Heroin is a highly addictive drug that has led to the death of numerous people and caused tremendous economic, health-care, and societal costs. The mu opioid receptor gene (OPRM1) is a prominent candidate gene in the underlying neurobiology of heroin addiction. The OPRM1 undergoes extensive alternative splicing, generating multiple splice isoforms or variants that are conserved across species including rodents and humans (Pan, 2005; Pasternak & Pan, 2013). These splice variants can be categorized into three main types based on the predicted receptor structure: (a) Full-length 7 transmembrane (TM) C-terminal variants that are identical except for a different