Epigenetic factors, such as DNA methylation, and histone modifications, have been shown to be altered by illicit drugs or in drug addiction (Kumar et al. 2005; Nielsen et al.2009; Novikova et al. 2008; Renthal et al. 2007, 2008; Zhang et al. 2007) and may play a role in an individual’s vulnerability to develop drug addiction, or to the response to pharmacotherapy. In genomic DNA, cytosine residues may be methylated at cytosine:guanine (CpG) dinucleotides by DNA methyltransferases. When CpG sites are methylated in promoter regions, in general, gene expression is decreased (Heller et al. 2008). A number of transcription binding sites contain CpG dinucleotides that, when methylated, alter transcription factor binding (e.g. Alikhani-Koopaei et al. 2004; Douet et al. 2007; Michelotti et al. 2007; Zhang et al. 2007).
