TIE-1 and TIE-2 comprise the cell-surface receptors that bind and are activated by the angiopoietins, (Ang1, Ang2, Ang3, Ang4). The angiopoietins are protein growth factors required for the formation of blood vessels (angiogenesis). The angiopoietins are protein growth factors that promote angiogenesis, the formation of blood vessels. There are now four identified angiopoietins: Ang1, Ang2, Ang3, Ang4. Ang1 and Ang4 function as agonistic or activating ligands for Tie2, whereas Ang2 and Ang3 behave as competitive antagonists. They function by binding their physiologic receptors, Tie-1 and Tie-2. These are receptor tyrosine kinases, so named because they mediate cell signals by inducing the phosphorylation of key tyrosines, thus initiating cell signalling.