– Activin proteins that belong to the transforming growth factor-beta (TGF-β) superfamily, exert their biological actions by binding to heteromeric receptor complexes of type I and type II serine/threonine kinase receptors. On ligand binding, type I and II receptors form a stable complex, resulting in phosphorylation of type I receptors by type II receptors with constitutive kinase activity, and subsequently initiates the activation of downstream molecules including the endogenous Smads. ActRIIB, also known as ActRIIB, is a type II receptor containing an extracellular domain (ECD), a transmembrane segment, and a cytoplasmic region that includes the kinase domain. ActRIIB is a receptor for activin A, activin B and inhibin A. Multiple ActRIIB isoforms can also be generated, which bind activin isoforms with different affinities.