Box plots of the AT3 of codons of lowly and highly expressed genes within narrow bins of AT-I, and thus presumably having similar background mutational pressures. Genes were binned into categories with similar AT-I content to ascertain differences in AT3 with respect to expression. Different letters in each pair of bars indicates P < 0.05 using MWU-tests. No statistically significant differences in AT-I were observed between highly and lowly expressed genes for any bins (MWU-test P > 0.30; with the exception of a minor AT-I difference in medians of 0.019 for category 3 (0.6–0.7)). *AT3 for this bar is statistically significant from all other bars. Only one gene had AT-I > 0.8 for lowly expressed genes and thus the bar for this category was excluded.