Rüdiger Klein1, Ronald Jeroen Pasterkamp2. 1. Department Molecules-Signaling-Development, Max Planck Institute of Neurobiology, Am Klopferspitz 18, 82152 Martinsried, Germany. Electronic address: rklein@neuro.mpg.de. 2. Department of Translational Neuroscience, University Medical Center (UMC) Utrecht Brain Center, Utrecht University, Universiteitsweg 100, 3584 CG, Utrecht, The Netherlands. Electronic address: R.J.Pasterkamp@umcutrecht.nl.