DRD2 and nicotine dependence: 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.