Fas is a type I membrane protein belonging to the TNF/NGF receptor family, and is expressed in various tissues and cell lines. Binding of FasL or agonistic anti-Fas antibody to Fas causes apoptosis in Fas-bearing cells. That is, the Fas engagement recruits pro-caspase 8 through an adapter molecule called FADD. Oligomerization of pro-caspase 8 induces processing of pro-caspase 8 to active forms. The caspase 8 then activates other caspases in the downstream of the caspase cascade. These caspases then cleave various cellular substrates to cause morphological changes of the cells, and to degrade chromosomal DNA. The Fas-mediated apoptosis is involved to kill the activated T cells and В cells, thus down regulating the immune reaction. The loss-of-function mutations in Fas in human and mouse cause lymphoproliferation. and accelerate autoimmune diseases.