Appendage formation is organized by signals from discrete sources that presumably act upon downstream genes to control growth and patterning. The Drosophila vestigial gene is selectively required for wing-cell proliferation, and is sufficient to induce outgrowths of wing tissue from eyes, legs and antennae. Different signals activate separate enhancers to control vestigial expression : first, in the dorsal/ventral organizer through the Notch pathway, and subsequently, in the developing wing blade by decapentaplegic and a signal from the dorsal/ventral organizer. Signal integration must be a general feature of genes like vestigial, that regulate growth or patterning along more than one axis.