Activin receptor like kinase 1 (ALK1) is an endothelial-specific serine/threonine kinase that belongs to the transforming growth factor beta (TGFb) superfamily. ALK1, which is predominantly expressed in endothelial cells, stimulates Smad1 and Smad5 phosphorylation. ALK-1 is essential for the regulation of vascular endothelial growth factor (VEGF), which plays a role in angiogenesis and the formation of endothelia.