Hypoxia-inducible factor-1 (HIF1) is a transcription factor found in mammalian cells cultured under reduced oxygen tension that plays an essential role in cellular and systemic homeostatic responses to hypoxia. HIF1 is a heterodimer composed of a 120-kD HIF1-alpha subunit complexed with a 91- to 94-kD HIF1-beta subunit. [ ]