Extrinsic and intrinsic pathways of apoptosis. The extrinsic pathway is initiated by ligation of death receptors with death ligands. This interaction results in the formation of the death inducing signaling complex (DISC), which contains the death receptor, an adaptor molecule, and pro-caspase 8. Adaptor molecules possess a death domain (DD) and a death effector domain (DED). At the DISC, caspase 8 is autocatalytically activated and subsequently transmits the death signal to effector executioner caspases, resulting in apoptotic cell death. The intrinsic pathway signaling cascade is triggered by a number of factors, including DNA damage, hypoxia, growth factor deprivation, and ER stress. The death signal is sensed initially by the BH3-only protein, which then interacts with the downstream mediators of apoptosis (BAX and BAK). BAX and BAK undergo distinct conformational changes, which lead to the formation of mitochondrial pores or increases the permeability of the mitochondrial outer membrane, thereby releasing apoptogenic compounds, e.g. cytochrome c. Released cytochrome c binds to APAF-1 to facilitate formation of the apoptosome, a wheel-shaped heptameric complex, which can then recruit and activate pro-caspase 9. As a consequence, caspase 9 activates effector caspases (caspase 3, 6, or 7) and eventually leading to apoptosis. Abbreviations: TNF, tumor necrosis factor; TRAIL, TNF related apoptosis inducing ligand; FADD, Fas-associated death domain protein; TRADD, TNF receptor-associated death domain protein; BID, BH3 interacting-domain death agonist; tBID, truncated BID; BAX, Bcl-2 homologous antagonist/killer; BAK, Bcl-2 associated X protein; APAF-1, apoptotic protease activating factor-1.