(A) Linear signal processing preserves information, whereas nonlinear signal processing does not. This can be seen by rescaling the gray-scale values in this image of Schrödinger with nonlinear signaling (top) and linear signaling (bottom). The nonlinear approach leads to saturation at high input levels, which means that some information is irretrievably lost, so the original image cannot be retrieved by reversing the rescaling process. The linear approach avoids these problems because the outputs are equal to the inputs, so information is not lost (Yu et al., 2008). The histograms below the images show the distribution of brightness values in the images. (B) The three linear signaling mechanisms that we are aware of. I) A mechanism that combines an ultrasensitive amplifier with negative feedback; this is used by EGF-ERK. II) A mechanism that modulates an unsaturated or unsaturable cycle: in this case the cycle involves protein synthesis (to form A), degradation (to produce Ø), resynthesis (to form A again), and so on: this mechanism is apparently used by the canonical Wnt and TGF-β pathways. III) A push-pull mechanism in which the active form of one protein (A) stimulates downstream activity (b → B) while the nominally inactive form (a) acts to reduce downstream activity (B → b). The signaling pathways examined by Nunns and Goentoro – the canonical Wnt, EGF-ERK and TGF-β pathways – appear to use the first two mechanisms to relay information in a linear manner.