Similar findings were reported in a case-control study of Han Chinese subjects including 121 heroin-dependent subjects and 154 controls [70]. There was an excess of longer (5-7 repeats) alleles in the cases (P = 0.023; OR: 2.30, 95% CI: 1.07-4.93) with the 7-repeat allele appearing exclusively in heroin dependent subjects [70]. Altogether, the findings of these studies provide strong evidence to support the hypothesis that the long repeat alleles of the DRD4 exon III are associated with a higher vulnerability to opioid dependence.
