Hypoxia-inducible factors (HIFs) are transcription factors that respond to decreases in available oxygen in the cellular environment, or hypoxia. They are only present in parahoxozoan animals. Most, if not all, oxygen-breathing species express the highly conserved transcriptional complex HIF-1, which is a heterodimer composed of an alpha and a beta subunit, the latter being a constitutively-expressed aryl hydrocarbon receptor nuclear translocator (ARNT). HIF-1 belongs to the PER-ARNT-SIM (PAS) subfamily of the basic helix-loop-helix (bHLH) family of transcription factors.
