DRD1 and substance dependence: Of relevance to our present pilot study, the work of Zhu et al. [35] showing that for human carriers of the DRD1 rs4532, the duration of the transition from the first use to dependence (DTFUD), subjective pleasure responses to opioid on first use and post-dependence use and their opioid dependence overdose risk was significantly associated with the frequency of the DRD1 rs4532 allele.