THE determination of specific cell fates and polarity within each segmental unit of the Drosophila embryo involves the products of the segment polarity genes1. One of these, wingless (wg), encodes a secreted protein 2,3 that is homologous to the mammalian protooncogene Wnt-1 (refs 4, 5). In the embryonic epidermis, wg is expressed in a single row of cells within each segmental unit, although its activity is required for the correct patterning of most of the epidermis4,6. Initially Wg signals to adjacent posterior cells, maintaining engrailed (en) expression7,8. Later during embryogenesis, wg specifies the differentiation of naked cuticle9. Wg signalling functions by inactivating or antagonizing the activity of zeste-white 3 (zw3)10. We have investigated the requirement in the Wg signal transduction pathway for the three genes armadillo (arm)11,12, dishevelled (dsh) and porcupine (porc)13, all of which have embryonic mutant phenotypes similar to wg. Our results indicate that dsh and porc act upstream of zw3, and arm acts down-stream of zw3.