The ability of each set of ESEs to predict trends in relative synonymous codon usage. We plot the difference in HPI versus the difference in slope of codon usage, as one approaches a boundary for all pairs of synonymous codons. A negative slope implies a codon is enriched near boundaries. Thus we expect those codons with a high HPI to have a more negative slope. Thus we expect a large difference in HPI to be reflected in a large negative difference in slope between two synonymous codons.