(Upper left) Theoretical predictions (yellow) illustrating how the rate of mRNA production from both enhancers, , varies with the sum of the activity of the individual enhancers, (yellow). Mean number of Pol II molecules transcribing per nucleus (NPol II) is proportional to the rate of mRNA production. The green line shows perfect additivity for comparison. The model predicts additive behavior () when the rate of production is low and sub-additive behavior ( as the production rate increases. As the interaction strength of individual enhancers increases so does the rate of mRNA production, but the combined activity of both enhancers becomes sub-additive. (Upper right, lower left, lower right) Transcriptional activity of intact loci vs the sum of activities of individual enhancers for hb, kni, and sna at different times. A green line has been drawn in to indicate where is equal to . For hb and kni, the plots show data taken at different AP positions at 10 min into nc 13 and 20 min into nc 14, respectively, while for sna the datapoints were at different times. Ellipses indicate standard error of the mean.