1. Institute of Diabetes and Regeneration Research, Helmholtz Zentrum München, Business Campus Garching, Parkring 11; Garching D-85748 Germany
2. Institute of Stem Cell Research, Helmholtz Zentrum München, Ingolstädter Landstraße 1; Neuherberg D-85764 Germany
3. Institute of Molecular Biology OE5250, Medizinische Hochschule Hannover, Carl-Neuberg-Str.1; Hannover 30625 Germany