Some of the first attempts to address this partial overlap for different aspects of addiction have compared the results of the drug dependence studies mentioned above with those of nicotine cessation studies (Drgon, Johnson, et al., 2009; Drgon, Montoya, et al., 2009; G. R. Uhl, et al., 2007). These studies have identified a highly significant overlap between the set of genes associated with nicotine dependence and nicotine cessation and the set of genes identified in four previous GWAS for drug dependence. Furthermore, there was a substantial, though far from complete, overlap between genes associated with nicotine dependence and nicotine cessation. Interestingly, as in previous GWAS for drug dependence, cell adhesion molecules were over-represented in the set of genes identified as being associated with nicotine dependence with respect to their representation in the genome as a whole (see Table 2). Importantly, and in contrast to other GWAS for drug dependence, a number of other genes that had been previously associated with nicotine or drug dependence using candidate gene or genome-wide linkage approaches were apparent in this list including a number of
