Knock-down of expression is associated with increased caspase activity in both EBV⁻ and EBV⁺ PEL cells. EBV⁻, KSHV⁺ PEL cells (BC-3; A) and dually EBV/KSHV-positive PEL cells (JSC-1; B,C) were transfected with synthetic siRNAs targeted at vIRF-3 (si463, A,B; si976, C). Mock-transfected cells (HiPerFect only) or scrambled siRNAs (siN) were used as negative controls. Cells treated with the apoptosis-inducing agent etoposide (etop.) served as a positive control for caspase induction. Cells were harvested 2 days after transfection, and activity of caspase-3/7 was measured in duplicates. Arithmetic means and total range of values are given from 4 (A), 6 (B), or 3 (C) independent transfection experiments (data were normalized to siN in individual experiments). Although a slight increase of caspase activity could be observed by transfection of scrambled siRNAs as compared with mock-transfected cells, this difference was statistically not significant (P = .713). In contrast, transfection of si463 as well as si976 in either BC-3 or JSC-1 cells resulted in an increase of caspase activity (siN compared with si463: range, 1.5- to 3.1-fold; P < .001 2-sided t test for paired samples. Absolute values were used for statistical calculations).