Extrinsic and intrinsic apoptotic pathways activate caspase 3 through caspases 8 and 9, respectively. To discern possible contributions of these two pathways, RAW 264.7 cells were treated with DON for 3 and 6 h and the presence of cleaved caspases 8, 9, and 3 was determined by Western blot analysis. Cleavage of caspases 3, 8, and 9 was observed (Figs. 7A and 7B), suggesting that extrinsic and intrinsic pathways could potentially be involved in DON-induced rRNA fragmentation (Fig. 8) .