelona, Parc Científic de Barcelona, and Departament de Química Orgànica, Universitat de Barcelona, Barcelona, Spain, and Institute of Biochemistry, University of Leipzig, Leipzig, Germany