Engagement of the cell death surface receptor Fas by Fas ligand (FasL) results in apoptotic cell death, mediated by caspase activation. Cell death mediated via Fas/FasL interaction is important for homeostasis of cells in the immune system and for maintaining immune-privileged sites in the body. Killing via the Fas/FasL pathway also constitutes an important pathway of killing for cytotoxic T cells. Fas ligand is induced in activated T cells, resulting in activation-induced cell death by the Fas/FasL pathway. Recently it has been shown that the Fas receptor can also be up-regulated following a lesion to the cell, particularly that induced by DNA-damaging agents. This can then result in killing of the cell by a Fas/FasL-dependent pathway. Up-regulation of Fas receptor following DNA damage appears to be p53 dependent.