Earlier studies, together with HapMap, have identified the same two GABRA2 haplotype blocks within Caucasians, Asians, Native Americans and African Americans. The previously reported significant association signals with alcoholism have been within the haplotype block that extends downstream from intron 3 (called block 2 in our study). In Caucasians, Asians and Native Americans there are two major yin-yang haplotypes within this block that account for nearly all of the haplotype diversity (24). Individuals of African origin also have these two yin-yang haplotypes however in addition they have two common haplotypes that are not present in other populations. In the present study these two unique haplotypes were associated with addiction vulnerability: one haplotype was associated with heroin dependence and the other haplotype was more common in controls and may be protective against addiction. In contrast, we found no association between the yin-yang haplotypes and alcohol dependence or drug dependence unlike the many earlier studies in Caucasians (12–21). The two previously reported studies in African Americans showed no association between the yin-yang haplotypes and alcohol dependence (22) or polysubstance abuse (23). Moreover,
