Mutual repression buffers against correlated variations in the activator levels.

<p>(<b>A</b>) Time- and circumference-averaged Hb (, solid lines) and Kni (, dashed lines) total copy-number profiles along the AP axis for various morphogen dosage factors . Inset: the corresponding average occupancy of the promoter states with five bound morphogen molecules as a function of . (<b>B</b>) The average Hb boundary position as a function of , the distance between the Hb and Kni boundaries without mutual repression, for the system with mutual repression (green,) and without it (red); is varied by changing the morphogen dosage factor . Note that mutual repression makes the gap-gene expression boundaries essentially insensitive to correlated changes in morphogen levels when . (<b>C</b>) AP profiles of the average standard deviation of the total Hb (, solid lines) and Kni (, dashed lines) copy numbers. Inset: as a function of , where is the average Hb copy number at and its maximum over . The grey dashed line represents the Poissonian limit. (<b>D</b>) The Hb boundary width as a function of with (green) and without (red) mutual repression. For , it was impossible to obtain a reliable error bar on , because of the weak pinning force on the <i>hb</i> and <i>kni</i> expression boundaries.