Also called APO-l/Fas, it is a transmembrane protein (embedded within the surface membrane of the cell) that transmits apoptosis (“programmed” cell death) “signal” into cells. Transduction of that apoptosis signal occurs when certain ligands or antigens (i.e., the APO-l/Fas antigen) bind to the extracellular (i.e., portion outside of cell membrane) part (i.e., receptor) of the CD95 protein.