We investigate a mechanism for the robust identification of the center of a developing biological system. We assume the existence of two morphogen gradients, an activator emanating from the anterior, and a corepressor from the posterior. The corepressor inhibits the action of the activator in switching on target genes. We apply this system to Drosophila embryos, where we predict the existence of a hitherto undetected posterior corepressor. Using mathematical modeling, we show that a symmetric activator-corepressor model can quantitatively explain the precise midembryo expression boundary of the hunchback gene, and the scaling of this pattern with embryo size.