Fas Ligand (FasL), also known as CD178, CD95L, or TNFSF6, is a 40 kDa type II transmembrane member of the TNF superfamily of proteins. Fas Ligand itself transmits reverse signals that costimulate the proliferation of freshly antigen-stimulated T cells. Apoptosis triggered by Fas-Fas ligand binding plays a fundamental role in the regulation of the immune system. Its functions include: T-cell homeostasis, cytotoxic T-cell activity, immune privilege, maternal tolerance, tumor counterattack. Defective Fas-mediated apoptosis may lead to oncogenesis as well as drug resistance in existing tumors. Germline mutation of Fas is associated with autoimmune lymphoproliferative syndrome (ALPS), a childhood disorder of apoptosis.