DRD1 and nicotine dependence: Huang et al. [27] demonstrated that the rs686 polymorphism of DRD1 is significantly associated with nicotine dependence in an African American population, which eventually affects the expression of DRD1. Therefore, our results are in line with previous reports, and the rs686 polymorphism of DRD1 is significantly associated with smoking behaviour in our Malay male population.