Despite the remarkable conservation of the Hedgehog (Hh) signalling pathway through evolution, Costal2 (Cos2), a vital component of this pathway in flies,has escaped detection in vertebrates. But now, Tay et al. report the identification of the first vertebrate homologue of Cos2, which they cloned from zebrafish by virtue of its sequence similarity to fly Cos2 (see p. 625). In morpholino knockdown experiments, the authors show that decreased cos2 levels cause ectopic Hh signalling, as expected from its role in flies where this kinesin-related protein is thought to assemble a protein complex, ultimately leading to the Cubitus interruptus (Ci)-mediated repression of Hh target genes. In vitro, zebrafish Cos2 can complex with Gli1, the fly homologue of Ci. Together with the mouse and human Cos2-like sequences reported here, these findings should shed new light on the mechanics of Hh signalling in vertebrates.