Signal transduction by FAS. Protein-protein interactions between the death domains (horizontal stripes) of FAS and FADD and the death-effector domains (stippled) of FADD and pro-caspase 8 are shown. Trimerization of receptor complexes after binding of FAS to FAS ligand results in the cleavage of pro-caspase 8 and the release of active caspase 8.