Fig. 7. Quantitative model for contribution of CtBP activity to repression by Knirps. Protein levels of Knirps (horizontal axis) are plotted against differential levels of repressor activity (vertical axis at top). With CtBP, Knirps repression levels increase more sharply with increasing protein levels, allowing the activity to cross critical thresholds at lower protein levels. The position of the Knirps protein levels in the embryo (lower part of figure indicated by % egg length) then dictates where appropriate stripe boundaries will form (vertical broken lines). This model predicts that, owing to the inherently high threshold of the eve stripe 4/6 enhancer, loss of CtBP activity will move the intercept off of the range of physiological Knirps concentrations, while having little effect on the stripe 3/7 position.