Since the divergence of opossum from the other 11 species, there appears to have been a shift in Asn and His, such that their frequency is greatest in the opossum (Figure 1). Rat α-LA is richest in Val (5.7%, compared with primates, who had a much lower frequency of 1.6%). Aside from rat and mouse, in general, we did not observe dramatic changes between closely related species, with divergence in AA composition typically increasing with phylogenetic distance (Figure 1).