FLT1 and neoplasm: This critical finding supports our hypothesis that VEGF121/rGel does not affect mature vessels in either normal tissues or tumors since both VEGFR-1 and VEGFR-2 are over-expressed on the endothelium of tumor neovasculature [33-36] but are almost undetectable in the vascular endothelium of adjacent normal tissues and in mature tumor vessels.