1. Département de mathématiques et statistique; Université Laval; Québec G1V 0A6 Québec Canada
2. Département de mathématiques; Université de Québec À Montréal; Montreal Québec Canada
3. Lady Davis Institute for Medical Research; Jewish General Hospital; Montreal Québec Canada
4. Department of Epidemiology, Biostatistics and Occupational Health; McGill University; Montreal Québec Canada
5. Department of Twin Research; King's College London; London U.K.
6. Departments of Oncology and Human Genetics; McGill University; Montreal Québec Canada