Angiogenesis describes the development of new vessels from existing blood vessels and is a crucial physiological process involved in embryo development, wound healing, and the female reproductive cycle. However, angiogenesis has also been shown to be required for tumor growth and metastasis (1 ). On the basis of experiments showing that tumors implanted into isolated perfused organs failed to develop whereas the same tumors implanted within 6 mm of blood vessels induced angiogenesis, grew, and metastasized (2 ,3 ), Folkman proposed that solid tumors are dependent on angiogenesis for growth beyond a few millimeters in size, and that increase in tumor diameter required a corresponding increase in vascularization (4 ). It is becoming clear that there is a complex balance between pro-angiogenic growth factors and endogenous inhibitors that determine the final angiogenic phenotype (5 ).
