In our study, we used a series of MHC class I expressing MM cell lines to more closely mimic the situation in patients because surface expression of MHC class I molecules is an important inhibitory factor for NK cells, and sensitivity of tumor cells for NK cells is regulated by MHC class I. The expression of MHC by our cell lines might also explain the relatively low levels of CD107a that we observed when analyzing NK cell degranulation. This evidence concerns the gene HLA-C and Miyoshi myopathy.