During vertebrate embryogenesis, a left-right axis is established. Secreted growth factors of the TGF-beta family, including gene products derived from nodal, lefty-1 and lefty-2, play crucial roles in establishing left-right asymmetries. TGF-beta (Transforming growth factor-beta) is a pleiotropic cytokine that regulates growth and differentiation of diverse types of cells. TGF-beta actions are directed by ligand-induced activation of TGF-beta receptors. Complexes formed move into the nucleus, where they act as components of a transcriptional complex. Lefty, a novel member of the TGF-beta superfamily, inhibits TGF-beta signaling. Lefty acts to inhibit phosphorylation of Smad2 following activation of the TGF-beta receptor. Lefty also inhibits events downstream from R-Smad phosphorylation. Lefty provides a repressed state of TGF-beta-responsive genes. The Lefty family is comprised of Lefty 1 and Lefty 2 in mouse, and Lefty A and Lefty B in humans. Members of the TGF-beta superfamily require processing for their activation. Cleavage is therefore an essential step for Lefty activation. Lefty is synthesized as a large inactive precursor (42kD) that must be endoproteolytically processed to release the bioactive polypeptide (28kD and 34kD forms). The 28kD form induces MAPK activity.