Fig. 7. Two steps in the generation of robust left-right (LR) asymmetry in the lateral plate mesoderm (LPM). In this model, robust LR asymmetry (asymmetric Nodal expression) in the LPM is generated in two steps. (A,B) First, nodal flow generates a small difference (pink) between the two sides of the LPM. (C) This small difference is then converted to a robust asymmetry by a reaction-diffusion (RD) system comprising Nodal and Lefty proteins. (D) Photomicrograph illustrating this asymmetry. A RD system requires that both an activator and inhibitor travel over a long distance and that an inhibitor diffuses faster than an activator does. Circumstantial evidence supports that Nodal and Lefty fulfill these requirements (Chen and Schier, 2001; Chen and Schier, 2002; Meno et al., 2001; Sakuma et al., 2002).