To see how a region acts when viewed as a whole, we also calculate the average selection acting on double coding regions, by weighting the expected counts for each mutation by the appropriate selection coefficient -in the case of a single non-synonymous change in gene A or B by the factor f A and f B respectively, and in the case of two nonsynonymous changes by the joint factor f AB . Table 1 shows the values obtained for the different regions, both single and double coding. We can see that when viewed like this, the double coding regions are on average under 0.41 selection, and thus not greatly different to the single coding ones at an average of 0.39.