1. Department of Molecular Cell Biology, Research Institute Brain and Behaviour (EURON), University of Maastricht, Maastricht, The Netherlands
2. Present address: Erasmus MC, Rotterdam, The Netherlands
3. Department of Clinical Pharmacology, University of Bonn, Bonn, Germany
4. Experimental Genetics Group, Department of Human Genetics, K.U. Leuven, Leuven, Belgium