Binds directly to activin and functions as an activin antagonist. Specific inhibitor of the biosynthesis and secretion of pituitary follicle stimulating hormone (FSH). Monomer. Embryonic expression first occurs in the primitive streak, followed by expression in head mesoderm, somites, and specific rhombomeres of the hindbrain, and later in midbrain and diencephalon. No expression is seen in the node or notochord. Induces muscle hypertrophy when injected into the tibialis anterior muscle with reduced response in older mice.