MHC class I molecules present short peptides derived from intracellular proteins at the cell surface, which enables CD8+ T cells to 'screen' these cells for signs of infection or cancer. Defining the exact peptide repertoire of MHC class I molecules could aid the design of vaccines and other immunotherapies. A new study describes an improved mass spectrometry-based method for identifying such peptides. Mommen et al. found that combined electron-transfer and higher-energy collision dissociation increased the detectable peptide repertoire by approximately threefold compared with more established techniques. This study offers some interesting new insights: in particular, although previous work suggested that each intracellular protein is represented by only a single peptide on MHC class I molecules, the authors' data indicate that at least two peptides from most proteins are presented at the cell surface.