Apoptosis is also referred to as 'programmed cell death'. Mitochondrial (Intrinsic) pathway of apoptosis is triggered due to cellular injury caused by: Growth factor withdrawal DNA damage (by radiation, toxins, free radicals) Protein misfolding (ER stress) Death receptor (Extrinsic) pathway of apoptosis is triggered by activation of Fas and TNF receptors. The ligand for Fas receptor is called Fas ligand (FasL). FasL is expressed on T cells that recognize self antigens and on some CTLs that kill virus-infected and tumor cells.