1. Division of Molecular Neurobiology, MRC National Institute for Medical Research, The Ridgeway, Mill Hill, London NW7 1AA, UK
2. Present address: GlaxoSmithKline Biologicals, Rue de l'Institut 89, B-1330 Rixensart, Belgium
3. Present addresses: Department of Clinical Genetics, Erasmus University, Dr Molewaterplein 50, 3015 GE Rotterdam, The Netherlands