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ACVR1 (activin receptor type I), also known as ALK2 or ACTRI, is a receptor for activin. It forms a stable complex with type II receptor after ligand binding. These receptors are all transmembrane proteins, composed of a ligand-binding extracellular domain with cysteine-rich region, a transmembrane domain, and a cytoplasmic domain with predicted serine/threonine specificity. Type I receptors are essential for signaling, and type II receptors are required for binding ligands and for expression of type I receptors. ACVR1 is expressed in many tissues including skeletal muscle and chondrocytes. It functions as a receptor for bone morphogenetic protein (BMP) and induces Indian hedgehog in chondrocytes during skeletal development. Mutations in ACVR1 gene are associated with fibrodysplasia ossificans progressive (PMID: 16642017).