Despite enormous efforts over the past few decades, the progress in finding the genes and causal variants underlying drug addiction has been slow. The variants examined in candidate gene association studies so far have been based on a rather imperfect understanding of biological pathways, and studies have often yielded inconsistent results. For example, while case-control association studies have associated the Taq1 A1 allele with a number of smoking and alcoholic phenotypes, several studies have also been negative and meta-analyses have generally failed to support an association (4, 21, 22), (Table 2, Supplementary Table 3).
