Amino acid residue differences in peptide sets and kingdoms. Graphics shows the differences in percentage points versus the overall residue frequencies in the respective kingdom (cf. Additional file 1). All peptides sets contain 100 peptide patterns, except ORP-A for which only 54 and 6 patterns passed the filtering step for the Swiss-Prot and genome datasets, respectively (cf. Table 3). Green indicates that the residue is more abundant than background, while red indicates less abundant than background. The difference in percentage points is shown by color intensities according to the scale.