Serine/threonine-protein kinase receptor R3 is an enzyme that in humans is encoded by the ALK1 gene. ALK1 is a receptor in the TGF beta signaling pathway. ALK1 protein is a receptor in the TGF beta signaling pathway. It plays an important role in vascular development, remodeling, and pathologic angiogenesis, play a role in stabilizing angiogenic vessels and contribute to resistance to anti-VEGF therapies, ALK1 blockade may represent an effective therapeutic opportunity complementary to the current antiangiogenic modalities in the clinic. Recently, researcher found that, ALK1-Fc inhibited BMP9-mediated Id-1 expression in human umbilical vein endothelial cells and inhibited cord formation by these cells on a Matrigel substrate, in a chick chorioallantoic membrane assay, ALK1-Fc reduced vascular endothelial growth factor-, fibroblast growth factor-, and BMP10-mediated vessel formation, and ALK1-Fc treatment reduced tumor burden in mice receiving orthotopic grafts of MCF7 mammary adenocarcinoma cells.