In the DRD4 gene, most studies have focused on single-nucleotide polymorphisms (SNP) and their association with nicotine addiction; previous studies have suggested that alleles of the dopamine receptor D4 gene moderates the effect of nicotine on cigarette reward [19], greater craving [20], and neuroticism [21] among smokers; however, there have been no studies evaluating susceptibility to tobacco smoking/nicotine addiction to date. The gene discussed is DRD4; the disease is nicotine dependence.