Therefore, our data provide no support for the idea that variations in OPRM1, OPRD1, PENK and POMC are associated with alcohol dependence or general illicit drug dependence, but variations in PENK and POMC appear to be associated with the narrower phenotype of opioid dependence in these families.
Single-nucleotide polymorphism in the human mu opioid receptor gene alters beta-endorphin binding and activity: possible implications for opiate addiction.