The proper functioning of tissues and organs requires that each cell differentiate appropriately for its position. In many cases, the positional information that instructs cells about their prospective fate is conveyed by a morphogen gradient. Morphogens are signaling molecules that are produced in a restricted region of a tissue and move away from their source to form a long-range concentration gradient. Cells differentiate in response to morphogen signaling depending on their position within the gradient and thus on their distance from the morphogen source. Two criteria have gained acceptance as the evidence needed to qualify a secreted signaling protein as a morphogen: concentration dependence and direct action at a distance. The Wingless, Hedgehog (Hh), and Dpp gradients in the Drosophila wing and leg imaginal discs fulfill both criteria (for literature, see