English: The regulation of HIF1α by pVHL. Under normal oxygen levels, HIF1α binds pVHL through 2 hydroxylated proline residues and is polyubiquitinated. This leads to its degredation via the proteasome. During hypoxia, the proline residues are not hydroxylated and pVHL cannot bind. HIF1α causes the transcription of genes that contain the hypoxia response element. In VHL disease, genetic mutations cause alterations to the pVHL protein, usually to the HIF1α binding site.