Fas ligand, also known as TNFSF6, FasL or CD95L, is a member of the tumor necrosis factor (TNF) family. It is a homotrimeric type II transmembrane protein expressed on cytotoxic T lymphocytes. Cleavage of membrane-bound FasL by the ecto-matrix metalloproteinase MMP-7 at a conserved cleavage site generates soluble Fas ligand.Fas ligand/receptor interactions play an important role in the regulation of the immune system and cancer progression. It generates signals through the trimerization of FasR, and this trimerization usually leads to apoptosis or cell death. Apoptosis induced by Fas-Fas ligand binding plays a crucial role in the regulation of the immune system, which functions in T-cell homeostasis, cytotoxic T-cell activity, immune privilege, maternal tolerance, and tumor counteraction. Defective Fas-mediated apoptosis can lead to tumorigenesis and drug resistance in existing tumors.Mutations in the Fas gene are associated with autoimmune lymphoproliferative syndrome (ALPS).