Cyclophilin-40 (CyP40) promotes the formation of the gel shift complex containing baculovirus aryl hydrocarbon receptor (AhR), AhR nuclear translocator (Arnt) and dioxin response element (DRE). CyP40 was found to play a role in the AhR signaling since when the CyP40 content in MCF-7 cells is reduced, up-regulation of cyp1a1 and cyp1b1 by 3-methylcholanthrene (3MC) is also reduced, suggesting that CyP40 is essential for maximal AhR function. The CyP40 region containing amino acids 186-215, but not the peptidylprolyl cis-trans isomerase and tetratricopeptide repeat domains, is essential for forming the AhR/Arnt/DRE complex. CyP40 is found in the cell nucleus after 3MC treatment and appears to promote the DRE binding form of the AhR/Arnt heterodimer. Coprecipitation data suggests CyP40 binds weakly to AhR, but not Arnt. We report on the progress of applying bioluminescence resonance energy transfer and chromatin immunoprecipitation techniques to further elucidate the role of CyP40 in the aryl hydrocarbon receptor signaling pathway.