Answer:
In low oxygen gene regulatory protein called HIF-1 stimulates the transcription of the VEGF gene, the Vascular endothelial growth factor. This causes the endothelial cells to use proteases to digest there way through and bring new blood to the cells. This can also happen in cancers cells and this how cancer tumors can become vascularized. In a disorder called VHL - Von hippel-lindau. They are only born with one functioning copy of the VHL gene, mutations occur and they get random cancer throughout there body.