Peptide processing, HLA binding, and TCR recognition. The proteasome degrades proteins to produce peptide fragments, which are transported from the cytosol into the endoplasmic reticulum (ER) by the transporter associated with antigen processing (TAP) complex. Inside the ER, peptides bind to HLA class I molecules, which are then transported to the cell surface where the HLA/peptide structure is recognized by TCRs. The peptide residues important for HLA binding are indicated in pink and yellow, and the mutated residue is indicated by a cross. Note that the proteasome may not cleave at the appropriate position and that the mutation-containing peptide may not be transported by TAP or fail to bind to HLA.