Vascular Endothelial Growth Factors 121 (VEGF121) is one of five VEGF splice variants of 121, 145, 165, 183, 189, and 206 amino acids (aa) in length. VEGF121 is the only VEGF which lacks heparin-binding activity and freely diffusible. VEGF binds the type I transmembrane receptor tyrosine kinases VEGF R1 (also called Flt-1) and VEGF R2 (Flk-1 /KDR) on endothelial cells to activate signal transduction and regulate both physiological and pathological angiogenesis.