THE product of the Delta gene, acting as ligand, and that of the Notch gene, acting as receptor, are key components in a lateral-inhibition signalling pathway that regulates the detailed patterning of many different tissues in Drosophila^1–8. During neurogenesis in particular, neural precursors, by expressing Delta, inhibit neighbouring Notch-expressing cells from becoming committed to a neural fate^5,9,10. Vertebrates are known to have several Notch genes ^11–14, but their functions are unclear and their ligands hitherto unidentified. Here we identify and describe a chick Deltahomologue, C-Delta-1. We show that C-Delta-1 is expressed in prospective neurons during neurogenesis, as new cells are being born and their fates decided. Our data from the chick, combined with parallel evidence fromXenopus¹⁵, suggest that both the Delta/ Notch signalling mechanism and its role in neurogenesis have been conserved in vertebrates.