Positional Information-A concept underpinning our understanding of developmental biology.

It is now 50 years since Lewis Wolpert published the paper in which he set out the concept of Positional Information to explain how spatial patterns of cellular differentiation are generated. This concept has provided a universal model for pattern formation in embryonic development and regeneration and become part of the fabric of the field of developmental biology. Here I outline how Wolpert devised the concept of Positional Information and describe landmark studies from his lab investigating how Positional Information is specified in the developing chick limb.