Inhibitor of apoptosis suppression sensitises cells to TRAIL-induced apoptosis. (a) BT20 and MDAMB468 cells were transfected with siRNA targeting XIAP, and 24 hours later cells were treated with TNF-related apoptosis-inducing ligand (TRAIL) (10 ng/ml) for 48 hours. Cells were then either (i) analysed by western blotting to confirm knockdown or (ii) cytospun and scored for apoptosis by staining for nuclear morphology. Data presented as fold changes in apoptosis (mean ± standard error of the mean (SEM)) from at least three independent experiments. (b) BT474 cells were either transfected with siRNA targeting XIAP and drug treated as above or treated with the Smac mimetic (10 nM) for 2 hours prior to TRAIL (10 ng/ml) addition for 48 hours: (i) knockdown was confirmed by western blotting (cIAP1 only, as cIAP2 was not detected in BT474 cells), and (ii) apoptosis was scored by examining nuclear morphology. Data presented as fold changes in apoptosis (mean ± SEM) from at least three independent experiments. (c) Effect of the Smac mimetic on MDAMB468 cells, examined as for BT474 cells in (b). Data presented as mean ± SEM from at least three experiments. nt, not treated; con, no IAP inhibitor.