Activins and Inhibins are TGF-β superfamily cytokines that are involved in tissue morphogenesis and repair, fibrosis, inflammation, neural development, hematopoiesis, reproductive system function, and carcinogenesis. Activin A protects the heart from hypoxic stress and promotes the differentiation of embryonic stem cells into cardiomyocytes. Activins are homodimers or heterodimers of various β subunits (βA, βB, βC, and βE), while Inhibins are heterodimers of a unique α subunit and one of the β subunits. Activin A is a homodimer of two βA chains. Human βA shares 100% amino acid sequence identity with mouse, rat, bovine, porcine, and feline βA. Activin A binds to Activin RIIA which then associates with Activin RIB/ALK-4. Activin A bioactivity is regulated by cell-associated molecules (BAMBI, Betaglycan, and Cripto) and soluble molecules (β2-Macroglobulin, Follistatin, and FLRG).