Transcription factor that functions as an obligate heterodimerization partner for several basic helix-loop-helix PER-ARNT- SIM (bHLH-PAS) proteins. Forms heterodimers with the aryl hydrocarbon receptor (AHR) to mediate the transcriptional response to xenobiotics by binding xenobiotic response elements (XREs) or dioxin response elements (DREs) in target gene promoters (PubMed:28396409, PubMed:34521881). Also heterodimerizes with hypoxia-inducible factor alpha subunits (HIF1A, EPAS1, NPAS1 or NPAS3) to form hypoxia-inducible transcription factors that bind hypoxia response elements (HREs) and activate genes involved in cellular and systemic responses to hypoxia, including angiogenesis, erythropoiesis, and metabolic adaptation (PubMed:16181639, PubMed:20699359). Plays a central role in environmental sensing and developmental and physiological regulation of gene expression (PubMed:34521881). {ECO:0000269|PubMed:16181639, ECO:0000269|PubMed:20699359, ECO:0000269|PubMed:28396409, ECO:0000269|PubMed:34521881}. This is the function of ARNT (aryl hydrocarbon receptor nuclear translocator, ENSG00000143437).