PDYN and heroin dependence: Therefore, the present study aimed to examine the possible association between rs2235749 and rs910080 variants in the 3`-untranslated regions (3`UTR), the rs2281285 variant in the second intron, and the 68-bp VNTR polymorphism in the promoter region of PDYN gene with the risk of heroin addiction in a sample of the southeast Iranian population.