Fig. 2. Extracellular regulation of TGFß ligand family activity. Ligand activity can be masked in latent complexes and activated by the action of extracellular metalloproteases in the BMP1/Tolloid family (Massague, 1998; Dale, 2000; Ashe, 2005). Activity can be modulated both positively and negatively by cell surface proteins, as discussed in the text. This figure provides a snapshot of systems discussed at the meeting, and is not intended to be a comprehensive review. Abbreviations: Sog, Short gastrulation, Tsg, Twisted gastrulation