Regulatory circuits of the anterior-posterior axis in a Drosophila egg. The bicoid and nanos mRNAs are localized near the anterior and posterior poles of the egg, respectively. The caudal, hunchback, and pumilio mRNAs are distributed throughout the cytoplasm. Gradients of Bicoid and Nanos proteins lead to accumulation of Hunchback protein in the egg’s anterior region and Caudal protein in its posterior. Because Pumilio requires Nanos for its activity as a translational repressor of hunchback mRNA, Pumilio functions only at the posterior end.