How does our immune system know whether a virus is lurking inside a cell, whether a normal cell has turned cancerous, or whether a cell is from an organ that was transplanted from an unrelated donor? Such cells are distinct from our own normal cells,but the differences are often hidden deep inside a cell's genome. However, our immune system has the remarkable ability to survey these differences through a process called antigen presentation. On page 1444 of this issue, Warren et al. (1) reveal an unexpected mechanism by which cells generate these presented antigenic peptides, increasing the breadth of proteins generated from the genetic code.
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