Comparison of apoptotic effect of Fas triggering on CD34⁺ BM cells from representative CML patients in chronic phase who showed optimal (A) or poor (B) response to IFN-α treatment. Values (mean ± SEM of triplicate measurements) represent percentage of apoptotic CD34⁺ BM CML cells evaluated by the Tdt assay. CD34⁺ cells were purified from CML BM and cultured in the presence of IFN-α and MoAb CH11. MoAb CH11 was used at a concentration of 1 μg/mL. TdT assay was performed as described in Materials and Methods. Each experiment was performed in triplicate. Statistical analysis (paired t-test) showed P < .05 only in the responder group, for control versus IFN-α 100 and 1,000 U/mL; control versus Fas triggering; any IFN-α concentration versus IFN-α + Fas triggering.