The waste system of living cells, the proteasome, not only shreds disused or damaged proteins. It also supports the immune system in recognizing virally infected or cancerous cells by producing protein fragments, so-called immunopeptides. In an international collaboration, researchers led by Juliane Liepe at the Max Planck Institute (MPI) for Multidisciplinary Sciences have now simulated protein degradation by the proteasome in the laboratory and identified and quantified the peptides thereby produced. In future, the resulting data set could help predict immunopeptides and develop new vaccines against infectious diseases or cancer.