OPRM1 and drug dependence: A common single-nucleotide polymorphism (SNP) in the human mu-opioid receptor gene (OPRM1), A118G, is associated with drug dependence in humans (Bart et al. 2004, 2006; Bond et al. 1998; Nishizawa et al. 2006), although some studies have found no association (Bergen et al. 1997; Coller et al. 2009), and others have suggested that it may be associated with drug dependence in some populations but protective in others (Schwantes-An et al. 2016).