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Group I Burkitt lymphoma (BL) lines retaining the original BL tumor cell phenotype are unable to present endogenously expressed antigens to HLA class I‐restricted cytotoxic T cells (CTL) but can be recognized if the relevant HLA class I/peptide epitope complex is reconstituted at the cell surface by exogenous addition of synthetic target peptide. Endogenous antigen‐processing function is restored in BL lines that have undergone Epstein‐Barr virus (EBV)‐induced drift in culture to the group III phenotype typically displayed by EBV‐transformed lymphoblastoid cell lines (LCL) of normal B cell origin. We compared group I versus group III cells for their expression of proteasome components, transporter proteins and HLA‐class I antigens, all of which are thought to be involved in the endogenous antigen processing pathway. By Western blot analysis, there were not consistent differences in the low molecular mass …