The vascular endothelial growth factor (VEGF) receptor (VEGFR-1, Flt-1) is a 180 kDa receptor tyrosine kinase belonging to the VEGFR (Flt) family (1-3). The receptor is comprised of seven extracellular Ig-like domains, a single transmembrane region and cytoplasmic tail containing the active kinase domain (1,2). VEGFR-1 plays an important role in endothelial cell function and normal vascular development, as well as in hematopoietic function (2,3). VEGF-A is a high affinity ligand of VEGFR-1. VEGFR-1 also binds VEGF-B and PLGF (2). Ligand binding results in very little VEGFR-1 kinase activation, and VEGFR-1/VEGF-A binding negatively regulates VEGF function by diverting the growth factor from other functional VEGF receptors (3).Two forms of the VEGF receptor 1 are found in cells. Both the membrane-bound form described above and a soluble isoform of VEGFR-1 (sVEGFR-1 or sFlt-1) bind the VEGF ligand with high affinity (4). Full-length VEGFR-1 and the truncated, soluble protein are encoded by the same gene and are generated through differential splicing. Both proteins are associated with an array of human disorders and are potential candidates for therapeutic study (5).