The authors are grateful to the subjects in the CGS, French CF, and DCCT studies for their participation. The authors also thank Drs. Hughes Aschard and Peter Kraft for providing their simulation results; the CF Gene Modifier Consortium and Drs. Michael Knowles, Garry Cutting, and Mitchell Drumm for helpful discussions; and the Canadian and US CF Foundations for their generous support of the genotyping. This data resource was also supported in part by Genome Canada, through the Ontario Genomics Institute per research agreement 2004-OGI-3-05 (to P.R.D.), with the Ontario Research Fund, Research Excellence Program. This work was funded by the Canadian Institutes of Health Research (CIHR; 201309MOP-310732-G-CEAA-117978 to L.S. and MOP-258916 to L.J.S.); the Natural Sciences and Engineering Research Council of Canada (NSERC; 250053-2013 to L.S. and 371399-2009 to L.J.S.); Cystic Fibrosis Canada #2626 (to L.J.S.); Training grant GET-101831; Université Pierre et Marie Curie Paris; Agence Nationale de la Recherche (R09186DS to H.C.), Direction Générale de la Santé; Association Vaincre la Mucoviscidose, Chancellerie des Universités (Legs Poix); Association Agir Informer Contre la Mucoviscidose; and Groupement d’Intérêt Scientifique (GIS)–Institut des Maladies Rares. A.D.P. held a Canada Research Chair in the Genetics of Complex Diseases. D.S. is a trainee of CIHR STAGE (Strategic Training for Advanced Genetic Epidemiology) program at the University of Toronto.