Alcohol use disorder is a common and complex disorder with a well-documented highly hereditary nature (Higuchi et al., 2006; Roh et al., 2008). Subjective response to alcohol is also known to be a genetically influenced characteristic (Schuckit et al., 2007; Viken et al., 2003). This suggests that genetic influences on individual variation in subjective response to alcohol may underlie the effects of genes on alcohol-related disorders. A low level of response to the acute effects of alcohol has been associated with an increased risk of both excessive alcohol intake (Hinckers et al., 2006; Schuckit et al., 2007) and alcohol dependence (Schuckit, 1994; Schuckit and Smith, 1996; Schuckit et al., 2004), which are well known to be highly heritable (Kendler, 2001). There is consistent evidence that the GABAA receptor regulates the alcohol self-administration in animal models, probably by stimulating reward circuitry in the mesolimbic system (Chester and Cunningham, 2002; Eiler and June, 2007; Harvey et al., 2002; June et al., 2003). Several GABAA receptor subunits have been implicated in alcohol effects, so that the specific subunit composition of the receptor may be an important determinant of alcohol’s CNS effects. The α2 subunit of the GABAA receptor mediates the anxiolytic effects of benzodiazepines (Low et al., 2000; Rudolph et al., 1999) and enhances the hypnotic, but not the sedative, effects of combined exposure to alcohol and benzodiazepines (Tauber et al., 2003). Two genome-wide scans in humans have provided evidence of linkage of alcohol dependence to a region of chromosome 4p that includes a cluster of 4 genes encoding γ-aminobutyric acid A (GABAA) receptor subunits (Long et al., 1998; Reich et al., 1998). A previous study (Edenberg et al., 2004) found that 31 single nucleotide polymorphisms (SNPs) in GABAA receptor α2 subunit gene (GABRA2), but only 1 of the 20 SNPs in the flanking genes, showed significant association with alcohol dependence. Additional studies have provided the replication of this association in a region of GABRA2 among various ethnic groups (Covault et al., 2004; Fehr et al., 2006; Lappalainen et al., 2005; Soyka et al., 2008); though, there is a negative association study between GABRA2 and alcohol dependence (Matthews et al., 2007). Previous studies have also provided evidence that GABAA receptors mediate several behavioral effects of alcohol such as ethanol self-administration and motor impairment (Davies, 2003; Grobin et al., 1998; Hanchar et al., 2005). GABRA2 gene was associated with the differences in the subjective effects of alcohol including blushing sensations, stimulant and sedative effects (Pierucci-Lagha et al., 2005), and the variance in drinking behavior (Bauer et al., 2007). In the former study (Pierucci-Lagha et al., 2005), the more common A allele of the rs279858 SNP within the GABRA2 gene showed greater subjective effects of alcohol than did individuals with 1 or 2 copies of the alcohol dependence–associated G allele. These findings underscore the potential contributions of variation at GABRA2 to the differences in the subjective responses to alcohol, the variance of drinking behavior, and the risk for alcohol dependence. This study examined the moderating effects of GABRA2 alleles on subjective and physiologic effects of alcohol in healthy social drinkers. We used the alcohol clamp method for alcohol administration, which uses an intravenous infusion of alcohol at rates adjusted online to close the gap between measurements of breath alcohol concentration (BrAC) and the target concentration. The clamp method reduces experimental variance in BrAC (O’Connor et al., 1998) which can be caused by the substantial pharmacokinetic variability following oral alcohol administration. Therefore, the alcohol clamp used in this study is a more exhaustive objective measure than the oral loading of alcohol that was used in the previous study of the effects of GABRA2 on the subjective effects of alcohol assessed only during the ascending limb of the BrAC (Pierucci-Lagha et al., 2005). Our method allowed the evaluation of the initial response to alcohol following the ascending limb of the BrAC as well as the adaptive response to alcohol during the clamped BrAC interval. Based on the previous studies, we hypothesized that GABRA2 alleles would moderate the subjective responses to alcohol measured during not only the ascending limb of BrAC curve but also the clamped BrAC interval. As we expected, subjects with ALDH2*1/*2 showed higher initial response than those with ALDH2*1/*1 (Matsushita et al., manuscript in preparation). Therefore, we divided subjects by ALDH2 genotype and performed further analyses separately in each ALDH2 genotypic group.