One of the targets of the VCB-CUL2 complex is a protein called hypoxia-inducible factor 2-alpha (HIF-2α). HIF-2α is one part (subunit) of a larger protein complex called HIF, which plays a critical role in the body's ability to adapt to changing oxygen levels. HIF controls several genes involved in cell division, the formation of new blood vessels, and the production of red blood cells. It is the major regulator of a hormone called erythropoietin, which controls red blood cell production. HIF's function is particularly important when oxygen levels are lower than normal (hypoxia). However, when adequate oxygen is available, the VCB-CUL2 complex keeps HIF from building up inappropriately in cells.