Activins are dimeric growth and differentiation factors which belong to the transforming growth factor-beta (TGF-beta);superfamily of structurally related signaling proteins. 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. Type II receptors are considered to be constitutively active kinases. This gene;encodes activin A type IIB receptor, which displays a 3- to 4-fold higher affinity for the ligand than activin A type;II receptor.