Future large analyses on the multiple influences of polymorphism–polymorphism interactions, i.e. among the functional genetic polymorphisms of SLC6A3, ANKK1/DRD2, CYP2A6, and other related molecules, on smoking behaviour and nicotine dependence in different ethnic groups could address the problem of small sample size and lead to conclusions that are more reliable. The gene discussed is SLC6A3; the disease is nicotine dependence.