The sample consists of 1842 active heroin users (1516 ethnic Bulgarian and 326 Roma), the majority of whom have not received regular care for their drug problem (such as methadone maintenance). Participants are aged 18 years and older, have used heroin daily or nearly daily for a minimum of one year prior to assessment and met DSM-IV criteria for a lifetime diagnosis of heroin dependence. The 1451 unrelated population-representative control samples include 1178 ethnic Bulgarians and 273 Romas. Recruitment (via snowball sampling) is still underway, with the targeted sample size of 2,500 cases and 2,500 controls to be reached in 2011. Face-to-face semi-structured psychiatric interviews were done by trained members of the research team. The number of control samples was balanced to cases on gender and ethnicity. Within gender and ethnicity, the control sample is population representative, being selected from the biobank of the PKU newborn screening program at the National Genetic Laboratory in Sofia (250,000 samples, > 85% of all live births since 1998). Given the low prevalence of heroin dependence in the population (under 1%), the use of
