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Cellular signaling pathways constitute series of genes widely used in a multitude of molecular processes within an organism. Generally, the activation of these pathways is relatively upstream in the sequence of developmental events leading to a particular phenotype. Because these pathways are used in such an array of developmental contexts, cellular differentiation must rely on the combinatorial effect of multiple pathways and their spatio-temporal regulation. In this way, we can imagine an enormous web of genetic interactions, with specific tissues types taking advantage of a particular "route" through this web.