Fas ligand (FasL) is a type II-membrane protein, belonging the tumor necrosis factor (TNF)/nerve growth factor (NGF) family. It is known that engagement of Fas (receptor of FasL) by FasL causes apoptotic cell death. FasL is predominantly expressed in activated T lymphocytes and Natural Killer (NK) cells and expressed in the tissues of the immune-privilege sites such as the testis and eye. On the other hand, Fas is expressed in various tissues with abundant expression in the thymus, liver, heart, and kidney. Furthermore, a variety of cancers such as melanomas were found to express FasL in addition to Fas. It is said that, Fas/FasL system is involved in the deletion of dangerous activated or autoreactive T and B cells, and in tern play a role in human diseases such as fulminant hepatitis, AIDS, cancers and other diseases involving CTL-induced tissue destruction. Furthermore, FasL can be used as an immune-suppressive agent. Rejection of transplants is mediated by activated T cells that may express functional Fas. If a transplanted tissue is engineered to express FasL or is co-transplanted with FasL-expressing cells, the transplant may be tolerated.Synonyms: APT1LG1, APTL, Apoptosis antigen ligand, CD95L protein, FASL, FASLG, Fas antigen ligand, TNFSF6, Tumor necrosis factor ligand superfamily member 6