1. Medical Research Council Integrative Epidemiology Unit, University of Bristol, Bristol BS8 2BN, UK.
2. Bristol Medical School, University of Bristol, Oakfield House, Oakfield Grove, Bristol BS8 2BN, UK.
3. K.G. Jebsen Center for Genetic Epidemiology, Department of Public Health and Nursing, NTNU, Norwegian University of Science and Technology, Norway.