Binds directly to activin and functions as an activin antagonist. Inhibits activin A signaling in the iris and regulates somatostatin phenotype in ciliary ganglion neurons. Specific inhibitor of the biosynthesis and secretion of pituitary follicle stimulating hormone (FSH). Monomer. Ciliary ganglion neurons. Levels are higher in the iris than the choroid. Levels increase in the iris from embryonic day 9 (E9) to E16 in contrast to the choroid where it remains low relative to iris. During early hindbrain development strongly expressed in rhombomeres R2, R4, R5 and R6 but not in R3. Expression in R3 is seen at later stages and is dependent on neighboring interactions.