Roles of Notch signalling during development. (a) The best known role of Notch in development is in a process known as lateral inhibition. This is a conserved developmental mechanism by which a particular fate (navy blue cell) is adopted by one or two cells from a larger group that all have the potential to adopt the fate (mid-blue cells). As the selected cells adopt their fate (represented by the darkening of the shade of blue), they emit an inhibitory signal that prevents neighbouring cells from adopting the same fate (represented by lightening of the blue shade until it is white). The DSL/Notch/CSL signalling pathway transduces the lateral inhibition signal. (b) In addition to its role in lateral inhibition, CSL-dependent Notch signalling also plays an important role in defining the two cell fates that arise from an asymmetric cell division. Typically, the inheritance of the Numb protein (green) within one of the two daughter cells inhibits Notch signalling within that cell. As a consequence, one daughter cell receives a Notch signal while the other does not, causing the two cells to adopt different fates. (c) CSL-dependent Notch signalling has also been shown to play a role in the formation of a boundary (orange cells) between two different populations of cells (green and blue cells). In this situation, Notch signalling (arrows) is restricted to the cells at the interface between the two populations of cells, causing them to adopt a boundary cell fate.