Activin belongs to the TGF-β protein superfamily, is produced in the gonads, pituitary gland, placenta, and other organs. Activin is a dimer composed of two identical or very similar beta subunits. In mammals, two activin βA subunits give rise to activin A. Activin binds to the Type II receptor and initiates a cascade reaction that leads to the recruitment, phosphorylation, and activation of Type I activin receptor. This then interacts with and then phosphorylates SMAD2 and SMAD3. Activin enhances FSH biosynthesis and secretion, and participates in the regulation of the menstrual cycle. Many other functions have been found to be exerted by activin, including roles in cell proliferation, differentiation, apoptosis, metabolism, homeostasis, immune response, wound repair, and endocrine function.