Based on the results of the human candidate gene and mouse KO studies discussed above, it might be thought that those genes are clearly implicated in addiction. Based on the marked alterations in survival of several of these KO strains, it was also unlikely that variations that produced marked alterations in these genes would survive to become common in the population. Thus, there were thoughts that GWAS might identify some of those genes, and also that many were unlikely to harbour variants that altered their functions to the extent found in KO mice. In conducting genome wide studies, as for candidate gene studies, both linkage and association methods can be used. Initial genome-wide linkage scans for alcohol dependence found possible linkage at as many as four loci (Foroud, et al., 2000; Long, et al., 1998; Reich, et al., 1998). Generally, these effects were relatively modest (LOD scores between 1.5 and 4) and the same loci were not identified in all samples. A genome-wide linkage scan for nicotine dependence identified only one locus with a strong linkage signal (Straub, et al.,
