The angiopoietins are protein growth factors that promote angiogenesis, the formation of blood vessels from pre-existing blood vessels. There are now four identified angiopoietins: Ang1, Ang2, Ang3, Ang4. In addition, there are a number of proteins that are closely related to angiopoietins (ANGPTL2, ANGPTL3, ANGPTL4, ANGPTL5, ANGPTL6, ANGPTL7). Ang1 and Ang2 are required for the formation of mature blood vessels, as demonstrated by mouse knock out studies.

The TIE receptors are tyrosine kinases, so named because they mediate cell signals by inducing the phosphorylation of key tyrosines, thus initiating the binding and activation of downstream, intracellular enzymes; this process is called cell signalling, and it is the method by which cells are induced to activate or inhibit key regulatory functions. It is somewhat controversial which of the TIE receptors mediate functional signals downstream of Ang stimulation - but it is clear that at least TIE-2 is capable of physiologic activation as a result of binding the angiopoietins. The angiopoietins are the secreted ligands of the TIE family of protein receptor tyrosine kinases.