The segment polarity network (SPN) as modeled (a) by von Dassow et al. [23, 24] and (b) in our reduction: A single cell is shown together with one of its cell faces and the face of a neighboring cell. All cells contain an identical network. Nodes are elliptical for mRNAs, rectangular for proteins, and hexagonal for protein-protein complexes (whilst a rhombus is used for the dummy node). Triangular-headed edges represent positive influence and round-headed edges represent negative influence between connected nodes. (c) Subgraph of partially reduced Figure 1(a).