VEGF is a potent growth and angiogenic cytokine. It stimulates proliferation and survival of endothelial cells, and promotes angiogenesis and vascular permeability. Expressed in vascularized tissues, VEGF plays a prominent role in normal and pathological angiogenesis. VEGF signals through three receptors; fms-like tyrosine kinase (flt-1), KDR gene product (the murine homolog of KDR is the flk-1 gene product) and the flt4 gene product. Due to its increased acidity, VEGF121 circulates more freely than other VEGF forms, which bind more tightly with vascular heparin sulfates. Recombinant human VEGF121 is a 28.4 kDa disulfide-linked homodimeric protein consisting of two 121 amino acid polypeptide chains.