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CLASS I molecules of the major histocompatibility complex (MHC) bind and present peptides derived from the degradation of intracellular, often cytoplasmic, proteins, whereas class II molecules usually present proteins from the extracellular environment^1–6. It is not known how peptides derived from cytoplasmic proteins cross a membrane before presentation at the cell surface. But certain mutations in the MHC can prevent presentation of antigens with class I molecules^7–9. In addition, mutations possibly in the MHC can affect presentation by class II molecules¹⁰. Here we report the finding of a new gene in the MHC that might have a role in antigen presentation and which is related to the ABC (ATP-binding cassette) superfamily of transporters^11,12. This superfamily includes the human multidrug-resistance protein, and a series of transporters from bacteria and eukaryotic cells capable of transporting a range of …