The existence of a blood vessel growth stimulating factor was proposed in the 1930s when it was observed that transplanted tumors had a remarkable ability to form new vasculature. It took 50 years for this factor, called vascular endothelial growth factor (VEGF), to be identified. VEGF is a key cytokine in the formation of normal blood vessels during embryonic development and in other processes where new blood vessels are needed, such as following injury or to bypass blocked vessels. However, VEGF is also involved in several forms of cancer including poor prognosis breast cancer.