Tie-1/Tie and Tie-2/Tek are receptor tyrosine kinases with unique structural characteristics including two immunoglobulin-like domains flanking three epidermal growth factor (EGF)-like domains, followed by three fibronectin type III-like repeats in the extracellular region, and a split tyrosine kinase domain in the cytoplasmic region. Tie-2 is involved in vascular stabilization and remodeling. Although less well understood, Tie-1 may also act as an ANG receptor, possibly in complex with Tie-2. Human Tie-2 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. Tie-2 is a receptor for the angiopoietin (ANG) family: ANG-1, ANG-2, and ANG-3 (mouse)/-4 (human). Ang-2 has been reported to act as an antagonist for Ang-1. Mice engineered to overexpress Ang-2 or to lack Ang-1 or Tie-2 display similar angiogenesis defects.