Abstract Since its initial identification in mouse mammary tumors, the proto-oncogene Wnt-1 has been shown to encode a member of a family of putative signalling molecules. Recent work has implicated Wnt-1, and other members of the Wnt gene family, in regulation of a number of basic developmental processes in Drosophila, Xenopus and the mouse. These studies are leading to a new appreciation of the normal and oncogenic actions of Wnt genes.