THE process of body prepatterning during Drosophila blastoderm formation relies on the localized activities of zygotic segmentation genes, which are controlled by asymmetrically distributed maternal determinants^1,2. The anterior determinant bicoid, a homeodomain transcription factor^3,4, forms an anterior-to-posterior concentration gradient^1-4. It interacts with the maternal transcription factor hunchback⁵ to activate the anterior zygotic patterning genes, including the central gap gene Krüppel (Kr)⁶. In contrast, the posterior maternal system^1,2 does not provide such a decisive transcription factor, but rather prevents the repressor hunchback from acting in the posterior half so that the gap genes giant (gt) and knirps (kni) are activated by an as yet unknown transcription factor^2,7. Here we show that caudal, a conserved homeodomain protein that forms a posterior-to-anterior concentration gradient^8,9, and the anterior determinant bicoid cooperate to form a partly redundant activator system in the posterior region of the embryo.