Figure 1

Induction of engrailed (en) expression by Wingless (Wg) protein at the anterior parasegment boundary in Drosophila. The wg gene is expressed at the posterior parasegment boundary and the protein diffuses to neighboring cells. Only the cells on the posterior side respond by expressing en, however, as they are within the en-competent domain. Ectopic wg expression will induce en expression only within this domain.