A critical step in antigen presentation is the degradative processing of peptides by aminopeptidases in the endoplasmic reticulum. It is unclear whether these enzymes act only on free peptides or on those bound to their major histocompatibility complex (MHC)-I-presenting molecules. A recent study examined the structure and biophysics of N-terminally extended peptides in complex with MHC-I, revealing the conformational adjustment of MHC to permit both binding of the peptide core and exposure of the peptide N terminus. These data suggest a mechanism by which aminopeptidase access is determined and offer an explanation for how longer peptides may be displayed at the cell surface.