Measurement of synergism of XIAP depletion and TRAIL on viability of SW620 cells. Cells were electroporated with varying concentrations of siRNA and TRAIL was added 40 hr post electroporation. A and B. Viability was measured using ATPlite 16 hr following addition of TRAIL. Combination index was determined using Compusyn software. A combination index of < 0.1 is indicative of very strong synergism. (Chou et. al., 2006). 1 μM s1456 XIAP siRNA (black circle). TRAIL (black square) TRAIL and 1 μM s1456 XIAP siRNA (black triangle). C. XIAP protein levels at 48 hr post electroporation.