Segmentation in the inset embryo is initiated by maternally provided information, which is stored in the developing oocyte. In Drosophila, the genes necessary for this process have been genetically characterized. The anterior segmented region is organized by the bicoid (bcd) gene product. The posterior segmented region is organized by several interacting gene products, among them the oskar (osk) gene product. The first zygotic group of genes, which are thought to respond to the spatial cues provided by the maternal genes, are the gap genes, whose members include hunchback (hb), Krüppel (Kr) and knirps (kni). To elucidate the role played by the maternal genes in expression of the gap gene hb, antibodies were raised against a fusion protein and were used for the cytological localization of the hb gene product in wild-type and mutant embryos. The hb protein is predominantly located in the nucleus. Its spatial expression includes the formation of an anterior-posterior gradient during the early cleavage stages and a strong zygotic expression in the anterior half of the embryo. Analysis of embryos mutant for the maternal genes affecting the anterior-posterior segmentation pattern shows that the formation of the early gradient is controlled by the osk group of genes, whereas efficient activation of the zygotic anterior expression domain is dependent on bcd activity.