also been investigated in regard to dependence on drugs other than opiates (Bergen, et al., 1997; A. C. H. Chen, et al., 2002; Dahl, et al., 2005; Franke, et al., 1999; Ide, Kobayashi, et al., 2004; Xuei, et al., 2006). Many genes in these systems have also been examined more broadly in drug dependence to multiple substances or combined groups of individuals addicted to different substances (Agrawal, et al., 2006; Berrettini, Hoehe, Ferraro, DeMaria, & Gottheil, 1997; Blomqvist, Gelernter, & Kranzler, 2000; Comings, Muhleman, Ahn, Gysin, & Flanagan, 1994; Gelernter, Kranzler, & Cubells, 1999; Hoehe, et al., 2000; H. R. Kranzler, Gelernter, O’Malley, Hernandez-Avila, & Kaufman, 1998; Krebs, et al., 1998; Luo, Kranzler, Zhao, & Gelernter, 2003; Smith, et al., 1992; Vandenbergh, et al., 2000; Vandenbergh, Rodriguez, Miller, Uhl, & Lachman, 1997; H. Zhang, Kranzler, Yang, Luo, & Gelernter, 2008). Serotonergic system genes have also been studied with regard to addiction to a number of substances (Cigler, et al., 2001; Dahl, et al., 2006; Hong, et al., 2003; H. Kranzler, Lappalainen, Nellissery, & Gelernter, 2002; H. R. Kranzler, Hernandez-Avila, & Gelernter, 2002; Patkar, et al., 2001; Sander, Harms, Lesch, et al., 1997). Few studies have looked at additive or interactive
