A hepatoma cell line, Hep G2, reveals the diminished HLA class I surface expression and the reduced expression of LIMPS, LMP7, and tapasin transcripts, suggesting that the reduced expression of these transcripts may be associated with the low expression of HLA class I molecules. Introduction of tapasin gene dramatically upregulates the surface expression of HLA class I molecules on Hep G2 cells, and unexpectedly, enhances the expression of LIMPS and LMP7 transcripts as well. Unlike Hep GS, these tapasin-transfected Hep G2 cells are recognized by allo-specific CTL. However, the transfectant is unable to endogenously present an HIV envelope peptide to an HIV-specific CTL clone, suggesting that a proteasome-independent antigen processing pathway exists and still remains defective in the transfectant, These data may provide significant evidence that the nonproteasomal antigen processing pathway as well as the proteasomal pathway may be impaired in tumor cells to escape immune surveillance performed by CTL.