Variable symmetry in amino acid abundance trends comparing 5' and 3' exon ends within species. Intraspecific correlations between the 5' (x-axis) and 3' (y-axis) slopes as extracted from individually fitted linear models considering all 23 amino acid groupings are shown. Approximately symmetric arrangements are particularly evident for some species (notably vertebrates) whereas nematode arrangements (Ce, Cb) are not symmetric. Further notable is the higher variability of slope coefficients in some species (vertebrates and nematodes) vis-à-vis others (Am, At). Amino acids are represented by their one letter code (two-fold blocks are denoted by '2'). The regression lines are from SMA regressions. Lines were not fitted for Arabidopsis, Cryptococcus and S. cerevisiae given concerns about the adequacy of this technique for these datasets (see Materials and methods). For associated statistics consult Table 5.