K562 cells (target cells) were labelled with CFSE and co-cultured with NK cells (effector cells) at the ratios indicated above each panel for four hours at 37°C. At the end of co-incubation, dead cells were labeled with 7-AAD. The percentage of K562 cell death was analyzed by flow cytometry. When cultured alone, there is little cell death (2%) on the K562 cell population. Addition of NK cells at a ratio as low as 6.25:1 caused a significant increase in K562 cell death (8.2%). At a ratio of NK cells to target cells of 25:1, there is 19.7% death of K562 cells.