The present invention provides for diagnostic kits for identifying cancer patients who are more susceptible to cancer therapies employing endostatin and other angiogenesis inhibitors, based upon the discovery that Nucleolin is a specific receptor for Endostatin. In particular, the diagnostic kits include antibody molecules against Nucleolin, DNA or RNA molecules that specifically bind to nucleic acid molecules encoding Nucleolin. The present invention also discloses methods of screening for angiogenesis inhibitors which specifically interact with Nucleolin, and act as angiogenesis inhibitors in an analogous manner as Endostatin. In addition, the present invention discloses methods of inhibiting the proliferation of endothelial cells or angiogenesis of tumor by administering an anti-nucleolin antibody linked to a cytotoxic agent such as tumor necrosis factor alpha to the endothelial cells.