More research is needed to determine which smoking-related characteristics are associated with the cluster. For example, ND was measured by SQ or SQ levels in the studies reported by Berrettini et al. (2008) and Thorgeirssone et al. (2008), whereas the study reported by Sacconne et al. (2007) focused on searching for susceptibility genes responsible for the transition from non-dependent to dependent smoking. On the other hand, Weiss et al. (2008) found a significant association of the same cluster with ND only in subjects who began daily smoking at or before the age of 16. Considering these findings in light of several published and unpublished studies that reveal no or weak evidence for the association of this cluster with ND (Vink and others 2009), additional research is needed to investigate the role of the cluster in a variety of smoking phenotypes. For example, it was recently reported that no association exists between this cluster and either smoking initiation or smoking cessation (Hung and others 2008; Thorgeirsson and others 2008; Uhl and others 2008).
