Fig. (3) Overview of apoptotic pathways and viral anti-apoptotic strategies: In the perforin/granzyme pathway (bottom left), effector caspases are directly activated by granzyme B, which enters the cells with the help of perforin. The death receptor mediated pathway (top left) is induced by death ligands of the TNF superfamily. Upon receptor engagement the DISC is formed and caspase-8 is activated which then activates effector caspases such as caspase-3, -6 and -7. The mitochondrial pathway (right) is initiated by various stress signals and mediated by BH3-only proteins. Once activated, BH3 only proteins inhibit the anti-apoptotic Bcl-2 like proteins, Bak and Bax can then breach the integrity of the outer mitochondrial membrane and release factors such as cytochrome c from the intermembrane space. Release of cytochtome c results in apoptosome formation, caspase-9 cleavage and effector caspase activation. The death receptor and the mitochondria mediated pathways are linked via the BH3 only protein Bid, which is cleaved to tBid by caspase-8 and amplify the extrinsic signal via the mitochondrion. Anti-apoptotic strategies are indicated in red at the appropriate points within the apoptotic pathway.