Activin receptor type-1, also known as Activin receptor type I, Activin receptor-like kinase 2, Serine/threonine-protein kinase receptor R1, TGF-B superfamily receptor type I, ACVRLK2 and ACVR1, is a single-pass type I membrane protein. ACVR1 is expressed in normal parenchymal cells, endothelial cells, fibroblasts and tumor-derived epithelial cells. ACVR1 belongs to the protein kinase superfamily. Activins signal through a heteromeric complex of receptor serine kinases which include at least two type I (I and IB) and two type II (II and IIB) receptors. 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. Type I and II receptors form a stable complex after ligand binding, resulting in phosphorylation of type I receptors by type II receptors. ACVR1 signals a particular transcriptional response in concert with activin type II receptors.