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). {ECO:0000250|UniProtKB:Q0VBL6, ECO:0000269|PubMed:11573933, ECO:0000269|PubMed:16126907, ECO:0000269|PubMed:19694616, ECO:0000269|PubMed:20416395, ECO:0000269|PubMed:21069422}. [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. {ECO:0000269|PubMed:19694616, ECO:0000269|PubMed:20416395, ECO:0000269|PubMed:21069422}. [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). {ECO:0000269|PubMed:16126907, ECO:0000269|PubMed:17998805, ECO:0000269|PubMed:19694616, ECO:0000269|PubMed:20416395}. [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. . This is the function of ENSG00000124440 (HIF3A, hypoxia inducible factor 3 subunit alpha).