Wnt7a, in turn, is restricted to dorsal ectoderm by En1 in the ventral ectoderm (Loomis et al. 1996; Cygan et al. 1997; Logan et al. 1997), whose expression marks a lineage boundary coincident with the dorsoventral midline of the apical ectodermal ridge (Altabef et al. 1997; Michaud et al. 1997; Kimmel et al. 2000).