Fas / APO1 (CD95) is an important member of the tumour necrosis factor (TNF) superfamily involved in membrane mediated apoptosis. Ligation of Fas by Fas Ligand or an anti Fas cross linking antibody, triggers activation of the caspase cascade. Functional impairment of the Fas / Fas Ligand system is associated with the development and progression of malignancies. Fas gene mutations have been suggested to have a role in testicular germ cell tumors. Tumor cells frequently exhibit de novo expression of Fas Ligand, which plays a significant role in local tissue destruction, metastatic spread, and immune escape of the tumor cells. The apoptosis of lymphocytes, which occurs in autoimmune diseases, is usually induced by the Fas/Fas Ligand system. The full length of FasL contains 281 amino acids, consisting of the transmembrane domain, extracellular domain, and the cytoplasmic region. A metalloproteinase cleaves the membrane-bound FasL to produce soluble Fas (sFasL), which exists as a homotrimer. The sFasL shows lower cytotoxicity than membrane bound FasL.

Cell membrane: single pass type II membrane protein. Secreted protein: may be released into the extracellular fluid, probably by cleavage form the cell surface.