Reaction-Diffusion Patterning Six decades ago, Alan Turing proposed the reaction-diffusion model to explain how complex patterns emerge during morphogenesis. In this model, an activator activates both itself and an inhibitor (the “reaction”), with the activator being less mobile than the inhibitor (“diffusion”). Many activator/inhibitor pairs have been described, but the role of differential mobility versus differential clearance of activators and inhibitors is often unclear. Now, Müller et al. (p. 721 , published online 12 April) show that during zebrafish embryogenesis, the activator Nodal has lower diffusivity than the inhibitor Lefty, while both molecules are cleared at similar rates, which supports the idea that Nodal and Lefty form a classical reaction-diffusion system.