Serine/threonine-protein kinase receptor R3 is an enzyme that in humans is encoded by the ACVRL1 gene. ACVRL1 is a receptor in the TGF beta signaling pathway. It is also known as activin receptor-like kinase 1, or ALK1. This gene encodes a type I cell-surface receptor for the TGF-beta superfamily of ligands. It shares with other type I receptors a high degree of similarity in serine-threonine kinase subdomains, a glycine- and serine-rich region (called the GS domain) preceding the kinase domain, and a short C-terminal tail. The encoded protein, sometimes termed ALK1, shares similar domain structures with other closely related ALK or activin receptor-like kinase proteins that form a subfamily of receptor serine/threonine kinases. Mutations in this gene are associated with hereditary hemorrhagic telangiectasia (HHT) type 2, also known as Rendu-Osler-Weber syndrome 2.