1. Department of Genetics, Groningen Biomolecular Sciences and Biotechnology Institute, 9751 NN Haren
2. Department of Genetics, Smurfit Institute, Trinity College, Dublin 2, Ireland
3. Institut de Génétique et Microbiologie, Université Paris-Sud, 91405 Orsay Cedex, France
4. Department of Pharmaceutical Biology, University of Groningen, 9713 AV Groningen, The Netherlands