In addition to more traditional genotyping approaches, research on addiction and other complex diseases have been extended to genome-wide association (GWA) studies during the past few years, where large numbers of SNPs (ranging from several hundred thousand to more than one million) spread across the genome are assessed in affected individuals and controls, and associated with disease outcome. SNPs that are in high linkage disequilibrium (i.e., tend to be inherited together more commonly than expected due to chance) can serve as proxies for each other, and as such not all of the markers within a given region need to be genotyped, as long as the marker panels can capture the variation at those loci that have not been genotyped (11). GWA studies have provided new insights into our understanding of the genetics of addiction as they are conducted without a prior hypothesis based on gene function or disease pathways as described above. As a result, a number of novel targets with potential biological relevance have been discovered. For instance, GWA studies have found associations for genes involved in cell adhesion
