Activin Receptor-Like Kinase 1 (ALK-1) is a type I cell-surface receptor for the TGF-β superfamily of ligands, which mediates signaling of BMP9 (bone morphogenetic protein) and BMP10. ALK1 signaling is necessary for angiogenesis during embryogenesis, wound healing, and tumor growth. ALK-1 has a high degree of similarity in serine-threonine kinase subdomains, a glycine and serine rich region preceding the kinase-domain, and a C-terminal tail with other activin receptor-like kinase proteins. ALK-1 is mainly expressed in endothelial cells regulating proliferation and migration in vitro and angiogenesis in vivo. Mutations in ALK-1 as well as in endoglin are associated with hereditary hemorrhagic telangiectasia (HHT), suggesting ALK-1 plays a critical role for in the control of blood vessel development or repair.