During development, cells can affect their neighbours by secreting factors called morphogens. In the fruitfly, one such factor — Sog — affects signalling by another morphogen, Dpp. But how? A clever study of Sog expression indicates that this protein can inhibit Dpp signalling in cells that are close to its source, yet stimulate Dpp in those cells that are further away. This is the first evidence for such a split personality
