Alcoholism and substance abuse have been associated with a polymorphism in a noncoding region of the D 2 receptor gene (the A1 allele of the Taq1 'A' system) in several, [1][2][3][4][5][6][7][8][9] but not all 10-13 studies. In addition, the presence of the A1 allele has been associated with lower density of D 2 receptors in the caudate nucleus in one postmortem study. 14 If the Taq1 'A1' allele is in linkage disequilibrium with a mutation that decreases the expression of the D 2 receptor gene, it is conceivable that subjects with the A1 allele might be predisposed to behaviors which stimulate dopamine transmission, such as alcoholism or substance abuse. In this study, we attempted to confirm the association between the A1 allele and lower D 2 receptor density, and explored a possible association between the B1 allele and D 2 receptor expression. Genotypes at the Taq1 'A' and 'B' systems were determined in 70 subjects who underwent in vivo measurement of D 2 receptors-binding potential with single photon emission computerized tomography (SPECT) and the selective dopamine D 2 receptor radiotracer [ 123 I]IBZM. [ 123 I]IBZM-binding potential was identical in A1 carriers (ie, subjects heterozygous or homozygous for that allele) (230 ± 85 ml g −1 , n = 27) and A1 noncarriers (231 ± 70 ml g −1 , n = 43). Similarly, we found no effect of the B1 allele on [ 123 I]IBZM-binding potential. In conclusion, the results of this study failed to replicate the previously reported association between A1 allele and lower D 2 receptor expression.In 1990, Blum et al 1 reported an association between alcoholism and a polymorphism in a noncoding region of the D 2 receptor gene (the A1 allele of the Taq1 'A' system). This association has been supported in several, 2-5 but not all subsequent studies. 10-13 The A1 allele was also found to be associated with prolonged P300 latency, reduced visuospatial preference, and low cerebral glucose utilization, all putative markers of alcoholism and dopamine function deficiency. [15][16][17] In addition, the A1 allele has also been associated with substance abuse, 6-9 which led to its conceptualization as a 'reward gene'. Together, these data suggest that this polymorphism might affect dopamine transmission in a way that predisposes subjects to 'reward seeking' behavior, i.e. behaviors aiming at increasing stimulation of dopamine transmission.The mechanism underlying the association between the A1 allele and vulnerability to addictive behaviors is unclear, since the Taq1 'A' polymorphism is not located in the coding region of the gene and a direct effect of this polymorphism on the structure of the D 2 receptor gene has been excluded by denaturing gradient gel electrophoresis mutational analysis study. 18 Noble et al 14 described an association between the A1 allele and decreased D 2 receptor density expressed in postmortem striatal samples from alcoholic and nonalcoholic subjects. If replicated, this observation might suggest that this polymorphism is in linkage disequilibrium with an unknown muta...