TRAIL-induced apoptosis of IL-2–activated NK cells (10 days) in the presence of anti-FLIP siRNAs. siRNAs were nucleofected into NK cells 24 hours before TRAIL addition. (A) Negative control was IL-2–treated NK cells + siRNA. The percentage of living cells (37.5%) was calculated by selecting the events with a PI fluorescence lower than 10° relative units. (Insert) Ten-day IL-2–stimulated control lymphocytes. (B) IL-2–treated NK cells + siRNA + TRAIL. The percentage of living cells was 15.0%. (C) TRAIL-induced apoptosis in IL-2–stimulated NK cells with or without siRNA. Results obtained from 3 unrelated donors are reported. Values are expressed as means ± standard deviation.