Required for activity of the Ah (dioxin) receptor. This protein is required for the ligand-binding subunit to translocate from the cytosol to the nucleus after ligand binding. The complex then initiates transcription of genes involved in the activation of PAH procarcinogens. The heterodimer binds to core DNA sequence 5'-TACGTG-3' within the hypoxia response element (HRE) of target gene promoters and functions as a transcriptional regulator of the adaptive response to hypoxia (By similarity). The heterodimer ARNT:AHR binds to core DNA sequence 5'-TGCGTG-3' within the dioxin response element (DRE) of target gene promoters and activates their transcription (PubMed:28396409). Monomer. Homodimer only upon binding to a DNA (By similarity). Efficient DNA binding requires dimerization with another bHLH protein. Interacts with TACC3 (By similarity). Interacts with HIF1A, EPAS1, NPAS1 and NPAS3; forms a heterodimer that binds core DNA sequence 5'-TACGTG-3' within the hypoxia response element (HRE) of target gene promoters (PubMed:20699359, PubMed:16181639) (By similarity). Forms a heterodimer with AHRR, as well as with other bHLH proteins (Probable). Interacts with NOCA7 (PubMed:10395741). Interacts with TACC3 (By similarity). Interacts with AHR; the heterodimer ARNT:AHR binds to core DNA sequence 5'-TGCGTG-3' within the dioxin response element (DRE) of target gene promoters and activates their transcription (PubMed:28396409). Interacts with SIM1 and NPAS4 (By similarity). Extended N-terminus. Extended N-terminus.