Effect of snake venom toxin on viability of human colon cancer cells. HCT116 cells and HT-29 cells were inoculated into 24-well plates (5 × 104 cells/well) and thereafter treated with snake venom toxin (0.1, 0.5, 1 μg/ml) at 37°C for 24 h. a, Cell viability of HCT 116 cell, HT-29 cell and CCD18Co cells was determined by direct counting viable cells in Neubauer chamber. The results were expressed as a percentage of viable cells. b, Analysis of apoptosis by TUNEL assay. The colon cancer cells (HCT116 and HT-29) were treated with snake venom toxin (0.1-1 μg/ml) for 24 h, and then labeled with TUNEL solution. Total number of cells in a given area was determined by using DAPI nuclear staining (fluorescent microscope). The apoptotic index was determined as the DAPI-stained TUNEL-positive cell number/total DAPI stained cell number (magnification, 200x). Columns, means of three experiments, with triplicates of each experiment; bars, SD. *, p <0.05, significantly different from snake venom toxin-untreated control cells.