HLA-C and neoplasm: The K562 tumor cell line was selected as stimulatory cells because they, (1) are mostly devoid of MHC determinants and thus sensitive to NK cell cytotoxicity, (2) grow in suspension culture easing the interaction with NK cells, (3) have a relatively short doubling time of 18–24 h, and (4) are easily and stably transduced by retrovirus to enforce expression of antigens as desired.