Endothelial cells express three different vascular endothelial growth factor （VEGF） receptors， belonging to the family of receptor tyrosine kinases （RTKs）. They are named VEGFR-1 （Flt-1）， VEGFR-2 （KDR/Flk-1）， and VEGFR-3 （Flt-4）. Their expression is almost exclusively restricted to endothelial cells， but VEGFR-1 can also be found on monocytes. All VEGF-receptors have seven immunoglobulin-like extracellular domains， a single transmembrane region and an intracellular split tyrosine kinase domain. VEGFR-2 has a lower affinity for VEGF than the Flt-1 receptor， but a higher signalling activity. Mitogenic activity in endothelial cells is mainly mediated by VEGFR-2 leading to their proliferation. Differential splicing of the flt-1 gene leads to the formation of a secreted， soluble variant of VEGFR-1 （sVEGFR-1）. No naturally occurring， secreted forms of VEGFR-2 have so far been reported. The binding of VEGF165 to VEGFR-2 is dependent on heparin.
