Department of Epidemiology, University of Groningen, University Medical Center Groningen, P.O. box 30.001, 9700 RB Groningen, The Netherlands and Cardiogenetics Lab, Human Genetics Research Centre, St. Georges Hospital Medical School, London SW17 0RE, UK Department of Epidemiology, University of Groningen, University Medical Center Groningen, P.O. box 30.001, 9700 RB Groningen, The Netherlands and Cardiogenetics Lab, Human Genetics Research Centre, St. Georges Hospital Medical School, London SW17 0RE, UK.