Fas Ligand (FasL), also known as CD178, CD95L, or TNFSF6, is a transmembrane protein homotrimer that binds to Fas/CD95 and triggers apoptosis in the Fas-expressing cell. Fas Ligand also binds the soluble decoy receptor DcR3. It is expressed on activated Th1 cells, CD8+ cytotoxic T cells, and NK cells. A soluble form can be released which remains trimeric and retains the ability to bind Fas. Fas Ligand-induced apoptosis plays a central role in the development of immune tolerance and the maintance of immune privileged sites. Tumor cells can evade immune surveillance by upregulating Fas Ligand to kill tumor infiltrating lymphocytes. In gld mice, a Fas Ligand point mutation is the cause of severe lymphoproliferation and systemic autoimmunity.