Candidate gene approaches have tended to focus upon specific sets of genes based upon a priori assumptions about the importance of particular genes in addiction, usually assessing a small number of genomic markers. These assumptions were based in part upon the mechanism of actions of particular drugs of abuse, e.g. dopamine systems (amphetamine, cocaine and other stimulants), opioid systems (heroin and other opiates), GABAergic systems (ethanol and benzodiazepines), etc. Thus, candidate gene studies examined association or linkage of dopaminergic system genes with addiction/dependence for cocaine and amphetamines (Comings, Gonzalez, et al., 1999; Gelernter, Kranzler, & Satel, 1999; Guindalini, et al., 2006; Hong, Cheng, Shu, Yang, & Tsai, 2003; H. C. Liu, et al., 2004; Noble, et al., 1993; Persico, Bird, Gabbay, & Uhl, 1996a; Sery, Vojtova, & Zvolsky, 2001; Tsai, et al., 2002), opioid system genes for opiate addiction/dependence (Bart, et al., 2004; Bond, et al., 1998; Comings, Blake, et al., 1999; Crowley, et al., 2003; Franke, et al., 1999; Li, et al., 2000; Mayer, et al., 1997; Shi, et al., 2002; Szeto, Tang, Lee, & Stadlin, 2001; Tan, Tan,
