Angiopoietin 1 and Angiopoietin 2 are important for development of the endothelium, by regulating tyrosine phosphorylation of the membrane receptor Tie 2. Angiopoietin 2 is only 60% homologous with Angiopoietin 1. Angiopoietin-2 is a naturally occurring antagonist of angiopoietin-1 that competes for binding to the TIE2 receptor and blocks ANGPT1-induced TIE2 autophosphorylation. Angiopoietin 1 binding to Tie 2 causes phosphorylation of the receptor. Angiopoietin 2 competes for this binding, and thus blocks receptor phosphorylation. Angiopoietin 2 expression occurs at sites of vascular remodelling: dorsal aorta and major aortic branches, ovary, placenta and uterus.