Limb development requires the formation of a proximal-distal axis perpendicular to the main anterior-posterior and dorsal-ventral body axes. The secreted signalling proteins Decapentaplegic and Wingless act in a concentration-dependent manner to organize the proximal-distal axis. Discrete domains of proximal-distal gene expression are defined by different thresholds of Decapentaplegic and Wingless activities. Subsequent modulation of the relative sires of these domains by growth of the leg is required to form the mature pattern.