Vascular endothelial growth factor (VEGF or VEGF-A), also known as vascular permeability factor (VPF), is a potent mediator of both angiogenesis and vasculogenesis in the fetus and adult (1-3). It is a member of the PDGF family that is characterized by the presence of eight conserved cysteine residues and a cystine knot structure (4). Humans express two sets of alternately spliced isoforms of 121, 145, 165, 183, 189, and 206 amino acids (aa) in length (4). VEGF₁₆₅ appears to be the most abundant and potent of the angiogenic isoform set, followed by VEGF₁₂₁ and VEGF₁₈₉ (3, 4). The anti-angiogenic or “b” set of isoforms is differentially spliced to contain six alternate amino acids at the C-terminus, and are the more highly expressed isoforms in normal adult tissue (5). VEGF_165b, like VEGF₁₂₁ but unlike most angiogenic isoforms, does not bind heparins and is therefore diffusible (4, 6). Human VEGF_165b shares 88% aa sequence identity with corresponding regions of mouse and rat, 96% with porcine, 95% with canine, and 93% with feline, equine and bovine VEGF_165b, respectively. VEGFs bind the type I transmembrane receptor tyrosine kinases VEGF R1 (also called Flt-1) and VEGF R2 (Flk-1/KDR) on endothelial cells (4). Although VEGF affinity is highest for binding to VEGF R1, VEGF R2 appears to be the primary mediator of VEGF angiogenic activity (3, 4). The affinity of VEGF_165b for VEGF R2 is similar to that of VEGF₁₆₅, but VEGF_165b only partially activates VEGF R2 such that the kinase regulatory site Y1054 is not phosphorylated (6). VEGF_165b also does not bind neuropilin-1, suggesting that the functional difference between VEGF₁₆₅ and VEGF_165b maybe due to either the lack of neuropilin-1 co-signaling or unique downstream signaling activated by VEGF_165b (8). Since VEGF_165b may compete with angiogenic VEGFs for VEGF R2 sites, its ectopic expression in tumors has been shown to inhibit their growth (7).