Activin B is the cytokine of the TGF-beta family, which is a 13 kDa protein containing 115 amino acid residues. Activin B has the wide variety of biological function including stem cell differentiation, inflammation, bone remodeling and neural development. They also have a role in regulation of FSH secretion. Activin B induces the phosphorylation of Smad which binds the smad binding element.