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 and two type II receptors. ACVR1B, also known as activin receptor-like kinase 4 (ALK4), is a type I receptor for activin and plays major roles in cell differentiation, growth arrest and apoptosis. Like another type I activin receptor ACVR1C, ACVR1B can mediate signaling by ligands such as Nodal, Xnr1, GDF-1/3, activin B and activin AB.  In Xenopus embryos, expression of a dominant-negative form of ACVR1B blocked all mesoderm-inducing ligands, while expression of a dominant negative ACVR1C only blocked Nodal and Xnr1 signaling, suggesting that the ACVR1B and ACVR1C possess distinct functions.  At least three isoforms of ACVR1B are known to exist.  This antibody is predicted to have no cross-reactivity to ACVR1 or ACVR1C.