The DNA collection and extraction procedure has been described in detail previously (Jern et al. 2012). Briefly, Oragene™ DNA self‐collection kits (DNA Genotek, Inc., Kanata, ON, Canada) were used when collecting saliva samples from participants. The kits were posted to the participants along with the manufacturer's instructions on collecting the samples (DNA Genotek, Inc.) and participants were instructed to deposit approximately 2 ml of saliva into the collection cup before returning it by mail. DNA was stabilized in the cup upon closure and stored at −20°C until used.
