Endostatin is a naturally-occurring 20-kDa C-terminal fragment derived from collagen alpha-1(XVIII) chain (COL18A1). It is reported to serve as an anti-angiogenic agent, similar to angiostatin and thrombospondin. It potently inhibits endothelial cell proliferation and inhibits endothelial cell angiogenesis by binding to the heparan sulfate proteoglycans involved in growth factor signaling. Endostatin is a broad spectrum angiogenesis inhibitor and may interfere with the pro-angiogenic action of growth factors such as basic fibroblast growth factor (bFGF/FGF-2) and vascular endothelial growth factor (VEGF).