VEGFR-3, also known as FLT4, is a member of the Tyr protein kinase family. The extracellular portion of VEGFR-3 contains 7 immunoglobulin (Ig)-like domains and the cytoplasmic portion contains a protein kinase domain. FLT4 regulates angiogenesis and lymphangiogenesis, its ligands are VEGF-C and D and its binding is mediated by the 2nd and 3rd Ig-like domains of FLT4. During fetal development VEGFR-3 is expressed on endothelial cells, however, in the adult mice, the vascular endothelial cells lose VEGFR-3 expression, but the lymphatic endothelium expresses it constitutively. In addition, VEGFR-3 expression can be induced in tumors with active angiogenesis.