GWAS for drug-dependence have attained a high degree of replication, e.g. the same genes or gene loci being associated with drug dependence across a large number of studies. This concordance is far from 100%, but such an expectation would be untenable given the likelihood of a high degree of both gene and loci heterogeneity, and substantial differences in subject pools in terms of features of their underlying addictions (drugs of preference, degree of dependence, psychiatric comorbidities, core behavioral symptoms, etc.). Furthermore, complete concordance between studies (rather than a probabilistic overlap) should not be expected because it has become abundantly apparent that addiction, like most common diseases perhaps, is highly polygenic. These studies have also shown that when the genes which have been identified are considered as a whole, they are not the genes which, a priori, would have been thought to be involved in addiction – e.g. the overabundance of cell adhesion molecules. These genes thus represent a vastly untapped resource for understanding addiction, and comorbid pleiotropically mediated psychiatric conditions, as well as for developing new treatments for these conditions.
