Individuals of the same or closely related species can vary substantially in size. Still, the proportions within and between tissues are precisely kept. This adaptation of pattern with size termed scaling, is receiving a growing attention. We review experimental evidence for scaling, and describe theoretical models for mechanisms that scale morphogen gradients. We particularly note the Expansion-Repression mechanism, in which a diffusible molecule that positively regulates the morphogen gradient width is repressed by morphogen signaling. The Expansion-Repression circuit provides scaling in a robust manner and is readily implemented by a host of molecular mechanisms. We suggest means for identifying such a circuit in a system of interest.
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