The aryl hydrocarbon receptor nuclear translocator (ARNT) is a member of the Per-ARNT-Sim (PAS) protein superfamily, transcription factors that mediate the cellular responses to various developmental signals and environmental conditions. A -class (partner) PAS protein, ARNT exhibits the capacity to form transcriptionally active heterodimers with several -class (sensor) proteins, including the aryl hydrocarbon receptors (AHRs), the hypoxia-inducible factors (HIFs), and the Single minded (Sim) proteins. Two genes encode different forms of ARNT in mammals: ARNT1, which is widely expressed, and ARNT2, which is limited to the brain and kidneys of adults and specific neural and branchial tissues of embryos. In contrast, fish apparently express only a single ARNT gene, although in different species, this may be either ARNT1 or ARNT2.