Angiopoietin-related protein 1 (Angiopoietin-3) is a secreted protein with homology to human angiopoietin-1 and human angiopoietin-2. Angiopoietin-3 mRNA is expressed in lung and cultured human umbilical vein endothelial cells. VEGF slightly increases Angiopoietin-3 mRNA expression in cultured human umbilical vein endothelial cells. Angiopoietin 3 (ANG3), also called ANG4, might represent counterparts in mouse and human. Based on chromosomal analysis, mouse ANG3 is thought to be an ortholog of human ANG4. Human ANG4 acts as an agonist that binds and activates Tie2, whereas mouse ANG3 is likely to be a Tie2 antagonist, exerting agonist or antagonist activities depending on the cell context.