The function of major histocompatibility complex (MHC) class I molecule is to sample peptides derived from intracellular proteins and to present these peptides to CD8+ T cells. We analyzed twenty-nine human cancer cell lines for expression of the TAP-1/2 and LMP-2/7 genes and identified whether defects in these genes were associated with the level of expression of MHC molecules in cancer cells. Expression of TAP-1 was positive in 86% (25/29) of the cell lines, TAP-2 expression was positive in 82% (24/29), LPM2 was positive in 86% (25/29), and LMP-7 was positive in 52% (15/29) at the cell lines. In fifteen cell lines that expressed LMP-7, the mean fluorescence intensity (MFI) was 825.1 +/- 123.2; however, in the cell lines that lacked LMP-7, the MFI was 314.7 +/- 77.2. These data suggest that the expression level of MHC class I molecules is associated, in most cases, with the loss of the LMP-7 gene but without the loss of the TAP-1, TAP-2 or LMP-2 genes.