The current study aims to examine the genetic contribution to methadone treatment response (continued opioid use) in individuals with opioid use disorder, with a specific focus on addiction-related genes, BDNF and DRD2. We hypothesize that carriers of the minor alleles of both rs6265 and rs1799978 will be more likely to engage in continued illicit opioid use during methadone treatment, indicating poor treatment response. The gene discussed is BDNF; the disease is opioid use disorder.