Size distribution of peptides following digestion of the yeast and human proteomes with trypsin. A) The distribution of peptides produced by the theoretical proteome (grey) is compared to that produced in experiment (black). Peptide masses are plotted at unit resolution (Da). The relatively lower numbers of peptides observed between 1600–2000 Da is an artefact resulting from MS runs being programmed to detect only ions with m/z in the range 400–1600. Identification of peptides with mass greater than 1600 is possible with ions of charge greater than one. B) Distribution of valine-, cysteine-, and tryptophan-containing peptides in the human proteome.