Several limitations must be considered when interpreting our findings. Our cases were ascertained entirely from maintenance clinics in the Greater Sydney Area. Additional study may be needed to determine whether similar results would be seen in samples not currently in treatment or from other areas. Non-dependent controls were interviewed via telephone; cases and neighborhood controls completed an in-person assessment. Given the low general population prevalence and extreme severity of heroin dependence, it seems highly unlikely that telephone administration, used at QIMR for more than 25,000 interviews to date, led to a substantial number of false negative diagnoses. Although both our cases and non-dependent controls included primarily individuals of European ancestry, the groups differed somewhat in ethnic composition with more Asians found among cases. It is possible that population stratification could have contributed to the significant differences that we observed. Since we found an excess of the same alleles among cases as observed in a prior report (Levran et al. 2008) in a predominately Caucasian sample with some Middle Eastern contribution , we consider this possibility unlikely. We also reran analyses
