Fas Ligand （FasL） is a 17.34 kDa tumor necrosis factor with 152 amino acid residues. FasL is expressed from lymphoid tissue and secreted to blood. Binding to its receptor, TNFRSF6/FAS, leads to induce apoptotic signal into cells. Involved in cytotoxic T-cell-mediated apoptosis, natural killer cell-mediated apoptosis and in T-cell development.