DRD4 and nicotine dependence: Currently, our study allows us to conclude that the C allele of rs1800955 in the DRD4 gene and the A allele of rs6311 in the HTR2A gene are associated with cigarette smoking, whereas the T allele of rs6313 (HTR2A) is associated with smoking and a higher degree of nicotine addiction in smokers of Mexican mestizo ancestry.