Once a cluster of cancer cells reaches a diameter of about 1-2 mm, it must develop a blood supply in order to grow larger. Given that diffusion is no longer adequate to supply the cancerous cells with oxygen and nutrients and to remove wastes. Therefore, these cells secrete substances that promote the formation of new blood vessels (angiogenesis), which are vital for the growth and persistence of solid tumors and their metastases.The production of several angiogenic factors (FGF-a, FGF-b, VEGF and VPF) is upregulated by tumors in order to stimulate angiogenesis. However, numerous malignant tumors also generate inhibitors of angiogenesis which include angiostatin and thrombospondin. The angiogenic phenotype is basically the end result of a net balance between the positive and negative regulators of neovascularization mentioned above. Endostatin's role is to inhibit endothelial proliferation, therefore it is an effective angiogenesis inhibitor.