The aryl hydrocarbon receptor (AhR) heterodimerizes with the aryl hydrocarbon receptor nuclear translocator (Arnt) for transcriptional regulation. We generated three N-terminal deletion constructs of the human AhR of 12-24 KDa in size—namely D1 (aa 84-295), D2 (aa 84-192) and D3 (aa 191-295)—to suppress the Arnt function. We observed that all three constructs interact with the human Arnt with similar affinities. D2, which contains part of the AhR PAS-A domain and interacts with the PAS-A domain of Arnt, inhibits the formation of the AhR gel shift complex. D2 suppresses the 3-methylcholanthrene-induced, dioxin response element (DRE)-driven luciferase activity in Hep3B cells and exogenous Arnt reverses this D2 suppression. D2 suppresses the induction of CYP1A1 at both the message and protein levels in Hep3B cells; however, the CYP1B1 induction is not affected. D2 suppresses the recruitment of Arnt to the cyp1a1 promoter but not to the cyp1b1 promoter, partly because the AhR/Arnt heterodimer binds better to the cyp1b1 DRE than to the cyp1a1 DRE. Interestingly, D2 has no effect on the cobalt chloride-induced, hypoxia inducible factor-1 (HIF-1)-dependent expression of vegf, aldolase c, and ldh-a messages. Our data reveal that the flanking sequences of the DRE contribute to the binding affinity of the AhR/Arnt heterodimer to its endogenous enhancers and the function of AhR and HIF-1 can be differentially suppressed by the D2 inhibitory molecule. In chapter 2, a Pichia Pastoris expression system was constructed expressing codon optimized human full length AhR. This codon optimization is necessary for overexpression of huAhR. RT-PCR data showed that the codon optimized mRNA was more stably expressed than wild types. Overexpressed huAhR protein was degraded by proteinase when using a regular P. Pastoris strain yJC100 whereas the proteinase deficient ySMD1163 maintained a much higher level of huAhR. P. Pastoris expressed huAhR was natively purified and analyzed. Coimmunopricipitation assay shows its interaction with endogenous Arnt. A ligand-dependent gel shift was also observed. In addition, we performed an in vitro coprecipitation assay to study its binding to endogenous cyp1b1 DREs. The result shows that the DRE3, known as a critical DRE for cyp1b1 transcriptional activity, has the highest binding affinity to AhR/Arnt complex. Taking together, we constructed a novel P. Pastoris expression system to overexpress human full length AhR. Purified huAhR is a good reagent for studing its ligand and DNA binding. In chapter 3, an adeno-associated virus (AAV) expression system was constructed to express an AhR deletion contruct CΔ553 (aa1-295) for tumor injection. Western blot shows the expression of CΔ553 (aa1-295) in hela cells infected by AAV-553, but the low yield of AAV-553 limited its application on tumor treatment. Possible solutions were discussed for future work.