This review does not exhaustively cover all genes that to date have been implicated in alcohol and illicit drug dependence. For example, several genes encoding receptors for the neurotransmitter dopamine have been suggested to determine at least in part a person’s susceptibility to various forms of drug dependence. In particular, the DRD2 gene has been associated with alcohol dependence (Blum et al. 1990) and, more broadly, with various forms of addiction (Blum et al. 1996). This association remains controversial, however, and more recent studies suggest that the observed association actually may not involve variants in the DRD2 gene but variants in a neighboring gene called ANKK1 (Dick et al. 2007b). Studies to identify candidate genes that influence dependence on illicit drugs, but not on alcohol, are particularly challenging because of the high co-morbidity between alcohol dependence and dependence on illicit drugs. Therefore, meaningful studies require large sample sizes to include enough drug-dependent people with no prior history of alcohol dependence.
