CASP9 and acute myeloid leukemia: To eliminate AML-MSC interactions as the sole cause, we first preconditioned MSCs with AML cell lines (transduced with an inducible caspase-9 [iCASP9] suicide gene) for 5 days and then cocultured them with normal CD34 after removal of the AML cell lines using AP1903 to kill specifically all AML cell lines (ref. 51, Figure 2D, and Supplemental Figure 2B).