Activin is a protein that plays a crucial role in regulating various biological processes in the body, including cell growth, differentiation, and reproduction. It is part of the transforming growth factor-beta (TGF-β) superfamily and is produced in many tissues, influencing functions such as inflammation and metabolism. Activin exists in several forms, primarily as Activin A, Activin B, and Activin AB, each composed of different combinations of inhibin subunits. It interacts with specific receptors on cell surfaces, triggering signaling pathways that affect gene expression and cellular responses, making it important in developmental biology and potential therapeutic applications.