TIE-1 （tyrosine kinase with Ig and EGF homology domains 1） and TIE-2/Tek comprise a receptor tyrosine kinase （RTK） subfamily with unique structural characteristics: two immunoglobulin-like domains flanking three epidermal growth factor （EGF）-like domains and followed by three fibronectin type III-like repeats in the extracellular region and a split tyrosine kinase domain in the cytoplasmic region. These receptors are expressed primarily on endothelial and hematopoietic progenitor cells and play critical roles in angiogenesis， vasculogenesis and hematopoiesis. Human TIE-1 cDNA encodes a 1124 amino acid （aa） residue precursor protein with an 18 residue putative signal peptide， a 727 residue extracellular domain and a 354 residue cytoplasmic domain. Whereas two ligands have been described for TIE-2 [angiopoietin-1 （Ang1） and angiopoietin-2 （Ang2）]， so far no ligand was found for TIE-1.
