The logic behind many candidate gene studies was generally rather sound from the point of view of biological relevance, and the consequences of some polymorphisms appeared to support the logic behind candidate gene studies. For instance, the μ opioid receptor (MOR) A118G SNP produces an amino acid change that alters endogenous ligand binding (Bond, et al., 1998). However, there was no significant association of this polymorphism with opiate dependence. Subsequent studies have both found (Bart, et al., 2004; Szeto, et al., 2001; Tan, et al., 2003) and failed to find (Crowley, et al., 2003; Franke, et al., 2001; Li, et al., 2000; Shi, et al., 2002; Tan, et al., 2003) significant associations for the same polymorphism. Population admixture has been suggested to be involved in positive associations with this polymorphism when they have been observed in some, though not all, cases (Kreek, et al., 2005).
