DNA methylation of the human DAT1 gene has been extensively investigated since 80% of its promoter is formed by GCs sequences and it has multiple CpG islands [68,69], and alterations in the epigenetic mark have been reported in different phenotypes, such as ADHD [70], alcohol dependence [71], mild Internet use [66], and nicotine dependence [72], showing at specific CpG sites both reductions as well as increases in methylation levels [72,73,74]. This evidence concerns the gene SLC6A3 and nicotine dependence.