Fig. 2. Reaction-diffusion mechanism for the generation of positional information. The generation of a stable Nodal signaling gradient (shown in blue) across a developmental field can result from a source of Nodal signals (at left) that undergo positive autoregulation and act at long-range (blue arrows). The expression of Lefty inhibitor (red) is also induced by the Nodal pathway, and has a greater range than Nodal signals. Cells in close proximity to the Nodal source thereby perceive high levels of signaling activity, whereas more distant cells perceive little or no signaling activity, as lateral inhibition by Lefty will prevail over a longer range. Such a regulatory mechanism for Nodal pathway activity may function during mesendoderm specification and left-right patterning. [Adapted from Branford and Yost (Branford and Yost, 2004).]