1 Department of Pediatrics, University of Minnesota, Minneapolis, MN, USA.
2 School of Public Health, University of Alberta, Edmonton, AB T6G 1C9, Canada.
3 School of Public Health, University of Alberta, Edmonton, AB T6G 1C9, Canada; Department of Epidemiology and Cancer Control, St. Jude Children's Research Hospital, Memphis, TN, USA.
4 Department of Epidemiology and Cancer Control, St. Jude Children's Research Hospital, Memphis, TN, USA.
5 Department of Pediatrics, Children's Hospital of Philadelphia, Philadelphia, PA, USA.
6 School of Public Health, University of Alberta, Edmonton, AB T6G 1C9, Canada. Electronic address: yan.yuan@ualberta.ca.