Expression and function of CD95 in K562-transfected cell lines. K562 cells were transfected with pcDNAfas, selected with G418, and cloned by limiting dilution. (A) Surface expression of CD95 detection by staining with UB2-FITC or IgG1-FITC isotype control, and flow cytometry analysis. Filled peaks represent isotype control. (B) Annexin V-FITC analysis of CH-11–mediated apoptosis. Cells were treated with 500 ng/mL CH-11 anti-CD95 antibody for 24 hours followed by Annexin-V staining and flow cytometric analysis of programmed cell death. Horizontal axis is Annexin V-FITC, y-axis is propidium iodide staining. Background levels of apoptosis with mouse IgM was determined in an identical manner, and found to range from 2% to 12% in all cell lines.