The onset of angiogenesis is believed to be an early event in tumorigenesis and may facilitate tumor progression and metastasis. Several growth factors with angiogenic activity have been described. These include fibroblast growth factor (FGF), platelet derived growth factor (PDGF) and vascular endothelial growth factor (VEGF). VEGF is a dimeric glycoprotein with struc- tural homology to PDGF. Several variants of VEGF have been described that arise by alternative mRNA splicing. It has been speculated that VEGF may function as a tumor angiogenesis factor in vivo. Two additional proteins des- ignated VEGF-B and VEGF-C share a significant degree of homology withVEGF. VEGF-B is abundantly expressed in heart and skeletal muscle and is frequently co-expressed with VEGF.