Rapidly growing tumors result in hypoxic regions. Adaptive responses of most cells to hypoxia are (1) to produce VEGF and other hypoxia-induced angiogenic cytokines that promote increased tissue vascularization, thereby increasing tissue oxygenation, and (2) to switch metabolically from oxidative phosphorylation to anaerobic glycolysis. Hypoxia-inducible factor 1 (HIF-1) is an oxygen-regulated transcriptional activator that plays essential roles in the process. The HIF-1alpha subunit is oxygen-dependent ubiquitination and proteasomal degradation. In addition, cytokines including interleukin-1 (IL-1) and tumor necrosis factor-alpha (TNF-alpha) stimulate HIF-1 dependent gene expression. HIF-1 increases the expression of several genes that promote blood flow and inflammation, such as vascular endothelial growth factor (VEGF). 
Signosis has developed a plate-based array for profiling 20+ HIF-regulated genes. By using this assay, researchers are able to compare gene expression in up to three samples on a single microtiter plate.