Fig. 1. Sketch of the model for the spatiotemporal evolution of ERES. A basic event in the simulation is the attachment of particles (e.g. COPII complexes) from the cytoplasm (blue) to the ER membrane (red), either to any place on the membrane (`free binding') or with a preference to patches near to pre-existing clusters (`biased binding'). While on the membrane, particles diffuse and are able to fuse to form larger clusters when touching each other. Particles can also detach from the membrane after hydrolysis of GTP. Upon reaching a certain size, clusters can pinch off vesicles (green). See main text and Materials and Methods for further details.