Binds directly to activin and functions as an activin antagonist which plays a role in neural induction. The short isoform is a more potent inhibitor of activin than the long isoform. Specific inhibitor of the biosynthesis and secretion of pituitary follicle stimulating hormone (FSH). Monomer. Spemann organizer and notochord. The short isoform is present maternally while the long isoform is expressed at gastrula stages. Detected in a few cells of the Spemann organizer at the onset of gastrulation. During gastrulation expression continues in the prechordal plate and the anterior portion of the notochord anlage. Beginning at early neurula stages, expression is initiated at new sites in the head mesoderm; hypochord; pronephros; eyes; fore-, mid-, and hindbrain; and the midbrain-hindbrain junction. By activin.