Hypoxia inducible factors (HIFs) are heterodimeric proteins belonging to the basic helix-loop-helix-PAS (bHLH/PAS) family of transcription factors that mediate the primary transcriptional response to stress caused by hypoxia (accessible O2 level < 2%). HIFs are characterized by containing two subunits, including O2-labile α subunit (HIFα) and constitutively expressed β subnit (HIFβ). Mammalian HIFα consists of three isoforms, including HIF1α, HIF2α and HIF3α, which play an essential role in the regulation of HIF activity by O2 availability. Under normal O2 tension, HIFα is hydroxylated at the two conserved proline residues within the O2-dependent degradation (ODD) domain by prolylhydroxylase domain proteins (PHDs) and undergoes proteosomal degradation catalyzed by a complex formed by E3 ubiquitin ligase and the von Hippel-Lindau protein (pVHL); while, under hypoxia, HIFα stabilizes with PHDs being deactivated and hence induce transcription of genes with adaptive functions.