According to the 2010 Global Burden of Disease Health Measurement Survey, around 15.5 million people worldwide were dependent on heroin and other opioid drugs (1). Three regions had prevalence rates significantly higher than the global rate: Australasia, western Europe, and North America (1). In the United States, the 2013 National Survey on Drug Use and Health estimated that 669,000 people aged 12 or older abused heroin in the past year, representing a 78% increase since 2007 (2). To address this public health burden, a better understanding of the pathogenesis leading to heroin addiction is needed. Genetic vulnerability is recognized as a major risk factor contributing to heroin and other opioid addiction, as evidenced by twin studies showing that genetic factors account for 40% to 60% of the population variability (3-6). A few genome-wide association (7-9) and numerous candidate gene studies (10-19) in humans have implicated genes encoding opioid receptors (OPRM1, OPRD1, and OPRK1), potassium channels (KCNG1 and KCNG2), and others as contributing to heroin/opioid addiction phenotypes. In supporting the biological plausibility of the candidate genes, particularly genes in the opioid
