Of the 1842 cases, 1528 (83%) were males and 314 (17%) females. Of the 1451 controls, 1142 (79%) were males and 309 (21%) females. The mean participant age was 30 (SD 5.7). Table 1 summarizes allelic and genotypic frequencies for 118A>G. Within ethnicity, we found no significant deviations from Hardy-Weinberg equilibrium in either cases or controls. The G allele is more common in Romas than in ethnic Bulgarians (20.2% versus 13.8%, χ2(1) = 11.2, p = 0.0009). The association analyses were thus carried out independently for the two ethnic groups to avoid possible stratification problems. Within ethnicity, we find no significant difference in allelic or genotypic frequencies between heroin dependent cases and controls. Among Roma women, there was a weak trend toward a higher frequency of the G allele in heroin dependent participants (45.1% compared to 31.8%, χ2(1) = 2.16, p = 0.14). However, the relevant samples sizes (N = 51 and 66, respectively) are too limited for definite conclusions.
