Activin A is a TGF-β family member that exhibits a wide range of biological activities including regulation of cellular proliferation and differentiation, and promotion of neuronal survival. Elevated levels of Activin A in human colorectal tumors and in post-menopausal woman have been implicated in colorectal and breast cancers, respectively. The biological activities of Activin A can be neutralized by inhibins and by the diffusible TGF-β antagonist, Follistatin. Human Activin A is a 26.0 kDa disulfide-linked homodimer of two β A chains, each containing 116 amino acid residues.