Individuals of the same or closely related species can vary substantially in size. However, the proportions within (and between) tissues are precisely kept. This adaptation of pattern with size, termed scaling, is receiving a growing attention in recent years. I will discuss experimental evidence for scaling during the early development of multicellular organisms, and describe theoretical ideas of what is required for scaling morphogen gradients. In particular, I will discuss the Expansion-Repression mechanism which we recently describe and present experimental evidence for its function in two biological systems: the early frog embryo and the fruit-fly wing disc.
