Immune cells patrol past, reading those samples. Fewer MHC class I molecules on a cell surface means fewer warning signals for CD8 T cells, the immune cells that kill infected and abnormal cells. The work builds on a 2023 finding from the same laboratory that depriving colon cancer cells of arginine increased the mutations they accumulated. Arginine scarcity suppresses production of MHC class I proteins, the molecules that let immune cells recognize infected or abnormal cells. Colon cancer, influenza, and SARS-CoV-2, in mice, plus a correlation observed in human colorectal cancer samples.