Antiapoptotic molecules regulate the sensitivity to apoptotic signals. In human lymphocytes apoptosis can be induced through at least 2 pathways. Stimulation of the Fas receptor results in recruitment of the initiator caspase 8, through interaction with the adaptor molecule Fas-associated death domain protein (FADD) by means of its death domains (DDs) and death effector domains (DEDs). In turn, activated caspase 8 activates the effector caspases 3, 6, and 7. Alternatively, disruption of the mitochondrial membrane results in the release of cytochrome c, which activates caspase 9 through interaction with the adaptor molecule, apoptotic protease-activating factor 1 (Apaf-1). Caspase 9 is then able to activate caspase 3. The death receptor–activated extrinsic pathway can crosstalk to the intrinsic mitochondrial pathway through the caspase 8–mediated cleavage of Bid (BH3 interacting domain death agonist). Inhibitors of apoptosis that engage at different steps of these pathways regulate the balance between apoptosis and survival. FLIP prevents the full activation of caspase 8. Antiapoptotic bcl-2 (B-cell CLL/lymphoma-2) family members prevent apoptosis initiated via the mitochondria. Inhibitors of apoptosis proteins (IAPs including X-linked IAP [XIAP]) can either prevent activation of caspases 3, 7, and 9 or inhibit their activated forms. DISC indicates death-inducing signaling complex; FasL, Fas ligand; c-FLIP, cellular FLICE-like inhibitory protein.