CD34 and leukemia: Table 1 shows the potency (IC50) of holotoxin A1 in each cell type. These results indicated that holotoxin A1 induced apoptosis in multiple leukemia types. Moreover, the holotoxin A1 concentrations used in this study (0.01–0.6 μM) did not alter the rate of apoptosis in normal human hematopoietic progenitor cells (CD34+ cells), demonstrated with annexin-V/PI staining (Figure 5B).