In the early chick embryo, the dorsal-ventral (DV) boundary organizes the apical ectodermal ridge (AER) structure in the limb bud field. Here it is reported that Engrailed-1 (En-l), a homolog of the Drosophila segment polarity gene engrailed expressed in the ventral limb ectoderm, participates in AER formation at the DV boundary of the limb bud. Restricted ectopic expression of En-1 in the dorsal side of the limb bud by transplantation of En-1-overexpressing ectoderm induces ectopic AER at the boundary of En-1-positive and -negative cells. The results suggest that En-1 is involved in AER formation at the DV boundary of the limb bud.