Figure 1. (a) Lateral inhibition. In wild-type embryos, clusters of cells or ‘equivalence groups’ (orange) first share the same potential to adopt a neural fate. As the embryo develops, one cell in each cluster (red) inhibits the neural fate potential of the neighbouring cells (light orange). The ‘winning cell’ will then become a neuroblast (dark red). In Notch mutant embryos, lateral inhibition does not occur; therefore, all cells adopt a neural fate. (b) Example of an inductive role of Notch. The Drosophila wing develops from a larval epithelial sac called wing imaginal disc. In response to gene expression differences, the wing disc divides into four compartments: anterior (A), posterior (P), dorsal (D) and ventral (V). Along the dorsoventral border of the disc, Notch is activated in ventral cells by dorsally expressed Serrate and in dorsal cells by ventrally expressed Delta. This inductive role of Notch will give rise to the specialized cell types of the wing margin.

Figure 2. (a) Structural domains of the Notch receptor. Notch is a large, modular protein, with 36 EGF repeats in its extracellular portion, to which its ligands bind. Several other domains along its length are important for effectors recognition, signal transduction and turnover. (b) Maturation of Notch. Drosophila Notch protein undergoes a complex cleavage during plasma membrane delivery and maturation. Three sites for cleavage have been identified (S1, S2 and S3). After S3 cleavage, the intracellular domain of Notch travels to the nucleus where it acts as a transcription factor together with Su(H).