An observed/expected ratio <1 indicates negative selection for NXS sequons. T-sequon, total NXS sequons; C-sequon, NXS sequons in the C-terminal region (70 ± 5 residues). The C-sequon density (NXS sequons/1,000 residues) for the C-terminal region was calculated from linear regression fits of plots of NXS sequons versus C-terminal distance. Expected sequon densities were calculated using the amino acid composition of the glycoproteins from each species. The density (glycans/1,000 residues) of NXS glycans in the C-terminal segment (70 ± 5 residues) of proteins was determined by linear regression of plots of NXS glycopeptides versus C-terminal distance. C-glycans, C-terminal glycans; T-glycans, total glycans. The apparent modification frequency was calculated by dividing the glycan density by the sequon density. The apparent modification frequency underestimates the actual percent modification caused by incomplete detection of all glycopeptides by mass spectrometry.