Many signal transduction mechanisms transmit information from the extracellular space or an intracellular compartment to the nucleus where they influence gene expression. This flow can be viewed metaphorically as the reverse of gene expression where information moves outward from the genome. However, the idea that signal transduction processes convey detailed amounts of information has been recently debated with respect to receptor tyrosine kinases (RTKs). Upon binding to their ligands, RTK molecules dimerize and undergo autophosphorylation on specific tyrosine residues. In turn, these phosphotyrosines and their adjacent sequences specifically recruit downstream signaling molecules via their SH2 domains, causing their subsequent activation. The activation of a single type of RTK thus leads to the activation of multiple intracellular signal transduction pathways. The naı̈ve expectation would be that each pathway would have a distinct function. However, a number of studies have called this presumption into question. For example, a study of mutants of the platelet-derived growth factor receptor RTK led to the surprising conclusion that the downstream pathways are in fact redundant in terms of what genes they activate (