FAS antigen is a cell surface protein that mediates apoptosis.
Several types of ligands, including antibody to this antigen, can
engage this antigen, thereby triggering apoptosis. The presence of
FAS on cell surface, thus, per se, does not indicates that the cell
is actually undergoing apoptosis.
ApopTag kit, and similar methods of labeling DNA strand breaks (e.g.
the most recent single step - kit provided by Phoenix Flow) detect
the presence of DNA strand breaks, i.e. they identify the cells which
are actually in the process of apoptosis. Thus, regardless whether
apoptosis is induced via FAS system or by other pathways (e.g DNA
damage), cells undergoing apoptosis can be detected by analysis of
DNA strand breaks.
Zbigniew Darzynkiewicz