Fas expression in the U-266-1970, U-266-1984, and U-1958 cell lines. Cells were incubated in the absence (U-266-1984) and presence (U-266-1970 and U-1958) of IL-6 (20 U/mL). Cells were incubated in the absence or in the presence of IFN-γ (1,000 U/mL) or IFN- (1,000 U/mL) for 96 hours and subjected to flow cytometry analysis using the UB2 antibody. The population of Fas-positive cells was gated in the analysis by the use of mouse anti-IgG1 antibodies.