Acts as a transcriptional regulator in adaptive response to low oxygen tension. Acts as a regulator of hypoxia-inducible gene expression (PubMed: 11573933, PubMed: 16126907, PubMed: 19694616, PubMed: 20416395, PubMed: 21069422). Functions as an inhibitor of angiogenesis in hypoxic cells of the cornea. Plays a role in the development of the cardiorespiratory system. May also be involved in apoptosis (By similarity). [Isoform 2]: Attenuates the ability of transcription factor HIF1A to bind to hypoxia-responsive elements (HRE) located within the enhancer/promoter of hypoxia-inducible target genes and hence inhibits HRE-driven transcriptional activation. Also inhibits hypoxia-inducible ARNT-mediated gene expression. [Isoform 3]: Attenuates the ability of transcription factor HIF1A to bind to hypoxia-responsive elements (HRE) located within the enhancer/promoter of hypoxia-inducible target genes and hence inhibits HRE-driven transcriptional activation. [Isoform 4]: Attenuates the ability of transcription factor HIF1A and EPAS1/HIF2A to bind to hypoxia-responsive elements (HRE) located within the enhancer/promoter of hypoxia-inducible target genes and hence inhibits HRE-driven transcriptional activation (PubMed: 16126907, PubMed: 17998805, PubMed: 19694616, PubMed: 20416395). May act as a tumor suppressor and inhibits malignant cell transformation (PubMed: 17998805). [Isoform 5]: Attenuates the ability of transcription factor HIF1A to bind to hypoxia-responsive elements (HRE) located within the enhancer/promoter of hypoxia-inducible target genes and hence inhibits HRE-driven transcriptional activation.