Fig. 7. Wntch as a transistor for cell fate assignments. Interactions between Wnt and Notch signalling occur at two levels, as indicated by the two colours. A conserved functional network, in which β-catenin activity leads to the expression of the Notch DSL ligands, is observed in many instances, suggesting that β-catenin acts as an element of a network motif (see text for details). Less often, Notch signalling can lead to the activation of Wnt proteins and antagonists, and so can modulate Wnt signalling (dashed line). This transcriptional circuit, or elements of it, can be found in many developmental contexts. In addition, studies in Drosophila have revealed that the two pathways mutually antagonise each other at the level of signal transduction (blue box). The two coloured boxes indicate the two modules that connect the two systems interlocking them into a single functional unit. Interestingly, the blue box has a much shorter time scale than the yellow one, and therefore can influence it. As a whole, the system is a unit that can be `plugged' into different processes.