Secreted polypeptide signalling molecules of the TGF- superfamily play critical roles during mouse development. There are 20 known members of this diverse family in the mouse, including the TGF-s, inhibin/activins, BMPs, GDFs, and nodal. Mutations in genes for some of these ligands have been identified or created by targeted mutagenesis. A complex network of binding proteins, proteases, and receptors is involved in the processing, presentation and biological activity of TGF-β-related ligands. It is likely that variations in these modifying factors will explain how different genetic backgrounds can influence the severity and penetrance of mutant phenotypes. © 1995 Academic Press Ltd.