(VTA) where they are highly expressed (Okada et al, 2004; Steiger and Russek, 2004; Wisden et al, 1992). This suggests that the chromosome 4 gene cluster may be implicated in addiction. Indeed, earlier genome-wide scans in American Indians and Caucasians have provided evidence for linkage of alcohol dependence to the location of the chromosome 4 GABAA gene cluster (Long et al, 1998; Reich et al, 1998; Zinn-Justin and Abel, 1999). Subsequently, several studies have found haplotype and SNP associations between GABRA2 and alcoholism. All these studies, together with HapMap, have identified the same two GABRA2 haplotype blocks, at least within Caucasians, American Indians and Asians. The significant association signals for alcoholism have been with two common yin-yang haplotypes within the haplotype block that extends downstream from intron 3 in the direction of the adjacent GABRG1 gene (Agrawal et al, 2006; Covault et al, 2004; Edenberg et al, 2004; Enoch et al, 2006a; Fehr et al, 2006; Lappalainen et al, 2005, Soyka et al, 2008). However, other studies have found no GABRA2 association with alcoholism (Covault et al, 2008 (Project MATCH dataset); Drgon et al, 2006; Matthews et al, 2007).
