A sensitive measure of repression

esearchers have proposed that complex embryonic patterns are generated by multiple enhancers that respond differentially to gradients of repressor proteins. In a recent report, Dorothy Clyde, Dmitri Papatsenko, Stephen Small, and colleagues (New York University, New York, NY) combine genetics and bioinformatics to demonstrate that this model may be essentially correct. The pattern of the fly embryo is influenced by combinations of enhancers and repressors that control expression of the pair rule gene eve in seven stripes along the anterior–posterior axis, which foreshadow the segmented body plan of the adult fly. Two enhancers, eve 3 7 and eve 4 6, control two stripes each. Ectopic expression of one of two repressor proteins that normally occur in opposing gradients, hunchback (Hb) or knirps (Kni), uniquely alters eve expression in different stripes. Thus, the enhancers respond differently to the same concentration of the repressors. Using computational approaches to analyze the Kni and Hb binding sites in the two enhancers, Clyde et al. found R