allelic contrast tests data was too limited to perform such an analysis. In the future we will continue to integrate new data toward a better understanding of drug addiction. In addition, recent re-sequencing efforts using next-generation deep sequencing technology support larger effects for at least some rarer variants in both Mendelian [36-40] and complex diseases [41,42], which would also be missed by the current analyses. Nevertheless, the interesting findings from these meta-analyses is complementary to recently published gene-based approach that was used to analyze primary GWAS data in ways that allow for substantial allelic and locus heterogeneities [25-27]. This study also provided an opportunity to study the relationship between addiction susceptible genes identified by traditional genetic association studies and rare addiction causal variants linked by "common disease, rare variants" approaches, when more genomic re-sequencing efforts become available [43-45].
