1. Laboratory of RNA Biology, Max Planck Institute of Biochemistry Am Klopferspitz 18 82152 Martinsried Germany
2. GSF—Institute of Molecular Immunology Marchioninistrasse 25 81377 Munich Germany
3. Bioanalytical Mass Spectrometry Group, Max Planck Institute of Biophysical Chemistry Am Fassberg 11 37077 Göttingen Germany