Vertebrate limb outgrowth requires a structure called the apical ectodermal ridge, formation of which follows the previous establishment of the dorsoventral limb axis. Radical fringe is expressed in the dorsal ectoderm before the ridge appears, and is repressed by Engrailed-I, which is expressed in the ventral ectoderm. Misexpression of these genes indicates that a ridge is formed wherever there is a boundary between cells expressing and not expressing Radical fringe, Thus, as in Drosophila, Radical fringe positions the ridge at the dorsoventral limb boundary.