A minority of NK cells were responsible for a majority of the killing events. (A) Percentage of NK cells forming increasing number of conjugates during the 12-hour assay. (B) Percentage of NK cells responsible for the increasing number of kills. (C) Cumulative percentage of NK cells (black bars, left axis) and cumulative percentage of kills (gray line, right axis) compared with the number of kills performed. (D) Chance of killing (%) at each successive interaction with a target cell for NK cells killing at least one target during the experiment. For panels A, B, and D, the numbers above the bars represent the number of NK cells.