Comparison of amino acid residue frequency in tryptic peptides from lung tissue proteome identified by electrospray mass spectrometry ('Identified tryptic proteome') and in the in silico trypsin digested combined human and mouse proteomes ('Theoretical tryptic proteome'). Residue frequencies are expressed as a fraction of the analyzed proteome (e.g. serine, S, comprises 8.5% of the theoretical tryptic proteome versus 6.8% of the identified tryptic proteome). Note that the occurrence of arginine (R) and lysine (K) are artificially reduced in the Identified tryptic proteome because non fully-tryptic peptides (i.e. peptides with residues other than R or K at their C-termini) form a significant fraction of peptides identified by SEQUEST.