The apical ectodermal ridge of the vertebrate limb bud lies at the junction of the dorsal and ventral ectoderm and directs patterning of the growing limb. Its formation is directed by the boundary between cells that do and cells that do not express the gene Radical fringe. This is similar to the establishment of the margin cells at the Drosophila wing dorsoventral border by fringe. Radical fringe expression in chick-limb dorsal ectoderm is established In part through repression by Engrailed-1 in the ventral ectoderm.