Immunofluorescence microscopy indicates that CX-1 cells (A) express lower quantities of cell-surface Fas than MIP-101 cells (B). Cells were labeled with mouse anti-Fas antibody and then treated with antimouse FITC-conjugated antibody. Note that CX-1 cells (A) show a discontinuous ring of fluorescence on their cell surface; in contrast, MIP-101 cells (B) stain very intensely with secondary antibody.