Fig. 5. A model for positive feedback. Initially BMP ligands, such as Dpp and Scw, bind to the type I and type II signaling receptors. This signal activates transcription of a cell surface BMP-binding protein (sbp), such as Cv-2, that helps to present ligand to the signaling receptors. This may account for the production of spatial bi-stability, as proposed by Ferguson and Wang (Ferguson and Wang, 2005). P, phosphorylation. The question mark indicates that the identity of this component is not yet established in the embryo.