Epitope I is produced in sufficient amounts by the proteasome, whereas epitope IV requires postproteasomal trimming. Shown are the relevant cleavage products detected in standard proteasome digestions of T-Ag_196-221 (A) and T-Ag_398-417 (B) in three independent experiments. Irrelevant peptide-products are indicated in black, and the epitope and epitope precursor peptides are indicated in red and blue, respectively. For pIV, peptide precursors for an alternative epitope with methionine on the C terminus are shown in green. Thickness of the cleavage products indicates relative cleavage intensity.