CASP3 and cancer: Figure 1b shows that cells actively proliferated in the absence of gemcitabine and at low concentrations of the drug; however, at gemcitabine concentrations above 4 nM, proliferating cells were no longer observed. Previous studies have shown that the main mode of action of gemcitabine is the induction of apoptosis in cancer cells [29]. Cleaved caspase-3 is a commonly used marker to detect apoptotic cells. As can be seen in Figure 1c, the number of cells positive for cleaved caspase-3 increased with rising concentrations of gemcitabine.