Binds directly to activin and functions as an activin antagonist. Specific inhibitor of the biosynthesis and secretion of pituitary follicle stimulating hormone (fsh) (By similarity). Inhibits bmp-signaling during later stages of development including late phases of dorsoventral patterning, to refine the early pattern set up by the interaction of chordino and bmp2/4. Not involved in organizer function or early phases of dorsoventral pattern formation. Monomer. Not expressed in the organizer region. Expression in gastrulating embryos is confined to anterior and paraxial regions, which give rise to head mesoderm and the first five somites. In addition, expressed transiently in a subset of cells in the posterior notochord anlage. Later, expression is seen in brain, eyes and somites. First expressed at mid-gastrulation.