Mass histograms. (a) Average molecular weight of unique peptide sequences in PeptideAtlas (solid filled bars) and the in silico tryptically digested SGD protein database (hashed bars) allowing one missed cleavage. (b-d) Mass histograms of spectra, separated by charge. The large number of peptides with masses less than 1,000 Da are difficult to identify in database searches, and hence are not present in the PeptideAtlas. CID, collision-induced dissociation.