Further steps are required to characterize the CHRNA5/A3/B4 cluster as a susceptibility locus for addictions and lung cancer. First, it is important to determine which variant(s) cause such significant associations. Resequencing might identify additional variants that explain the complex patterns, but functional analysis also will be necessary. The nonsynomous variant rs16969968 (Asp398Asn) causes the substitution of a negatively charged residue within the M3–M4 intracellular loop of CHRNA3 (Box 2), a region thought to be involved with receptor trafficking. Indeed, the variant form of CHRNA5 alters receptor function without affecting expression.55 Second, it is important to determine whether the CHRNA5/A3/B4 cluster also has a significant role in non-European populations. Third, more research is needed to determine which smoking-related behavior drives the association with this cluster. For example, ND was measured by smoking quantity in two studies,48,50 whereas another focused on searching for susceptibility genes underlying the transition from regular non-dependent to dependent smokers.49 Weiss et al.51 found a significant association of the cluster with ND only in early-onset smokers. Fourth, other genes in the region, such as a hypothetical locus LOC123688,
