On p. 5929 Estella et al. shed new light on how the morphological distinction between ventral and dorsal structures is achieved during development. In Drosophila, legs and antennae develop from the ventral imaginal discs, and Estella and co-workers report that the product of a single gene called buttonhead(btd) can trigger the entire genetic network needed for leg and antennal development. RNAi downregulation of btd, which encodes a zinc-finger transcription factor, together with an adjacent related transcription factor, Sp1, resulted in underdeveloped legs and antennae. Conversely, when btd was ectopically expressed in dorsal imaginal discs, the normal products of these discs – eyes, wings and halteres – were replaced by antennae and legs. This transformation involved the de novo activation of the engrailed/hedgehog/wingless/decapentaplegiccascade responsible for the growth and pattern of ventral imaginal discs.